How is polymorphysm look like by using Interfaces? - c#

I don't get the connection of Interfaces To polymorphism.
Polymorphism for me is about executing a method in a different way for some different concrete classes using abstract methods or virtual methods+ overriding and therefore this is only linked to inheritance in my vision, but how do you override methods With Interfaces??
How do you use Interfaces for doing same method in different ways and giving the object to decide what to do according to its concrete type?
Thanks


As stated by Andreas Hartl in his article on Inheritance Vs. Interfaces:
Many high-level languages support inheritance and interfaces, and for
someone new to the concepts, it's sometimes not clear which one to
choose. Although languages differ in their exact handling of
inheritance and interfaces, the basics are usually the same, so this
tip should be valid for most languages.
Inheritance means that we derive one class (the derived class) from
another class (the base class). The derived class is an extension of
the base class. It contains all the features (methods and data
members) of the base class, can extend it with new features, and can
reimplement virtual methods of the base class. Some languages, like
C++, support multiple inheritance, where a derived class can have
multiple base classes, but usually inheritance is restricted to a
single base class.
Interfaces can usually only define methods and no data members (but C#
for example allows data members in the form of properties within
interfaces), and a class can always implement multiple interfaces. An
interface contains only method definitions without implementations,
and a class that implements an interface supplies the implementation.
So, using inheritance, you write a base class with method
implementations, and when you derive a class from it, this class will
inherit everything from the base class, and is immediately able to use
its features. An interface on the other hand is just a contract of
method signatures, and a class that wants to implement an interface is
forced to supply the implementations for all methods of the interface.
So when do you use which? In some cases, the language already dictates
what you use: if you need your class to have multiple 'parents', you
cannot use inheritance in languages that don't support multiple
inheritance. And if you want to reuse a library object, you have to
use the fitting concept, depending on if that library object is a
class or an interface.
But which to use if you are free to choose? Basically, base classes
describe and implement common behavior of related types, while
interfaces describe functionality that unrelated types can implement.
Inheritance describes 'is a' relationships, interfaces describe
'behaves like' relationships. For example, say that you are writing a
flight simulator. Your basic entity, which you will for example store
in a list, will be 'Airplane'. Your concrete types will be 'Concorde'
and 'Phantom'. So how should you model the three types? Concorde and
Phantom are related, they both are airplanes and share data, like
'Weight' or 'MaxSpeed' and functionality, like 'Accelerate', so we can
model them with inheritance. 'Airplane' will be the base class with
common data and methods, and 'Concorde' and 'Phantom' will derive from
'Airplane'. We could say that both are specialized airplanes, which is
why it's often said that inheritance means specialization. Now assume
that we also add a class 'Pilot' to our program, and we want to give
the user the ability to save the game and load it later. So when he
saves the game, we need to save the state of all Aircrafts and the
state of all Pilots. And we want to do this in one function that takes
just a list of all saveable objects. So how do we model this? To
answer this, we must take a look at the different types we want to
save. Pilots and Airplanes. It's obvious that they are not related at
all. They share no common data and no common functionality. We can see
that writing a base class 'Saveable' and derive both Pilot and
Airplane from it would make little sense, since no code in Saveable
could be reused by Airplane or Pilot, since both have no common
properties. In this case, an interface is the best solution. We can
write an interface 'ISaveable' with a method Save(). Pilot could then
implement ISaveable.Save() by saving his name, while Airplane could
save its current speed and coordinates.
As you can see, a clear image of the relationship between classes
often makes the choice clear: Use inheritance for related types, where
each derived class 'is a' base class. Use interfaces for unrelated
types which have some common functionality.
Here are some more points to consider with inheritance and interfaces:
Interfaces are fixed. When you change an interface, you have to change every class implementing that interface. But when you change a
base class, every derived class will gain the new functionality, which
can both be good (if you make a bugfix in some base class method
implementation, a derived class using that method will gain the bugfix
without you needing to change it) or bad (if a change in the baseclass
introduces a new bug, all derived classes using the method will be
bugged too).
Interfaces are usually more flexible, since in most languages you can only derive from one class, but implement many interfaces
Interfaces help to protect internal classes: Assume class A has an internal object b of class B. When a method in A returns a pointer or
reference to b, the code that called this method now has access to the
whole object b, which can be dangerous if A only wants to expose
certain members of b. This problem can be solved if you create an
interface I with just the members which are safe to expose. When B
implements this interface, and your method in A returns b via an I
pointer or reference, the outside code can only do what you allow
through the interface.


Polymorphism as a concept does not require inheritance, although in many languages inheritance is the only way to achieve it. Some languages, like smalltalk allow you to polymorphically use any type that implements the same set of members and properties. If it looks like a duck, quacks like a duck, and walks like a duck, you can treat it like a duck.
Polymorphism is simply the ability to treat one object as another object, by providing the same way to access and use it as the original object. This is best illustrated by the Liskov Substitution Principle. This is called the "Interface" or sometimes "Contract", because it defines a "signature" that another object can use to do interesting things to the object.
in C#, you can inherit from interfaces or other (non-sealed) classes. The difference is that an interface does not provide any actual storage or methods (only their "signature"), it is merely a definition. You can't instantiate an interface, you can only instantiate an object that implements an interface.
Classes implement an interface (IDisposable, for instance) in the same way you build a house based on blue prints. If you build two houses with the same blueprints, then each house has the exact same "interface", they may have different color paint, or carpeting, but they function in exactly the same way, yet they are two distinctly different houses, with many differences in how various things might function.
When it comes to C#, just know that an interface says what properties or members an object that implements it MUST have. Likewise, in C#, a big difference is that you can inherit multiple interfaces but only ever a single class. (ie public class Test : BaseClass, IDisposable, ITest, IFooBar)


consider this...
public int SomeMethod(SomeBaseClass object)
{
// Pass in a descendant classe that implements / overrides some method in SomebaseClass
}
public int SomeMethod(ISomeInterface intf)
{
// pass in concrete classes that implement some ISomeInterface function
}
This is the basic essence of polymorphic behavior, a common contract, implemented specifically by a specialist class.

Related

Why is the List Class Implementing IList Interface?

My Question might feel a little stupid but i really want to know why was the collection hierarchy designed in such a way that it contains Interfaces and classes whereas it could have been only classes or just one interface at the top and rest classes.
Can someone explain me the reason?

Each class can be derived from only one class but multiple interfaces.
Why do we need multiple interfaces?
Generally there are two reasons that we have multiple Interfaces for one class, 1. One class can have different types of behaviors, 2. Open-Closed Principle.
A short definition for Interface is: Interface is an entity that determines how the derived classes must behave, and since there are different types of behaviors, we need possible different Interfaces for each class.
Moreover, according to the 2nd principle of SOLID which is short for Open-Closed Principle, meaning that entities must be open for extensions but closed for modifications, even if you want to add a new behavior (e.g. CanBark) to a specific class and you already have a relevant Interface (IDogActions) for that as a parent of your class, it is recommended not to modify The IDogActions Interface, because there might be many other classes implementing this Interface and don't need the new action, instead you can create a new Interface (e.g. IDogAuditoryActions).

Think, if you only define an IList interface then you need classes to implement it. List is just one such implementation, a default if you will.
On the other hand if you only had a list as abstract class (i.e. without an IList interface) then all your custom list collection classes would have to inherit. I would say it would have been a bad design choice, as the members of IList are more suited for contracts (individual add/remove methods can have different implementations, e.g. priority list, etc.). By making List available as an abstract class you would be forcing and controlling people's design choice.
Also in C# you would be additionally constraint to not be able to inherit from any other class for your custom collections.
Basically you should try and see the different features offered by an interface and an abstract class. That would make you realize the difference between IList and List.

Is a base class sufficient, or is it ALWAYS worth creating an interface also? [duplicate]

When should I use an interface and when should I use a base class?
Should it always be an interface if I don't want to actually define a base implementation of the methods?
If I have a Dog and Cat class. Why would I want to implement IPet instead of PetBase? I can understand having interfaces for ISheds or IBarks (IMakesNoise?), because those can be placed on a pet by pet basis, but I don't understand which to use for a generic Pet.

Let's take your example of a Dog and a Cat class, and let's illustrate using C#:
Both a dog and a cat are animals, specifically, quadruped mammals (animals are waaay too general). Let us assume that you have an abstract class Mammal, for both of them:
public abstract class Mammal
This base class will probably have default methods such as:
Feed
Mate
All of which are behavior that have more or less the same implementation between either species. To define this you will have:
public class Dog : Mammal
public class Cat : Mammal
Now let's suppose there are other mammals, which we will usually see in a zoo:
public class Giraffe : Mammal
public class Rhinoceros : Mammal
public class Hippopotamus : Mammal
This will still be valid because at the core of the functionality Feed() and Mate() will still be the same.
However, giraffes, rhinoceros, and hippos are not exactly animals that you can make pets out of. That's where an interface will be useful:
public interface IPettable
{
IList<Trick> Tricks{get; set;}
void Bathe();
void Train(Trick t);
}
The implementation for the above contract will not be the same between a cat and dog; putting their implementations in an abstract class to inherit will be a bad idea.
Your Dog and Cat definitions should now look like:
public class Dog : Mammal, IPettable
public class Cat : Mammal, IPettable
Theoretically you can override them from a higher base class, but essentially an interface allows you to add on only the things you need into a class without the need for inheritance.
Consequently, because you can usually only inherit from one abstract class (in most statically typed OO languages that is... exceptions include C++) but be able to implement multiple interfaces, it allows you to construct objects in a strictly as required basis.

