How to define a must inherit class? in C#
You mark the class as abstract (this is the C# analogue to the VB.NET Must Inherit).
This will ensure it can't be instantiated directly.
From the linked MSDN article:
The abstract modifier indicates that the thing being modified has a missing or incomplete implementation. The abstract modifier can be used with classes, methods, properties, indexers, and events. Use the abstract modifier in a class declaration to indicate that a class is intended only to be a base class of other classes. Members marked as abstract, or included in an abstract class, must be implemented by classes that derive from the abstract class.
(emphasis mine)
Use the abstract modifier.
public abstract class MyClass()
{
...
}
You can define a class as abstract, or give it a protected-only constructor. abstract is better.
If u want to create a class, that has to be inherited, you'll need to mark it with the abstract modifier.
public abstract MyClass
{
}
It's not possible enforse needness of derivation or implementation in code, if that was a question.
But:
You can define an interface to force consumer to implement it.
Or you can define abstract class with only abstract members to force consumer to override all of them.
Hope this helps.
An interface would be best.
If you need to simulate the functionality , and its not a requirement that it fail at compile time...
define a method in the base class. Throw a an exception as the only line in the implementation. You might want to make the message very very clear about what the problem is.
override the method in the super class(es) and implement them.
If you fail to implement in a super class, you will get the exception.
Not perfect, but say you are trying to port code from vb.net... this could work.
Related
I read an article from some site and in that article i read this :
Abstract classes can add more functionality without destroying the child classes that were using the old version. In an interface, creation of additional functions will have an effect on its child classes, due to the necessary implementation of interface methods to classes.
I don't understand what it means. Can anyone explain this more specifically , with a Good Example ?
this is the article which i read Link
Because interfaces only define members that types must implement, adding any new member to an interface will break any class that implements the old version because it inherently doesn't implement the new member. Any time you change the definition of an interface you must change every single class that implements that interface. Adding an abstract member to an abstract class does the same for derived classes but if you add a virtual member to the abstract class then it will have no impact on derived classes. They can be changed to override that member but they don't have to.
What it means is that if you consider a Abstract Class called Phone
and it has 3 virtual functions i.e.
AddPhonePrice , AddAccessoryPrice, AddAuxillaryPrice
and if there are two child classes
1) SamsungPhone 2) Iphone
now SamsungPhone will have implementation for all 3 functions.
while Iphone will have implementation for only AddPhonePrice, since they dont provide anything else with the phone
if we make interface called IMainPhone with
AddPhonePrice , AddAccessoryPrice, AddAuxillaryPrice functions
then both SamsungPhone and Iphone will need to implement all 3 functions
irrespective of whether they need them or not.
This means that you are able to add new members (methods, properties, fields, ...) to abstract classes that do not lead to changes in the derived classes - as long as the members are not abstract this is correct. For instance, consider this example:
internal abstract class MyBaseClass
{
public abstract void DoSomething();
// This method can also be added later without having an effect on the derived classes
public virtual void DoSomethingElse()
{
// Do something else...
}
}
internal class MyDerivedClass : MyBaseClass
{
public override void DoSomething()
{
// Do something...
}
}
In this case, the derived class has to implement the method DoSomething. But you can add non-abstract functions to the abstract base class later on. However, as soon as you add another abstract member to the base class, this also affects all non-abstract derived classes because the must implement the new members.
An interface on the other hand does not define concrete implementations at all but does only contain the abstract signature that all implementors must provide. Therefore, if you add a new member (method, property) to an interface it forces all implementors of the interface to also provide an implementation of the new members.
Interface: When you add a method to the base interface class, then you have to (manually) make sure all of the derived classes implement that method.
Abstract Base: When you add a method to an abstract base class, you aren't required by the compiler to implement that method in the derived classes.
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 a 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. Abstract class can also have class variables beside Events, Delegates, Properties and Methods. A class can only implement one abstract class only due non-existence of Multi-inheritance in C#.
So even that difference doesn't explain the question
1) What if you had an Abstract class with only abstract methods? How would that be different from an interface?
2) What if you had a Public variable inside the interface, how would that be different than in Abstract Class?
So any explanation will be vary help full.
Besides the technical differences it is mainly the intension of your design that should lead you to the decision to use one or the other:
Interfaces define the public API of the classes implementing them. Your goal of using an interface should be to show the usage of the classes that implement it. It is no side effect but a central design goal that a class can implement different interfaces to show the different roles it can act in.
An abstract class should implement some basic algorithm or common behaviour. It is mainly to join the common functionality of the subclasses in one place. Its purpose is to define the internal usage or flow and not the public interface. If you want to publish the usage of an abstract class it should implement a separate interface.