Well, Josh Bloch said himself in Effective Java 2d:
Prefer interfaces over abstract classes
Some main points:
Existing classes can be easily retrofitted to implement a new
interface. All you have to do is add
the required methods if they don’t yet
exist and add an implements clause to
the class declaration.
Interfaces are ideal for defining mixins. Loosely speaking, a
mixin is a type that a class can
implement in addition to its “primary
type” to declare that it provides
some optional behavior. For example,
Comparable is a mixin interface that
allows a class to declare that its
instances are ordered with respect to
other mutually comparable objects.
Interfaces allow the construction of nonhierarchical type
frameworks. Type hierarchies are
great for organizing some things, but
other things don’t fall neatly into a
rigid hierarchy.
Interfaces enable safe, powerful functionality enhancements via the
wrap- per class idiom. If you use
abstract classes to define types, you
leave the programmer who wants to add
functionality with no alternative but
to use inheritance.
Moreover, you can combine the virtues
of interfaces and abstract classes by
providing an abstract skeletal
implementation class to go with each
nontrivial interface that you export.
On the other hand, interfaces are very hard to evolve. If you add a method to an interface it'll break all of it's implementations.
PS.: Buy the book. It's a lot more detailed.

Interfaces and base classes represent two different forms of relationships.
Inheritance (base classes) represent an "is-a" relationship. E.g. a dog or a cat "is-a" pet. This relationship always represents the (single) purpose of the class (in conjunction with the "single responsibility principle").
Interfaces, on the other hand, represent additional features of a class. I'd call it an "is" relationship, like in "Foo is disposable", hence the IDisposable interface in C#.

Modern style is to define IPet and PetBase.
The advantage of the interface is that other code can use it without any ties whatsoever to other executable code. Completely "clean." Also interfaces can be mixed.
But base classes are useful for simple implementations and common utilities. So provide an abstract base class as well to save time and code.

Interfaces
Most languages allow you to implement multiple interfaces
Modifying an interface is a breaking change. All implementations need to be recompiled/modified.
All members are public. Implementations have to implement all members.
Interfaces help in Decoupling. You can use mock frameworks to mock out anything behind an interface
Interfaces normally indicate a kind of behavior
Interface implementations are decoupled / isolated from each other
Base classes
Allows you to add some default implementation that you get for free by derivation (From C# 8.0 by interface you can have default implementation)
Except C++, you can only derive from one class. Even if could from multiple classes, it is usually a bad idea.
Changing the base class is relatively easy. Derivations do not need to do anything special
Base classes can declare protected and public functions that can be accessed by derivations
Abstract Base classes can't be mocked easily like interfaces
Base classes normally indicate type hierarchy (IS A)
Class derivations may come to depend on some base behavior (have intricate knowledge of parent implementation). Things can be messy if you make a change to the base implementation for one guy and break the others.

In general, you should favor interfaces over abstract classes. One reason to use an abstract class is if you have common implementation among concrete classes. Of course, you should still declare an interface (IPet) and have an abstract class (PetBase) implement that interface.Using small, distinct interfaces, you can use multiples to further improve flexibility. Interfaces allow the maximum amount of flexibility and portability of types across boundaries. When passing references across boundaries, always pass the interface and not the concrete type. This allows the receiving end to determine concrete implementation and provides maximum flexibility. This is absolutely true when programming in a TDD/BDD fashion.
The Gang of Four stated in their book "Because inheritance exposes a subclass to details of its parent's implementation, it's often said that 'inheritance breaks encapsulation". I believe this to be true.

This is pretty .NET specific, but the Framework Design Guidelines book argues that in general classes give more flexibility in an evolving framework. Once an interface is shipped, you don't get the chance to change it without breaking code that used that interface. With a class however, you can modify it and not break code that links to it. As long you make the right modifications, which includes adding new functionality, you will be able to extend and evolve your code.
Krzysztof Cwalina says on page 81:
Over the course of the three versions of the .NET Framework, I have talked about this guideline with quite a few developers on our team. Many of them, including those who initially disagreed with the guidelines, have said that they regret having shipped some API as an interface. I have not heard of even one case in which somebody regretted that they shipped a class.
That being said there certainly is a place for interfaces. As a general guideline always provide an abstract base class implementation of an interface if for nothing else as an example of a way to implement the interface. In the best case that base class will save a lot of work.

Juan,
I like to think of interfaces as a way to characterize a class. A particular dog breed class, say a YorkshireTerrier, may be a descended of the parent dog class, but it is also implements IFurry, IStubby, and IYippieDog. So the class defines what the class is but the interface tells us things about it.
The advantage of this is it allows me to, for example, gather all the IYippieDog's and throw them into my Ocean collection. So now I can reach across a particular set of objects and find ones that meet the criteria I am looking at without inspecting the class too closely.
I find that interfaces really should define a sub-set of the public behavior of a class. If it defines all the public behavior for all the classes that implement then it usually does not need to exist. They do not tell me anything useful.
This thought though goes counter to the idea that every class should have an interface and you should code to the interface. That's fine, but you end up with a lot of one to one interfaces to classes and it makes things confusing. I understand that the idea is it does not really cost anything to do and now you can swap things in and out with ease. However, I find that I rarely do that. Most of the time I am just modifying the existing class in place and have the exact same issues I always did if the public interface of that class needs changing, except I now have to change it in two places.
So if you think like me you would definitely say that Cat and Dog are IPettable. It is a characterization that matches them both.
The other piece of this though is should they have the same base class? The question is do they need to be broadly treated as the same thing. Certainly they are both Animals, but does that fit how we are going to use them together.
Say I want to gather all Animal classes and put them in my Ark container.
Or do they need to be Mammals? Perhaps we need some kind of cross animal milking factory?
Do they even need to be linked together at all? Is it enough to just know they are both IPettable?
I often feel the desire to derive a whole class hierarchy when I really just need one class. I do it in anticipation someday I might need it and usually I never do. Even when I do, I usually find I have to do a lot to fix it. That’s because the first class I am creating is not the Dog, I am not that lucky, it is instead the Platypus. Now my entire class hierarchy is based on the bizarre case and I have a lot of wasted code.
You might also find at some point that not all Cats are IPettable (like that hairless one). Now you can move that Interface to all the derivative classes that fit. You will find that a much less breaking change that all of a sudden Cats are no longer derived from PettableBase.

Here is the basic and simple definiton of interface and base class:
Base class = object inheritance.
Interface = functional inheritance.
cheers

It is explained well in this Java World article.
Personally, I tend to use interfaces to define interfaces - i.e. parts of the system design that specify how something should be accessed.
It's not uncommon that I will have a class implementing one or more interfaces.
Abstract classes I use as a basis for something else.
The following is an extract from the above mentioned article JavaWorld.com article, author Tony Sintes, 04/20/01
Interface vs. abstract class
Choosing interfaces and abstract classes is not an either/or proposition. If you need to change your design, make it an interface. However, you may have abstract classes that provide some default behavior. Abstract classes are excellent candidates inside of application frameworks.
Abstract classes let you define some behaviors; they force your subclasses to provide others. For example, if you have an application framework, an abstract class may provide default services such as event and message handling. Those services allow your application to plug in to your application framework. However, there is some application-specific functionality that only your application can perform. Such functionality might include startup and shutdown tasks, which are often application-dependent. So instead of trying to define that behavior itself, the abstract base class can declare abstract shutdown and startup methods. The base class knows that it needs those methods, but an abstract class lets your class admit that it doesn't know how to perform those actions; it only knows that it must initiate the actions. When it is time to start up, the abstract class can call the startup method. When the base class calls this method, Java calls the method defined by the child class.
Many developers forget that a class that defines an abstract method can call that method as well. Abstract classes are an excellent way to create planned inheritance hierarchies. They're also a good choice for nonleaf classes in class hierarchies.
Class vs. interface
Some say you should define all classes in terms of interfaces, but I think recommendation seems a bit extreme. I use interfaces when I see that something in my design will change frequently.
For example, the Strategy pattern lets you swap new algorithms and processes into your program without altering the objects that use them. A media player might know how to play CDs, MP3s, and wav files. Of course, you don't want to hardcode those playback algorithms into the player; that will make it difficult to add a new format like AVI. Furthermore, your code will be littered with useless case statements. And to add insult to injury, you will need to update those case statements each time you add a new algorithm. All in all, this is not a very object-oriented way to program.
With the Strategy pattern, you can simply encapsulate the algorithm behind an object. If you do that, you can provide new media plug-ins at any time. Let's call the plug-in class MediaStrategy. That object would have one method: playStream(Stream s). So to add a new algorithm, we simply extend our algorithm class. Now, when the program encounters the new media type, it simply delegates the playing of the stream to our media strategy. Of course, you'll need some plumbing to properly instantiate the algorithm strategies you will need.
This is an excellent place to use an interface. We've used the Strategy pattern, which clearly indicates a place in the design that will change. Thus, you should define the strategy as an interface. You should generally favor interfaces over inheritance when you want an object to have a certain type; in this case, MediaStrategy. Relying on inheritance for type identity is dangerous; it locks you into a particular inheritance hierarchy. Java doesn't allow multiple inheritance, so you can't extend something that gives you a useful implementation or more type identity.

I recommend using composition instead of inheritence whenever possible. Use interfaces but use member objects for base implementation. That way, you can define a factory that constructs your objects to behave in a certain way. If you want to change the behavior then you make a new factory method (or abstract factory) that creates different types of sub-objects.
In some cases, you may find that your primary objects don't need interfaces at all, if all of the mutable behavior is defined in helper objects.
So instead of IPet or PetBase, you might end up with a Pet which has an IFurBehavior parameter. The IFurBehavior parameter is set by the CreateDog() method of the PetFactory. It is this parameter which is called for the shed() method.
If you do this you'll find your code is much more flexible and most of your simple objects deal with very basic system-wide behaviors.
I recommend this pattern even in multiple-inheritence languages.

Also keep in mind not to get swept away in OO (see blog) and always model objects based on behavior required, if you were designing an app where the only behavior you required was a generic name and species for an animal then you would only need one class Animal with a property for the name, instead of millions of classes for every possible animal in the world.

I have a rough rule-of-thumb
Functionality: likely to be different in all parts: Interface.
Data, and functionality, parts will be mostly the same, parts different: abstract class.
Data, and functionality, actually working, if extended only with slight changes: ordinary (concrete) class
Data and functionality, no changes planned: ordinary (concrete) class with final modifier.
Data, and maybe functionality: read-only: enum members.
This is very rough and ready and not at all strictly defined, but there is a spectrum from interfaces where everything is intended to be changed to enums where everything is fixed a bit like a read-only file.

Source: http://jasonroell.com/2014/12/09/interfaces-vs-abstract-classes-what-should-you-use/
C# is a wonderful language that has matured and evolved over the last 14 years. This is great for us developers because a mature language provides us with a plethora of language features that are at our disposal.
However, with much power becomes much responsibility. Some of these features can be misused, or sometimes it is hard to understand why you would choose to use one feature over another. Over the years, a feature that I have seen many developers struggle with is when to choose to use an interface or to choose to use an abstract class. Both have there advantages and disadvantages and the correct time and place to use each. But how to we decide???
Both provide for reuse of common functionality between types. The most obvious difference right away is that interfaces provide no implementation for their functionality whereas abstract classes allow you to implement some “base” or “default” behavior and then have the ability to “override” this default behavior with the classes derived types if necessary.
This is all well and good and provides for great reuse of code and adheres to the DRY (Don’t Repeat Yourself) principle of software development. Abstract classes are great to use when you have an “is a” relationship.
For example: A golden retriever “is a” type of dog. So is a poodle. They both can bark, as all dogs can. However, you might want to state that the poodle park is significantly different than the “default” dog bark. Therefor, it could make sense for you to implement something as follows:
public abstract class Dog
{
public virtual void Bark()
{
Console.WriteLine("Base Class implementation of Bark");
}
}
public class GoldenRetriever : Dog
{
// the Bark method is inherited from the Dog class
}
public class Poodle : Dog
{
// here we are overriding the base functionality of Bark with our new implementation
// specific to the Poodle class
public override void Bark()
{
Console.WriteLine("Poodle's implementation of Bark");
}
}
// Add a list of dogs to a collection and call the bark method.
void Main()
{
var poodle = new Poodle();
var goldenRetriever = new GoldenRetriever();
var dogs = new List<Dog>();
dogs.Add(poodle);
dogs.Add(goldenRetriever);
foreach (var dog in dogs)
{
dog.Bark();
}
}
// Output will be:
// Poodle's implementation of Bark
// Base Class implementation of Bark
//
As you can see, this would be a great way to keep your code DRY and allow for the base class implementation be called when any of the types can just rely on the default Bark instead of a special case implementation. The classes like GoldenRetriever, Boxer, Lab could all could inherit the “default” (bass class) Bark at no charge just because they implement the Dog abstract class.
But I’m sure you already knew that.
You are here because you want to understand why you might want to choose an interface over an abstract class or vice versa. Well one reason you may want to choose an interface over an abstract class is when you don’t have or want to prevent a default implementation. This is usually because the types that are implementing the interface not related in an “is a” relationship. Actually, they don’t have to be related at all except for the fact that each type “is able” or has “the ablity” to do something or have something.
Now what the heck does that mean? Well, for example: A human is not a duck…and a duck is not a human. Pretty obvious. However, both a duck and a human have “the ability” to swim (given that the human passed his swimming lessons in 1st grade :) ). Also, since a duck is not a human or vice versa, this is not an “is a” realationship, but instead an “is able” relationship and we can use an interface to illustrate that:
// Create ISwimable interface
public interface ISwimable
{
public void Swim();
}
// Have Human implement ISwimable Interface
public class Human : ISwimable
public void Swim()
{
//Human's implementation of Swim
Console.WriteLine("I'm a human swimming!");
}
// Have Duck implement ISwimable interface
public class Duck: ISwimable
{
public void Swim()
{
// Duck's implementation of Swim
Console.WriteLine("Quack! Quack! I'm a Duck swimming!")
}
}
//Now they can both be used in places where you just need an object that has the ability "to swim"
public void ShowHowYouSwim(ISwimable somethingThatCanSwim)
{
somethingThatCanSwim.Swim();
}
public void Main()
{
var human = new Human();
var duck = new Duck();
var listOfThingsThatCanSwim = new List<ISwimable>();
listOfThingsThatCanSwim.Add(duck);
listOfThingsThatCanSwim.Add(human);
foreach (var something in listOfThingsThatCanSwim)
{
ShowHowYouSwim(something);
}
}
// So at runtime the correct implementation of something.Swim() will be called
// Output:
// Quack! Quack! I'm a Duck swimming!
// I'm a human swimming!
Using interfaces like the code above will allow you to pass an object into a method that “is able” to do something. The code doesn’t care how it does it…All it knows is that it can call the Swim method on that object and that object will know which behavior take at run-time based on its type.
Once again, this helps your code stay DRY so that you would not have to write multiple methods that are calling the object to preform the same core function (ShowHowHumanSwims(human), ShowHowDuckSwims(duck), etc.)
Using an interface here allows the calling methods to not have to worry about what type is which or how the behavior is implemented. It just knows that given the interface, each object will have to have implemented the Swim method so it is safe to call it in its own code and allow the behavior of the Swim method be handled within its own class.
Summary:
So my main rule of thumb is use an abstract class when you want to implement a “default” functionality for a class hierarchy or/and the classes or types you are working with share a “is a” relationship (ex. poodle “is a” type of dog).
On the other hand use an interface when you do not have an “is a” relationship but have types that share “the ability” to do something or have something (ex. Duck “is not” a human. However, duck and human share “the ability” to swim).
Another difference to note between abstract classes and interfaces is that a class can implement one to many interfaces but a class can only inherit from ONE abstract class (or any class for that matter). Yes, you can nest classes and have an inheritance hierarchy (which many programs do and should have) but you cannot inherit two classes in one derived class definition (this rule applies to C#. In some other languages you are able to do this, usually only because of the lack of interfaces in these languages).
Also remember when using interfaces to adhere to the Interface Segregation Principle (ISP). ISP states that no client should be forced to depend on methods it does not use. For this reason interfaces should be focused on specific tasks and are usually very small (ex. IDisposable, IComparable ).
Another tip is if you are developing small, concise bits of functionality, use interfaces. If you are designing large functional units, use an abstract class.
Hope this clears things up for some people!
Also if you can think of any better examples or want to point something out, please do so in the comments below!

Interfaces should be small. Really small. If you're really breaking down your objects, then your interfaces will probably only contain a few very specific methods and properties.
Abstract classes are shortcuts. Are there things that all derivatives of PetBase share that you can code once and be done with? If yes, then it's time for an abstract class.
Abstract classes are also limiting. While they give you a great shortcut to producing child objects, any given object can only implement one abstract class. Many times, I find this a limitation of Abstract classes, and this is why I use lots of interfaces.
Abstract classes may contain several interfaces. Your PetBase abstract class may implement IPet (pets have owners) and IDigestion (pets eat, or at least they should). However, PetBase will probably not implement IMammal, since not all pets are mammals and not all mammals are pets. You may add a MammalPetBase that extends PetBase and add IMammal. FishBase could have PetBase and add IFish. IFish would have ISwim and IUnderwaterBreather as interfaces.
Yes, my example is extensively over-complicated for the simple example, but that's part of the great thing about how interfaces and abstract classes work together.

The case for Base Classes over Interfaces was explained well in the Submain .NET Coding Guidelines:
Base Classes vs. Interfaces
An interface type is a partial
description of a value, potentially
supported by many object types. Use
base classes instead of interfaces
whenever possible. From a versioning
perspective, classes are more flexible
than interfaces. With a class, you can
ship Version 1.0 and then in Version
2.0 add a new method to the class. As long as the method is not abstract,
any existing derived classes continue
to function unchanged.
Because interfaces do not support
implementation inheritance, the
pattern that applies to classes does
not apply to interfaces. Adding a
method to an interface is equivalent
to adding an abstract method to a base
class; any class that implements the
interface will break because the class
does not implement the new method.
Interfaces are appropriate in the
following situations:
Several unrelated classes want to support the protocol.
These classes already have established base classes (for
example,
some are user interface (UI) controls,
and some are XML Web services).
Aggregation is not appropriate or practicable. In all other
situations,
class inheritance is a better model.

One important difference is that you can only inherit one base class, but you can implement many interfaces. So you only want to use a base class if you are absolutely certain that you won't need to also inherit a different base class. Additionally, if you find your interface is getting large then you should start looking to break it up into a few logical pieces that define independent functionality, since there's no rule that your class can't implement them all (or that you can define a different interface that just inherits them all to group them).