So:
1) An abstract class with only public abstract methods does not make any sense when you use the guidelines above. An abstract class can define protected abstract methods to define a flow or algorithm. But that is not possible with an interface.
2) Aditionally to the public properties abstract classes can define protected instance variables and therefor have many more usage scenarios (see explanation above).
EDIT: The "java" tag was removed by the author. I tried to make this as general as possible and it should be true for both java and C#
In Java:
An abstract class can implement an interface.
An interface cannot extend an abstract class.
BTW: Strangely - an abstract class can implement and interface without actually doing so.
interface I {
public String hello ();
}
interface J {
public String goodbye ();
}
abstract class A implements I, J {
#Override
abstract public String hello ();
}
class B extends A {
#Override
public String hello() {
return "Hello";
}
#Override
public String goodbye() {
return "goodbye";
}
}
All the variables of an Interface are by default public and static, you can not have a only public variable in an interface, whereas in an Abstract class you can declare a public variable.
If a class extends an Abstract class there is no any contract between them. Class which extends it may or may not override the abstract methods, however in case of interface there is a strict contract between the interface and the class that implements it, i.e the class will have to override all the method of that interface. So from the abstract method point of view they appears to be same, but are having completely different properties and advantages.
While your question indicates it's for "general OO", it really seems to be focusing on .NET use of these terms.
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 itslef)
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
What if you had an Abstract class with only abstract methods? How
would that be different from an interface?
You can implement multiple interfaces but extend only one class
Abstract class are more immune to changes then interface becuase if you change an interface it would break the class implementing it.
Interface can have only static final fields..Abstract class can have any type of fields.
interface don't have constructor but abstract class can have it
But java docs say this
If an abstract class contains only abstract method declarations, it
should be declared as an interface instead.
Even if all the methods in today's version of an abstract class are abstract, future version of the class could add virtual or non-virtual methods without forcing modifications to implementations nor recompilation of consumers. By contrast, adding any member to an interface will generally require all classes which implement the interface be modified to implement that member, and both implementations and consumers will generally have to be recompiled regardless of whether the change added anything that wasn't already implemented.
The fact that abstract changes may be changed without breaking implementations or consumers is a big advantage in favor of abstract classes. On the other hand, an abstract class will force any implementing class to derive from it alone and no other class. By contrast, an interface will pose almost restrictions on what its implementers are allowed to inherit or derive from. That is a big advantage in favor of interfaces.
Because abstract classes and interfaces each have definite advantages, there are times when either may be better than the other. Conceptually, it would be possible to add a couple features to the way interfaces work that would give them the advantages presently enjoyed only by abstract classes, but I know of no particular plans to do so.
Your class can extends only one abstract class and implements many interfaces.
Well, in an abstract class you could also have fields, and auto-properties wouldn't need to be reimplemented. You can also specify access specifiers that aren't public. Also, it has better scalability (e.g. you can use [Obsolete] to mark an old implementation, and then make the new one call the old one by default). Also, it would stop you from having any more inheritance of classes. Another thing is that you can set static fields in abstract classes.
Also, interfaces are usually something that performs an action, while classes are about being that.
*1) What if you had an Abstract class with only abstract methods? How would that be different from an interface?*
By default the methods in an interface are 'public abstract' and the abstract class will also have the abstract methods as 'public abstract'.
If the abstract class contains only abstracts methods then it's better to make it an interface.
*2) What if you had a Public variable inside the interface, how would that be different than in Abstract Class?*
Interfaces can't have variables. If you meant properties, events, delegates etc... they would be by default 'Public'. If nothing is specified in the abstract class it would be 'Private'(In regards to members of the interface/abstract class only).
An interface is used when you want your class to be able to do something.
Your class extends an abstract class when there is a 'is a' relationship.
There is a semantic difference.
In case of abstract class.
class Dog : abstractAnimal
When we create object of Dog, we will have to create object of abstractAnimal to, it will lead to extra object creation.
In case of interface.
class Dog : IAnimal
When we create object of Dog, we will not be creating any extra object of anything.
In that case you can say:
1) We can specify different access modifier to methods present in class,
but we can't change access modifier of Interface member.
2) Derived class from abstract will not have a compulsion of
implementation.
I have some user controls which I want to specify properties and methods for.
They inherit from a base class, because they all have properties such as "Foo" and "Bar", and the reason I used a base class is so that I dont have to manually implement all of these properties in each derived class.