When I first started learning about object-oriented programming, I made the easy and probably common mistake of using inheritance to share common behavior - even where that behavior was not essential to the nature of the object.
To further build on an example much used in this particular question, there are lots of things that are petable - girlfriends, cars, fuzzy blankets... - so I might have had a Petable class that provided this common behavior, and various classes inheriting from it.
However, being petable is not part of the nature of any of these objects. There are vastly more important concepts that are essential to their nature - the girlfriend is a person, the car is a land vehicle, the cat is a mammal...
Behaviors should be assigned first to interfaces (including the default interface of the class), and promoted to a base class only if they are (a) common to a large group of classes that are subsets of a larger class - in the same sense that "cat" and "person" are subsets of "mammal".
The catch is, after you understand object-oriented design sufficiently better than I did at first, you'll normally do this automatically without even thinking about it. So the bare truth of the statement "code to an interface, not an abstract class" becomes so obvious you have a hard time believing anyone would bother to say it - and start trying to read other meanings into it.
Another thing I'd add is that if a class is purely abstract - with no non-abstract, non-inherited members or methods exposed to child, parent, or client - then why is it a class? It could be replaced, in some cases by an interface and in other cases by Null.

Prefer interfaces over abstract classes
Rationale,
the main points to consider [two already mentioned here] are :
Interfaces are more flexible, because a class can implement multiple
interfaces. Since Java does not have multiple inheritance, using
abstract classes prevents your users from using any other class
hierarchy. In general, prefer interfaces when there are no default
implementations or state. Java collections offer good examples of
this (Map, Set, etc.).
Abstract classes have the advantage of allowing better forward
compatibility. Once clients use an interface, you cannot change it;
if they use an abstract class, you can still add behavior without
breaking existing code. If compatibility is a concern, consider using
abstract classes.
Even if you do have default implementations or internal state,
consider offering an interface and an abstract implementation of it.
This will assist clients, but still allow them greater freedom if
desired [1].
Of course, the subject has been discussed at length
elsewhere [2,3].
[1] It adds more code, of course, but if brevity is your primary concern, you probably should have avoided Java in the first place!
[2] Joshua Bloch, Effective Java, items 16-18.
[3] http://www.codeproject.com/KB/ar...

Previous comments about using abstract classes for common implementation is definitely on the mark. One benefit I haven't seen mentioned yet is that the use of interfaces makes it much easier to implement mock objects for the purpose of unit testing. Defining IPet and PetBase as Jason Cohen described enables you to mock different data conditions easily, without the overhead of a physical database (until you decide it's time to test the real thing).

Don't use a base class unless you know what it means, and that it applies in this case. If it applies, use it, otherwise, use interfaces. But note the answer about small interfaces.
Public Inheritance is overused in OOD and expresses a lot more than most developers realize or are willing to live up to. See the Liskov Substitutablity Principle
In short, if A "is a" B then A requires no more than B and delivers no less than B, for every method it exposes.

Another option to keep in mind is using the "has-a" relationship, aka "is implemented in terms of" or "composition." Sometimes this is a cleaner, more flexible way to structure things than using "is-a" inheritance.
It may not make as much sense logically to say that Dog and Cat both "have" a Pet, but it avoids common multiple inheritance pitfalls:
public class Pet
{
void Bathe();
void Train(Trick t);
}
public class Dog
{
private Pet pet;
public void Bathe() { pet.Bathe(); }
public void Train(Trick t) { pet.Train(t); }
}
public class Cat
{
private Pet pet;
public void Bathe() { pet.Bathe(); }
public void Train(Trick t) { pet.Train(t); }
}
Yes, this example shows that there is a lot of code duplication and lack of elegance involved in doing things this way. But one should also appreciate that this helps to keep Dog and Cat decoupled from the Pet class (in that Dog and Cat do not have access to the private members of Pet), and it leaves room for Dog and Cat to inherit from something else--possibly the Mammal class.
Composition is preferable when no private access is required and you don't need to refer to Dog and Cat using generic Pet references/pointers. Interfaces give you that generic reference capability and can help cut down on the verbosity of your code, but they can also obfuscate things when they are poorly organized. Inheritance is useful when you need private member access, and in using it you are committing yourself to highly coupling your Dog and Cat classes to your Pet class, which is a steep cost to pay.
Between inheritance, composition, and interfaces there is no one way that is always right, and it helps to consider how all three options can be used in harmony. Of the three, inheritance is typically the option that should be used the least often.

Conceptually, an interface is used to formally and semi-formally define a set of methods that an object will provide. Formally means a set of method names and signatures, and semi-formally means human readable documentation associated with those methods.
Interfaces are only descriptions of an API (after all, API stands for application programming interface), they can't contain any implementation, and it's not possible to use or run an interface. They only make explicit the contract of how you should interact with an object.
Classes provide an implementation, and they can declare that they implement zero, one or more Interfaces. If a class is intended to be inherited, the convention is to prefix the class name with "Base".
There is a distinction between a base class and an abstract base classes (ABC). ABCs mix interface and implementation together. Abstract outside of computer programming means "summary", that is "abstract == interface". An abstract base class can then describe both an interface, as well as an empty, partial or complete implementation that is intended to be inherited.
Opinions on when to use interfaces versus abstract base classes versus just classes is going to vary wildly based on both what you are developing, and which language you are developing in. Interfaces are often associated only with statically typed languages such as Java or C#, but dynamically typed languages can also have interfaces and abstract base classes. In Python for example, the distinction is made clear between a Class, which declares that it implements an interface, and an object, which is an instance of a class, and is said to provide that interface. It's possible in a dynamic language that two objects that are both instances of the same class, can declare that they provide completely different interfaces. In Python this is only possible for object attributes, while methods are shared state between all objects of a class. However, in Ruby, objects can have per-instance methods, so it's possible that the interface between two objects of the same class can vary as much as the programmer desires (however, Ruby doesn't have any explicit way of declaring Interfaces).
In dynamic languages the interface to an object is often implicitly assumed, either by introspecting an object and asking it what methods it provides (look before you leap) or preferably by simply attempting to use the desired interface on an object and catching exceptions if the object doesn't provide that interface (easier to ask forgiveness than permission). This can lead to "false positives" where two interfaces have the same method name, but are semantically different. However, the trade-off is that your code is more flexible since you don't need to over specify up-front to anticipate all possible uses of your code.

It depends on your requirements. If IPet is simple enough, I would prefer to implement that. Otherwise, if PetBase implements a ton of functionality you don't want to duplicate, then have at it.
The downside to implementing a base class is the requirement to override (or new) existing methods. This makes them virtual methods which means you have to be careful about how you use the object instance.
Lastly, the single inheritance of .NET kills me. A naive example: Say you're making a user control, so you inherit UserControl. But, now you're locked out of also inheriting PetBase. This forces you to reorganize, such as to make a PetBase class member, instead.

I usually don't implement either until I need one. I favor interfaces over abstract classes because that gives a little more flexibility. If there's common behavior in some of the inheriting classes I move that up and make an abstract base class. I don't see the need for both, since they essentially server the same purpose, and having both is a bad code smell (imho) that the solution has been over-engineered.

Regarding C#, in some senses interfaces and abstract classes can be interchangeable. However, the differences are: i) interfaces cannot implement code; ii) because of this, interfaces cannot call further up the stack to subclass; and iii) only can abstract class may be inherited on a class, whereas multiple interfaces may be implemented on a class.

By def, interface provides a layer to communicate with other code. All the public properties and methods of a class are by default implementing implicit interface. We can also define an interface as a role, when ever any class needs to play that role, it has to implement it giving it different forms of implementation depending on the class implementing it. Hence when you talk about interface, you are talking about polymorphism and when you are talking about base class, you are talking about inheritance. Two concepts of oops !!!

I've found that a pattern of Interface > Abstract > Concrete works in the following use-case:
1. You have a general interface (eg IPet)
2. You have a implementation that is less general (eg Mammal)
3. You have many concrete members (eg Cat, Dog, Ape)
The abstract class defines default shared attributes of the concrete classes, yet enforces the interface. For example:
public interface IPet{
public boolean hasHair();
public boolean walksUprights();
public boolean hasNipples();
}
Now, since all mammals have hair and nipples (AFAIK, I'm not a zoologist), we can roll this into the abstract base class
public abstract class Mammal() implements IPet{
#override
public walksUpright(){
throw new NotSupportedException("Walks Upright not implemented");
}
#override
public hasNipples(){return true}
#override
public hasHair(){return true}
And then the concrete classes merely define that they walk upright.
public class Ape extends Mammal(){
#override
public walksUpright(return true)
}
public class Catextends Mammal(){
#override
public walksUpright(return false)
}
This design is nice when there are lots of concrete classes, and you don't want to maintain boilerplate just to program to an interface. If new methods were added to the interface, it would break all of the resulting classes, so you are still getting the advantages of the interface approach.
In this case, the abstract could just as well be concrete; however, the abstract designation helps to emphasize that this pattern is being employed.

An inheritor of a base class should have an "is a" relationship. Interface represents An "implements a" relationship.
So only use a base class when your inheritors will maintain the is a relationship.

Use Interfaces to enforce a contract ACROSS families of unrelated classes. For example, you might have common access methods for classes that represent collections, but contain radically different data i.e. one class might represent a result set from a query, while the other might represent the images in a gallery. Also, you can implement multiple interfaces, thus allowing you to blend (and signify) the capabilities of the class.
Use Inheritance when the classes bear a common relationship and therefore have a similair structural and behavioural signature, i.e. Car, Motorbike, Truck and SUV are all types of road vehicle that might contain a number of wheels, a top speed

Is there any difference between interfaces and abstract classes that have abstract methods only C#? [duplicate]

I have recently had two telephone interviews where I've been asked about the differences between an Interface and an Abstract class. I have explained every aspect of them I could think of, but it seems they are waiting for me to mention something specific, and I don't know what it is.
From my experience I think the following is true. If I am missing a major point please let me know.
Interface:
Every single Method declared in an Interface will have to be implemented in the subclass.
Only Events, Delegates, Properties (C#) and Methods can exist in an Interface. A class can implement multiple Interfaces.
Abstract Class:
Only Abstract methods have to be implemented by the subclass. An Abstract class can have normal methods with implementations. An Abstract class can also have class variables besides Events, Delegates, Properties and Methods. A class can implement one abstract class only due to the non-existence of Multi-inheritance in C#.
After all that, the interviewer came up with the question "What if you had an Abstract class with only abstract methods? How would that be different from an interface?" I didn't know the answer but I think it's the inheritance as mentioned above right?
Another interviewer asked me, "What if you had a Public variable inside the interface, how would that be different than in a Abstract Class?" I insisted you can't have a public variable inside an interface. I didn't know what he wanted to hear but he wasn't satisfied either.
See Also:
When to use an interface instead of an abstract class and vice versa
Interfaces vs. Abstract Classes
How do you decide between using an Abstract Class and an Interface?
What is the difference between an interface and abstract class?

How about an analogy: when I was in the Air Force, I went to pilot training and became a USAF (US Air Force) pilot. At that point I wasn't qualified to fly anything, and had to attend aircraft type training. Once I qualified, I was a pilot (Abstract class) and a C-141 pilot (concrete class). At one of my assignments, I was given an additional duty: Safety Officer. Now I was still a pilot and a C-141 pilot, but I also performed Safety Officer duties (I implemented ISafetyOfficer, so to speak). A pilot wasn't required to be a safety officer, other people could have done it as well.
All USAF pilots have to follow certain Air Force-wide regulations, and all C-141 (or F-16, or T-38) pilots 'are' USAF pilots. Anyone can be a safety officer. So, to summarize:
Pilot: abstract class
C-141 Pilot: concrete class
ISafety Officer: interface
added note: this was meant to be an analogy to help explain the concept, not a coding recommendation. See the various comments below, the discussion is interesting.

While your question indicates it's for "general OO", it really seems to be focusing on .NET use of these terms.
In .NET (similar for Java):
interfaces can have no state or implementation
a class that implements an interface must provide an implementation of all the methods of that interface
abstract classes may contain state (data members) and/or implementation (methods)
abstract classes can be inherited without implementing the abstract methods (though such a derived class is abstract itself)
interfaces may be multiple-inherited, abstract classes may not (this is probably the key concrete reason for interfaces to exist separately from abtract classes - they permit an implementation of multiple inheritance that removes many of the problems of general MI).
As general OO terms, the differences are not necessarily well-defined. For example, there are C++ programmers who may hold similar rigid definitions (interfaces are a strict subset of abstract classes that cannot contain implementation), while some may say that an abstract class with some default implementations is still an interface or that a non-abstract class can still define an interface.
Indeed, there is a C++ idiom called the Non-Virtual Interface (NVI) where the public methods are non-virtual methods that 'thunk' to private virtual methods:
http://www.gotw.ca/publications/mill18.htm
http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Non-Virtual_Interface

I think the answer they are looking for is the fundamental or OPPS philosophical difference.
The abstract class inheritance is used when the derived class shares the core properties and behaviour of the abstract class. The kind of behaviour that actually defines the class.
On the other hand interface inheritance is used when the classes share peripheral behaviour, ones which do not necessarily define the derived class.
For eg. A Car and a Truck share a lot of core properties and behaviour of an Automobile abstract class, but they also share some peripheral behaviour like Generate exhaust which even non automobile classes like Drillers or PowerGenerators share and doesn't necessarily defines a Car or a Truck, so Car, Truck, Driller and PowerGenerator can all share the same interface IExhaust.

Short: Abstract classes are used for Modelling a class hierarchy of similar looking classes (For example Animal can be abstract class and Human , Lion, Tiger can be concrete derived classes)
AND
Interface is used for Communication between 2 similar / non similar classes which does not care about type of the class implementing Interface(e.g. Height can be interface property and it can be implemented by Human , Building , Tree. It does not matter if you can eat , you can swim you can die or anything.. it matters only a thing that you need to have Height (implementation in you class) ).

There are a couple of other differences -
Interfaces can't have any concrete implementations. Abstract base classes can. This allows you to provide concrete implementations there. This can allow an abstract base class to actually provide a more rigorous contract, wheras an interface really only describes how a class is used. (The abstract base class can have non-virtual members defining the behavior, which gives more control to the base class author.)
More than one interface can be implemented on a class. A class can only derive from a single abstract base class. This allows for polymorphic hierarchy using interfaces, but not abstract base classes. This also allows for a pseudo-multi-inheritance using interfaces.
Abstract base classes can be modified in v2+ without breaking the API. Changes to interfaces are breaking changes.
[C#/.NET Specific] Interfaces, unlike abstract base classes, can be applied to value types (structs). Structs cannot inherit from abstract base classes. This allows behavioral contracts/usage guidelines to be applied on value types.

Inheritance
Consider a car and a bus. They are two different vehicles. But still, they share some common properties like they have a steering, brakes, gears, engine etc.
So with the inheritance concept, this can be represented as following ...
public class Vehicle {
private Driver driver;
private Seat[] seatArray; //In java and most of the Object Oriented Programming(OOP) languages, square brackets are used to denote arrays(Collections).
//You can define as many properties as you want here ...
}
Now a Bicycle ...
public class Bicycle extends Vehicle {
//You define properties which are unique to bicycles here ...
private Pedal pedal;
}
And a Car ...
public class Car extends Vehicle {
private Engine engine;
private Door[] doors;
}
That's all about Inheritance. We use them to classify objects into simpler Base forms and their children as we saw above.
Abstract Classes
Abstract classes are incomplete objects. To understand it further, let's consider the vehicle analogy once again.
A vehicle can be driven. Right? But different vehicles are driven in different ways ... For example, You cannot drive a car just as you drive a Bicycle.
So how to represent the drive function of a vehicle? It is harder to check what type of vehicle it is and drive it with its own function; you would have to change the Driver class again and again when adding a new type of vehicle.
Here comes the role of abstract classes and methods. You can define the drive method as abstract to tell that every inheriting children must implement this function.
So if you modify the vehicle class ...
//......Code of Vehicle Class
abstract public void drive();
//.....Code continues
The Bicycle and Car must also specify how to drive it. Otherwise, the code won't compile and an error is thrown.
In short.. an abstract class is a partially incomplete class with some incomplete functions, which the inheriting children must specify their own.
Interfaces
Interfaces are totally incomplete. They do not have any properties. They just indicate that the inheriting children are capable of doing something ...
Suppose you have different types of mobile phones with you. Each of them has different ways to do different functions; Ex: call a person. The maker of the phone specifies how to do it. Here the mobile phones can dial a number - that is, it is dial-able. Let's represent this as an interface.
public interface Dialable {
public void dial(Number n);
}
Here the maker of the Dialable defines how to dial a number. You just need to give it a number to dial.
// Makers define how exactly dialable work inside.
Dialable PHONE1 = new Dialable() {
public void dial(Number n) {
//Do the phone1's own way to dial a number
}
}
Dialable PHONE2 = new Dialable() {
public void dial(Number n) {
//Do the phone2's own way to dial a number
}
}
//Suppose there is a function written by someone else, which expects a Dialable
......
public static void main(String[] args) {
Dialable myDialable = SomeLibrary.PHONE1;
SomeOtherLibrary.doSomethingUsingADialable(myDialable);
}
.....
Hereby using interfaces instead of abstract classes, the writer of the function which uses a Dialable need not worry about its properties. Ex: Does it have a touch-screen or dial pad, Is it a fixed landline phone or mobile phone. You just need to know if it is dialable; does it inherit(or implement) the Dialable interface.
And more importantly, if someday you switch the Dialable with a different one
......
public static void main(String[] args) {
Dialable myDialable = SomeLibrary.PHONE2; // <-- changed from PHONE1 to PHONE2
SomeOtherLibrary.doSomethingUsingADialable(myDialable);
}
.....
You can be sure that the code still works perfectly because the function which uses the dialable does not (and cannot) depend on the details other than those specified in the Dialable interface. They both implement a Dialable interface and that's the only thing the function cares about.
Interfaces are commonly used by developers to ensure interoperability(use interchangeably) between objects, as far as they share a common function (just like you may change to a landline or mobile phone, as far as you just need to dial a number). In short, interfaces are a much simpler version of abstract classes, without any properties.
Also, note that you may implement(inherit) as many interfaces as you want but you may only extend(inherit) a single parent class.
More Info
Abstract classes vs Interfaces

If you consider java as OOP language to answer this question, Java 8 release causes some of the content in above answers as obsolete. Now java interface can have default methods with concrete implementation.
Oracle website provides key differences between interface and abstract class.
Consider using abstract classes if :
You want to share code among several closely related classes.
You expect that classes that extend your abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).
You want to declare non-static or non-final fields.
Consider using interfaces if :
You expect that unrelated classes would implement your interface. For example,many unrelated objects can implement Serializable interface.
You want to specify the behaviour of a particular data type, but not concerned about who implements its behaviour.
You want to take advantage of multiple inheritance of type.
In simple terms, I would like to use
interface: To implement a contract by multiple unrelated objects
abstract class: To implement the same or different behaviour among multiple related objects
Have a look at code example to understand things in clear way : How should I have explained the difference between an Interface and an Abstract class?

The interviewers are barking up an odd tree. For languages like C# and Java, there is a difference, but in other languages like C++ there is not. OO theory doesn't differentiate the two, merely the syntax of language.
An abstract class is a class with both implementation and interface (pure virtual methods) that will be inherited. Interfaces generally do not have any implementation but only pure virtual functions.
In C# or Java an abstract class without any implementation differs from an interface only in the syntax used to inherit from it and the fact you can only inherit from one.

By implementing interfaces you are achieving composition ("has-a" relationships) instead of inheritance ("is-a" relationships). That is an important principle to remember when it comes to things like design patterns where you need to use interfaces to achieve a composition of behaviors instead of an inheritance.

These answers are all too long.
Interfaces are for defining behaviors.
Abstract classes are for defining a thing itself, including its behaviors. That's why we sometimes create an abstract class with some extra properties inheriting an interface.
This also explains why Java only supports single inheritance for classes but puts no restriction on interfaces. Because a concrete object can not be different things, but it can have different behaviors.

Conceptually speaking, keeping the language specific implementation, rules, benefits and achieving any programming goal by using anyone or both, can or cant have code/data/property, blah blah, single or multiple inheritances, all aside
1- Abstract (or pure abstract) Class is meant to implement hierarchy. If your business objects look somewhat structurally similar, representing a parent-child (hierarchy) kind of relationship only then inheritance/Abstract classes will be used. If your business model does not have a hierarchy then inheritance should not be used (here I am not talking about programming logic e.g. some design patterns require inheritance). Conceptually, abstract class is a method to implement hierarchy of a business model in OOP, it has nothing to do with Interfaces, actually comparing Abstract class with Interface is meaningless because both are conceptually totally different things, it is asked in interviews just to check the concepts because it looks both provide somewhat same functionality when implementation is concerned and we programmers usually emphasize more on coding. [Keep this in mind as well that Abstraction is different than Abstract Class].
2- an Interface is a contract, a complete business functionality represented by one or more set of functions. That is why it is implemented and not inherited. A business object (part of a hierarchy or not) can have any number of complete business functionality. It has nothing to do with abstract classes means inheritance in general. For example, a human can RUN, an elephant can RUN, a bird can RUN, and so on, all these objects of different hierarchy would implement the RUN interface or EAT or SPEAK interface. Don't go into implementation as you might implement it as having abstract classes for each type implementing these interfaces. An object of any hierarchy can have a functionality(interface) which has nothing to do with its hierarchy.
I believe, Interfaces were not invented to achieve multiple inheritances or to expose public behavior, and similarly, pure abstract classes are not to overrule interfaces but Interface is a functionality that an object can do (via functions of that interface) and Abstract Class represents a parent of a hierarchy to produce children having core structure (property+functionality) of the parent
When you are asked about the difference, it is actually conceptual difference not the difference in language-specific implementation unless asked explicitly.
I believe, both interviewers were expecting one line straightforward difference between these two and when you failed they tried to drove you towards this difference by implementing ONE as the OTHER
What if you had an Abstract class with only abstract methods?

i will explain Depth Details of interface and Abstract class.if you know overview about interface and abstract class, then first question arrive in your mind when we should use Interface and when we should use Abstract class.
So please check below explanation of Interface and Abstract class.
When we should use Interface?
if you don't know about implementation just we have requirement specification then we go with Interface
When we should use Abstract Class?
if you know implementation but not completely (partially implementation) then we go with Abstract class.
Interface
every method by default public abstract means interface is 100% pure abstract.
Abstract
can have Concrete method and Abstract method, what is Concrete method, which have implementation in Abstract class,
An abstract class is a class that is declared abstract—it may or may not include abstract methods.
Interface
We cannot declared interface as a private, protected
Q. Why we are not declaring Interface a private and protected?
Because by default interface method is public abstract so and so that reason that we are not declaring the interface as private and protected.
Interface method
also we cannot declared interface as private,protected,final,static,synchronized,native.....
i will give the reason:
why we are not declaring synchronized method because we cannot create object of interface and synchronize are work on object so and son reason that we are not declaring the synchronized method
Transient concept are also not applicable because transient work with synchronized.
Abstract
we are happily use with public,private final static.... means no restriction are applicable in abstract.
Interface
Variables are declared in Interface as a by default public static final so we are also not declared variable as a private, protected.
Volatile modifier is also not applicable in interface because interface variable is by default public static final and final variable you cannot change the value once it assign the value into variable and once you declared variable into interface you must to assign the variable.
And volatile variable is keep on changes so it is opp. to final that is reason we are not use volatile variable in interface.
Abstract
Abstract variable no need to declared public static final.
i hope this article is useful.

For .Net,
Your answer to The second interviewer is also the answer to the first one... Abstract classes can have implementation, AND state, interfaces cannot...
EDIT: On another note, I wouldn't even use the phrase 'subclass' (or the 'inheritance' phrase) to describe classes that are 'defined to implement' an interface. To me, an interface is a definition of a contract that a class must conform to if it has been defined to 'implement' that interface. It does not inherit anything... You have to add everything yourself, explicitly.

Interface : should be used if you want to imply a rule on the components which may or may not be
related to each other
Pros:
Allows multiple inheritance
Provides abstraction by not exposing what exact kind of object is being used in the context
provides consistency by a specific signature of the contract
Cons:
Must implement all the contracts defined
Cannot have variables or delegates
Once defined cannot be changed without breaking all the classes
Abstract Class : should be used where you want to have some basic or default behaviour or implementation for components related to each other
Pros:
Faster than interface
Has flexibility in the implementation (you can implement it fully or partially)
Can be easily changed without breaking the derived classes
Cons:
Cannot be instantiated
Does not support multiple inheritance

I think they didn't like your response because you gave the technical differences instead of design ones. The question is like a troll question for me. In fact, interfaces and abstract classes have a completely different nature so you cannot really compare them. I will give you my vision of what is the role of an interface and what is the role of an abstract class.
interface: is used to ensure a contract and make a low coupling between classes in order to have a more maintainable, scalable and testable application.
abstract class: is only used to factorize some code between classes of the same responsability. Note that this is the main reason why multiple-inheritance is a bad thing in OOP, because a class shouldn't handle many responsabilities (use composition instead).
So interfaces have a real architectural role whereas abstract classes are almost only a detail of implementation (if you use it correctly of course).

Interface:
We do not implement (or define) methods, we do that in derived classes.
We do not declare member variables in interfaces.
Interfaces express the HAS-A relationship. That means they are a mask of objects.
Abstract class:
We can declare and define methods in abstract class.
We hide constructors of it. That means there is no object created from it directly.
Abstract class can hold member variables.
Derived classes inherit to abstract class that mean objects from derived classes are not masked, it inherit to abstract class. The relationship in this case is IS-A.
This is my opinion.

After all that, the interviewer came up with the question "What if you had an
Abstract class with only abstract methods? How would that be different
from an interface?"
Docs clearly say that if an abstract class contains only abstract method declarations, it should be declared as an interface instead.
An another interviewer asked me what if you had a Public variable inside
the interface, how would that be different than in Abstract Class?
Variables in Interfaces are by default public static and final. Question could be framed like what if all variables in abstract class are public? Well they can still be non static and non final unlike the variables in interfaces.
Finally I would add one more point to those mentioned above - abstract classes are still classes and fall in a single inheritance tree whereas interfaces can be present in multiple inheritance.

Copied from CLR via C# by Jeffrey Richter...
I often hear the question, “Should I design a base type or an interface?” The answer isn’t always clearcut.
Here are some guidelines that might help you:
■■ IS-A vs. CAN-DO relationship A type can inherit only one implementation. If the derived
type can’t claim an IS-A relationship with the base type, don’t use a base type; use an interface.
Interfaces imply a CAN-DO relationship. If the CAN-DO functionality appears to belong
with various object types, use an interface. For example, a type can convert instances of itself
to another type (IConvertible), a type can serialize an instance of itself (ISerializable),
etc. Note that value types must be derived from System.ValueType, and therefore, they cannot
be derived from an arbitrary base class. In this case, you must use a CAN-DO relationship
and define an interface.
■■ Ease of use It’s generally easier for you as a developer to define a new type derived from a
base type than to implement all of the methods of an interface. The base type can provide a
lot of functionality, so the derived type probably needs only relatively small modifications to its behavior. If you supply an interface, the new type must implement all of the members.
■■ Consistent implementation No matter how well an interface contract is documented, it’s
very unlikely that everyone will implement the contract 100 percent correctly. In fact, COM
suffers from this very problem, which is why some COM objects work correctly only with
Microsoft
Word or with Windows Internet Explorer. By providing a base type with a good
default implementation, you start off using a type that works and is well tested; you can then
modify parts that need modification.
■■ Versioning If you add a method to the base type, the derived type inherits the new method,
you start off using a type that works, and the user’s source code doesn’t even have to be recompiled.
Adding a new member to an interface forces the inheritor of the interface to change
its source code and recompile.

tl;dr; When you see “Is A” relationship use inheritance/abstract class. when you see “has a” relationship create member variables. When you see “relies on external provider” implement (not inherit) an interface.
Interview Question: What is the difference between an interface and an abstract class? And how do you decide when to use what? I mostly get one or all of the below answers: Answer 1: You cannot create an object of abstract class and interfaces.
ZK (That’s my initials): You cannot create an object of either. So this is not a difference. This is a similarity between an interface and an abstract class. Counter Question: Why can’t you create an object of abstract class or interface?
Answer 2: Abstract classes can have a function body as partial/default implementation.
ZK: Counter Question: So if I change it to a pure abstract class, marking all the virtual functions as abstract and provide no default implementation for any virtual function. Would that make abstract classes and interfaces the same? And could they be used interchangeably after that?
Answer 3: Interfaces allow multi-inheritance and abstract classes don’t.
ZK: Counter Question: Do you really inherit from an interface? or do you just implement an interface and, inherit from an abstract class? What’s the difference between implementing and inheriting? These counter questions throw candidates off and make most scratch their heads or just pass to the next question. That makes me think people need help with these basic building blocks of Object-Oriented Programming. The answer to the original question and all the counter questions is found in the English language and the UML. You must know at least below to understand these two constructs better.
Common Noun: A common noun is a name given “in common” to things of the same class or kind. For e.g. fruits, animals, city, car etc.
Proper Noun: A proper noun is the name of an object, place or thing. Apple, Cat, New York, Honda Accord etc.
Car is a Common Noun. And Honda Accord is a Proper Noun, and probably a Composit Proper noun, a proper noun made using two nouns.
Coming to the UML Part. You should be familiar with below relationships:
Is A
Has A
Uses
Let’s consider the below two sentences. - HondaAccord Is A Car? - HondaAccord Has A Car?
Which one sounds correct? Plain English and comprehension. HondaAccord and Cars share an “Is A” relationship. Honda accord doesn’t have a car in it. It “is a” car. Honda Accord “has a” music player in it.
When two entities share the “Is A” relationship it’s a better candidate for inheritance. And Has a relationship is a better candidate for creating member variables. With this established our code looks like this:
abstract class Car
{
string color;
int speed;
}
class HondaAccord : Car
{
MusicPlayer musicPlayer;
}
Now Honda doesn't manufacture music players. Or at least it’s not their main business.
So they reach out to other companies and sign a contract. If you receive power here and the output signal on these two wires it’ll play just fine on these speakers.
This makes Music Player a perfect candidate for an interface. You don’t care who provides support for it as long as the connections work just fine.
You can replace the MusicPlayer of LG with Sony or the other way. And it won’t change a thing in Honda Accord.
Why can’t you create an object of abstract classes?
Because you can’t walk into a showroom and say give me a car. You’ll have to provide a proper noun. What car? Probably a honda accord. And that’s when a sales agent could get you something.
Why can’t you create an object of an interface? Because you can’t walk into a showroom and say give me a contract of music player. It won’t help. Interfaces sit between consumers and providers just to facilitate an agreement. What will you do with a copy of the agreement? It won’t play music.
Why do interfaces allow multiple inheritance?
Interfaces are not inherited. Interfaces are implemented. The interface is a candidate for interaction with the external world. Honda Accord has an interface for refueling. It has interfaces for inflating tires. And the same hose that is used to inflate a football. So the new code will look like below:
abstract class Car
{
string color;
int speed;
}
class HondaAccord : Car, IInflateAir, IRefueling
{
MusicPlayer musicPlayer;
}
And the English will read like this “Honda Accord is a Car that supports inflating tire and refueling”.

An interface defines a contract for a service or set of services. They provide polymorphism in a horizontal manner in that two completely unrelated classes can implement the same interface but be used interchangeably as a parameter of the type of interface they implement, as both classes have promised to satisfy the set of services defined by the interface. Interfaces provide no implementation details.
An abstract class defines a base structure for its sublcasses, and optionally partial implementation. Abstract classes provide polymorphism in a vertical, but directional manner, in that any class that inherits the abstract class can be treated as an instance of that abstract class but not the other way around. Abstract classes can and often do contain implementation details, but cannot be instantiated on their own- only their subclasses can be "newed up".
C# does allow for interface inheritance as well, mind you.

Most answers focus on the technical difference between Abstract Class and Interface, but since technically, an interface is basically a kind of abstract class (one without any data or implementation), I think the conceptual difference is far more interesting, and that might be what the interviewers are after.
An Interface is an agreement. It specifies: "this is how we're going to talk to each other". It can't have any implementation because it's not supposed to have any implementation. It's a contract. It's like the .h header files in C.
An Abstract Class is an incomplete implementation. A class may or may not implement an interface, and an abstract class doesn't have to implement it completely. An abstract class without any implementation is kind of useless, but totally legal.
Basically any class, abstract or not, is about what it is, whereas an interface is about how you use it. For example: Animal might be an abstract class implementing some basic metabolic functions, and specifying abstract methods for breathing and locomotion without giving an implementation, because it has no idea whether it should breathe through gills or lungs, and whether it flies, swims, walks or crawls. Mount, on the other hand, might be an Interface, which specifies that you can ride the animal, without knowing what kind of animal it is (or whether it's an animal at all!).
The fact that behind the scenes, an interface is basically an abstract class with only abstract methods, doesn't matter. Conceptually, they fill totally different roles.

Interfaces are light weight way to enforce a particular behavior. That is one way to think of.

As you might have got the theoretical knowledge from the experts, I am not spending much words in repeating all those here, rather let me explain with a simple example where we can use/cannot use Interface and Abstract class.
Consider you are designing an application to list all the features of Cars. In various points you need inheritance in common, as some of the properties like DigitalFuelMeter, Air Conditioning, Seat adjustment, etc are common for all the cars. Likewise, we need inheritance for some classes only as some of the properties like the Braking system (ABS,EBD) are applicable only for some cars.
The below class acts as a base class for all the cars:
public class Cars
{
public string DigitalFuelMeter()
{
return "I have DigitalFuelMeter";
}
public string AirCondition()
{
return "I have AC";
}
public string SeatAdjust()
{
return "I can Adjust seat";
}
}
Consider we have a separate class for each Cars.
public class Alto : Cars
{
// Have all the features of Car class
}
public class Verna : Cars
{
// Have all the features of Car class + Car need to inherit ABS as the Braking technology feature which is not in Cars
}
public class Cruze : Cars
{
// Have all the features of Car class + Car need to inherit EBD as the Braking technology feature which is not in Cars
}
Consider we need a method for inheriting the Braking technology for the cars Verna and Cruze (not applicable for Alto). Though both uses braking technology, the "technology" is different. So we are creating an abstract class in which the method will be declared as Abstract and it should be implemented in its child classes.
public abstract class Brake
{
public abstract string GetBrakeTechnology();
}
Now we are trying to inherit from this abstract class and the type of braking system is implemented in Verna and Cruze:
public class Verna : Cars,Brake
{
public override string GetBrakeTechnology()
{
return "I use ABS system for braking";
}
}
public class Cruze : Cars,Brake
{
public override string GetBrakeTechnology()
{
return "I use EBD system for braking";
}
}
See the problem in the above two classes? They inherit from multiple classes which C#.Net doesn't allow even though the method is implemented in the children. Here it comes the need of Interface.
interface IBrakeTechnology
{
string GetBrakeTechnology();
}
And the implementation is given below:
public class Verna : Cars, IBrakeTechnology
{
public string GetBrakeTechnology()
{
return "I use ABS system for braking";
}
}
public class Cruze : Cars, IBrakeTechnology
{
public string GetBrakeTechnology()
{
return "I use EBD system for braking";
}
}
Now Verna and Cruze can achieve multiple inheritance with its own kind of braking technologies with the help of Interface.

1) An interface can be seen as a pure Abstract Class, is the same, but despite this, is not the same to implement an interface and inheriting from an abstract class. When you inherit from this pure abstract class you are defining a hierarchy -> inheritance, if you implement the interface you are not, and you can implement as many interfaces as you want, but you can only inherit from one class.
2) You can define a property in an interface, so the class that implements that interface must have that property.
For example:
public interface IVariable
{
string name {get; set;}
}
The class that implements that interface must have a property like that.

Though this question is quite old, I would like to add one other point in favor of interfaces:
Interfaces can be injected using any Dependency Injection tools where as Abstract class injection supported by very few.

From another answer of mine, mostly dealing with when to use one versus the other:
In my experience, interfaces are best
used when you have several classes
which each need to respond to the same
method or methods so that they can be
used interchangeably by other code
which will be written against those
classes' common interface. The best
use of an interface is when the
protocol is important but the
underlying logic may be different for
each class. If you would otherwise be
duplicating logic, consider abstract
classes or standard class inheritance
instead.

Interface Types vs. Abstract Base Classes
Adapted from the Pro C# 5.0 and the .NET 4.5 Framework book.
The interface type might seem very similar to an abstract base class. Recall
that when a class is marked as abstract, it may define any number of abstract members to provide a
polymorphic interface to all derived types. However, even when a class does define a set of abstract
members, it is also free to define any number of constructors, field data, nonabstract members (with
implementation), and so on. Interfaces, on the other hand, contain only abstract member definitions.
The polymorphic interface established by an abstract parent class suffers from one major limitation
in that only derived types support the members defined by the abstract parent. However, in larger
software systems, it is very common to develop multiple class hierarchies that have no common parent
beyond System.Object. Given that abstract members in an abstract base class apply only to derived
types, we have no way to configure types in different hierarchies to support the same polymorphic
interface. By way of example, assume you have defined the following abstract class:
public abstract class CloneableType
{
// Only derived types can support this
// "polymorphic interface." Classes in other
// hierarchies have no access to this abstract
// member.
public abstract object Clone();
}
Given this definition, only members that extend CloneableType are able to support the Clone()
method. If you create a new set of classes that do not extend this base class, you can’t gain this
polymorphic interface. Also, you might recall that C# does not support multiple inheritance for classes.
Therefore, if you wanted to create a MiniVan that is-a Car and is-a CloneableType, you are unable to do so:
// Nope! Multiple inheritance is not possible in C#
// for classes.
public class MiniVan : Car, CloneableType
{
}
As you would guess, interface types come to the rescue. After an interface has been defined, it can
be implemented by any class or structure, in any hierarchy, within any namespace or any assembly
(written in any .NET programming language). As you can see, interfaces are highly polymorphic.
Consider the standard .NET interface named ICloneable, defined in the System namespace. This
interface defines a single method named Clone():
public interface ICloneable
{
object Clone();
}

Answer to the second question : public variable defined in interface is static final by default while the public variable in abstract class is an instance variable.

From Coding Perspective
An Interface can replace an Abstract Class if the Abstract Class has only abstract methods. Otherwise changing Abstract class to interface means that you will be losing out on code re-usability which Inheritance provides.
From Design Perspective
Keep it as an Abstract Class if it's an "Is a" relationship and you need a subset or all of the functionality. Keep it as Interface if it's a "Should Do" relationship.
Decide what you need: just the policy enforcement, or code re-usability AND policy.

For sure it is important to understand the behavior of interface and abstract class in OOP (and how languages handle them), but I think it is also important to understand what exactly each term means. Can you imagine the if command not working exactly as the meaning of the term? Also, actually some languages are reducing, even more, the differences between an interface and an abstract... if by chance one day the two terms operate almost identically, at least you can define yourself where (and why) should any of them be used for.
If you read through some dictionaries and other fonts you may find different meanings for the same term but having some common definitions. I think these two meanings I found in this site are really, really good and suitable.
Interface:
A thing or circumstance that enables separate and sometimes incompatible elements to coordinate effectively.
Abstract:
Something that concentrates in itself the essential qualities of anything more extensive or more general, or of several things; essence.
Example:
You bought a car and it needs fuel.
Your car model is XYZ, which is of genre ABC, so it is a concrete car, a specific instance of a car. A car is not a real object. In fact, it is an abstract set of standards (qualities) to create a specific object. In short, Car is an abstract class, it is "something that concentrates in itself the essential qualities of anything more extensive or more general".
The only fuel that matches the car manual specification should be used to fill up the car tank. In reality, there is nothing to restrict you to put any fuel but the engine will work properly only with the specified fuel, so it is better to follow its requirements. The requirements say that it accepts, as other cars of the same genre ABC, a standard set of fuel.
In an Object Oriented view, fuel for genre ABC should not be declared as a class because there is no concrete fuel for a specific genre of car out there. Although your car could accept an abstract class Fuel or VehicularFuel, you must remember that your only some of the existing vehicular fuel meet the specification, those that implement the requirements in your car manual. In short, they should implement the interface ABCGenreFuel, which "... enables separate and sometimes incompatible elements to coordinate effectively".
Addendum
In addition, I think you should keep in mind the meaning of the term class, which is (from the same site previously mentioned):
Class:
A number of persons or things regarded as forming a group by reason of common attributes, characteristics, qualities, or traits; kind;
This way, a class (or abstract class) should not represent only common attributes (like an interface), but some kind of group with common attributes. An interface doesn't need to represent a kind. It must represent common attributes. This way, I think classes and abstract classes may be used to represent things that should not change its aspects often, like a human being a Mammal, because it represents some kinds. Kinds should not change themselves that often.

Efficiently managing similar objects with varying properties and methods

I am working on a text-based game using C# and WPF. In this game the player can build different structures, probably around 20 unique ones in total when it's all done. Each structure will share some of the same properties, though with different values (such as the cost to build, etc.), and some will have properties and methods unique to themselves. I am trying to figure out the best, most efficient way to implement this.
I have explored using one class for all of them and using a static method with a parameter determining the desired type to generate an instance of the chosen structure, but think that the unique properties and methods some of the structures have would end up making that approach impractical. I could obviously make each different structure its own class and have about 20 of them, all inheriting from an interface containing common members, but I'm hoping that there's a more efficient way to do this that I'm overlooking.
I'm not looking for any code at this point, just concepts that might simplify this process and aid me in the future.

I recommend you look into Inheritance and Polymorphism.
Inheritance, together with encapsulation and polymorphism, is one of the three primary characteristics (or pillars) of object-oriented programming. Inheritance enables you to create new classes that reuse, extend, and modify the behavior that is defined in other classes. The class whose members are inherited is called the base class, and the class that inherits those members is called the derived class. A derived class can have only one direct base class. However, inheritance is transitive. If ClassC is derived from ClassB, and ClassB is derived from ClassA, ClassC inherits the members declared in ClassB and ClassA.
Specifically I wouldn't use an interface, but an abstract class (Polymorphism) that has the shared code for multiple methods, but mark them as virtual so that derived(inherited) classes can overwrite the code. Using only an Interface means that you could be duplicating a lot of code.

C# - What should I use, an Interface, Abstract class, or Both?

So, hypothetically, I'm building some sort of real estate application in C#. For each type of property, I'm going to create a class such as ResidentialProperty and CommercialProperty. These two classes as well as all other property classes will share some common properties, such as Id, Title, Description, and Address information.
What I would like to be able to do is:
a) return a collection of objects that contain just the basic information
b) be able to either call a method such as GetProperty(id) which will create and return either a ResidentialProperty or CommercialProperty, or call GetProperties() which will return a collection of one or the other, or both.
So with that said, it would probably make sense to create an abstract class called BasicProperty (or PropertyBase) which contains all of the common attributes, and have the ResidentialProperty and CommercialProperty extend from it. This would take care of problem #1, as I could create a method that returns a collection of BasicProperties.
But for #2, being able to return either one property type or the other, I would need an Interface (IProperty), and have the Residential and Commercial classes inherit from it, and then have the GetProperty(id) and GetProperties() return an IProperty object (or because they inherit from IProperty, can I return them as is and not as the Interface?)?
Now if I should use an Interface, what do I do with the BasicProperty class?
- Do I leave it as an abstract and implement the Interface? Or
- Do I leave it as an abstract and all 3 classes implement the Interface? Or
- Do I not create it as an abstract, put all of the basic information into the Interface, and the BasicProperty, ResidentialProperty and CommercialProperty all implement the Interface?
Thanks in advance,
Carl J.

While I feel that defining an interface to begin with is almost always a good idea, just because it helps your code to be flexible in the future, it sounds like in this case you don't actually need to do that. Your GetProperty and GetProperties methods can use your abstract base class as a return value.
Think of it like this: What if I had a method called GetShape? It would presumably return a Shape, right? Let's say Shape is an abstract base class, and some derived classes are Triangle, Square, Circle, etc.
But a triangle is a shape, a square is a shape, and so on--each of these happens to be more than just a shape, but they are shapes nonetheless. So if I say "give me a shape" and you hand me a square, you're doing just as I asked. No funny business there.
This is one of the core underlying principles of OOP: an instance of a derived class is an instance of its base class; it's just also more than that.

From what I can gather, you are talking about two different things here.
Class structure
Data Access of those classes
You are correct in thinking that you should create an abstract class to contain the common properties, that's what inheritance is for :) (among other things)
But I dont see why you can't create a data access class that has a method GetProperty(id) that specifies a return type of PropertyBase
i.e.
public PropertyBase GetProperty(long id)
in the implementation of GetProperty you can construct a ResidentialProperty or CommercialProperty (based on what ever business/database logic you want) then return it, c# allows you to do that.
Perhaps I miss-understood you?
HTH
EDIT::
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
}
}
class DataAccessLayer
{
public PropertyBase GetSomething(int id)
{
if (id > 10)
return new CommercialProperty();
else
return new ResidentialProperty();
}
}
class PropertyBase { }
class ResidentialProperty : PropertyBase { }
class CommercialProperty : PropertyBase { }
}

An abstract class is used to provide common behaviour. An interface is used to provide a specific set of methods and properties, regardless of how they behave.
If your ResidentialProperty and CommercialProperty provide some common behaviour then it probably makes sense to implement this behaviour in an abstract class and have each of them inherit from this class. Presumably they also will have some custom behaviour ,otherwise there is no need to sub-class, it would then be sufficient just to have a PropertyType property to describe which type of Property the instance is.
You can then provide as many interfaces as you feel would be useful, IPropertyBase, IResidentialProperty and/or ICommercialProperty. It really depends on whether you expect this library to be used a base for other implementations which may have the same interface as one or more of your classes, but not the same behaviour as your base abstract class. The other benefit of exposing interfaces which represent your types is easier mocking for unit testing.
It's not really possible to answer this question absolutely because it really depends on how your objects are likely to be used, but I hope this answer provides you with a useful guideline.

It is my opinion that you should avoid using abstract classes unless it absolutely makes sense you should.
A lot of the common behaviour can be given to your entities through aggregation, using components and you can publicise this behaviour through the use of interfaces.
The reason I tend to go down this route, is that once you have an abstract base class, you're tied to using it, as you can't have multiple inheritance.
Sooner or later, you end up with a situation in which you DO want multiple inheritance and you're screwed.
Not that I'm a hardliner on this, because plenty of our code-base does utilise base abstract classes for the above, but those implement the interfaces and all the code enacting on those classes talk to them through the interfaces, so we can switch out the base classes for something more flexible later if necessary.

A quick not about the difference as I see it. You can always use an abstract base class even when you implement interfaces. Interfaces does not help you avoid code duplication which you should (see the DRY principle) but it doesn't force you to derive from anything special which makes them easier to combine with other base classes or interfaces.
An abstract base class on the other hand can remove some duplication and it is easier to change some things in the base without touching the derived classes. The latter is very nice when you implement a class library that others use. If you change things in interfaces in a library, all implementations of that interface needs to change! This might be a very small problem if you only implement an application with a small group of developers. But as other has said, a base class forces you to derive from it and then you cannot derive from something else if that need should appear.

Don't call your base class or interface BasicProperty or PropertyBase, just call it Property. You will not have both a Property and a BasicProperty, will you? You will act with Property classes or interfaces.
An abstract class is almost the same as an interface with the difference that the abstract class can store state in field variables. When your Properties have data like the address that is stored an abstract class with a field is one way to do that.
Now the subclassing of a class is one of the picture book examples of OOD, but there are other ways of differentiating objects than that, look at the decorator and behavior patterns. You should subclass only if you need to override methods of the base class. Have a look at this for example.

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