However, I want to have a method that is only in the derived classes, not in the base class, as the base class doesn't know how to "do" the method, so I am thinking of using an interface for this. If i put it in the base class, I have to define some body to return a value (which would be invalid), and always make sure that the overriding method is not calling the base. method
Is the right way to go about this to use both the base class and an interface to expose the method? It seems very round-about, but every way i think about doing it seems wrong...
Let me know if the question is not clear, it's probably a dumb question but I want to do this right.
EDIT : Thanks to all the people with your excellent abstract suggestions, but this breaks the designer. If abstract was not a selectable option, what would you do?
Alternatively you could define the method as 'abstract' in the base class, which will not require the class to implement it. For example:
abstract class A
{
public abstract void B();
}
Of course this will force your base class to be abstract as well, but it sounds like this would work just fine for you.
See Abstract methods on MSDN.
Update
Since abstract is not an option for you due to designer issues, you could just define the method as part of your base class, and have it throw a NotImplementedException if it is called directly from the base class:
void DerivMethod()
{
// Must be implemented by derived class
throw new NotImplementedException();
}
Otherwise, using an interface would be fine, especially if the above leaves a bad taste in your mouth...
You should make your base class an Abstract class. Then the base class can implement the Interface by marking the method abstract.
http://msdn.microsoft.com/en-us/library/aa664435(VS.71).aspx
Mark the method as abstract in your base class. You'll be forced to implement it in the derived classes, but the base class will not need to have a method definition.
I agree with with others, but making your user control abstract has some issues for the designer. The designer will often not display the abstract user control.
I would implement the interface methods in the base class. You can throw a NotImplemented exception or Assert.Fail in the methods if you want to make sure the inheritors are overriding these methods properly.
Declare the function signature in the base class and use the "abstract" modifier.
I've stumbled upon this "feature" of C# - the base class that implements interface methods does not have to derive from it.
Example:
public interface IContract
{
void Func();
}
// Note that Base does **not** derive from IContract
public abstract class Base
{
public void Func()
{
Console.WriteLine("Base.Func");
}
}
// Note that Derived does *not* provide implementation for IContract
public class Derived : Base, IContract
{
}
What happens is that Derived magically picks-up a public method, Base.Func, and decides that it will implement IContract.Func.
What is the reason behind this magic?
IMHO: this "quasi-implementation" feature is very-unintuitive and make code-inspection much harder. What do you think?
The reason is that your comment is simply incorrect:
// Note that Derived does not provide implementation for IContract
Sure it does. Follow the logic through.
Derived is required to provide a public member corresponding to each member of IContract.
All inheritable members of a base class are also members of a derived class; that's the definition of inheritance.
Therefore Derived provides an implementation for IContract; its inherited member is a member that fulfills the requirement
Therefore, no error.
this feature is very-unintuitive and make code-inspection much harder. What do you think?
I think you shouldn't use the feature if you don't like it. If you find it confusing and weird to read code that uses this feature then encourage your coworkers who use this feature to stop doing so.
How is this feature different from any other feature where a method from a base class is used from a derived class? There are a number of different ways in which a method from a base class may be used or mentioned in a derived class -- method calls, overrides, method group conversions, and so on.
Furthermore, this is relatively speaking a simple, straightforward case. If you really want to complain about confusing interface semantics in C#, I'd spend my time complaining about interface reimplementation semantics. That's the one that really seems to bake people's noodles. I always have to look that thing up in the spec to make sure I'm getting the semantics right.
Why do you think that this is strange and unnatural? Every public member of base class is also a public member of derived class. So there is no contradiction here. Anyhow you can implement interface explicitely if you like.
All members of an Interface are public by default. But there are some properties in my interface that I want to be used as private members of some subclasses that implement my interface. Is this something that can and is done or am I way off basis here. I'm working on using more Interfaces in my architecture these days so I'm not that well versed yet.
The point of interfaces is that they provide a contract that other objects can use to communicate with your object. If you change a member which is declared as public in an interface to private then you're not fulfilling the contract - another object may need to read that property / call that method, and you must allow them to.
An interface will never have private members as an interface is for "interfacing" between two objects. Your internal private members don't matter to it as long as you hold up your end of the contract.
Going on your question, and your use of the word "subclass", I don't think you've fully understood Interfaces yet.
I know you've probably heard this a million times but, an Interface describes what an object DOES, and a Class is HOW it does it. A Class IMPLEMENTS, an interface, it does not INHERIT from it.
So, if you want, have an Interface for you base Class, or for your SubClasses, but your question makes me think you're thinking about a base Class (Abstract Class), not an Interface.
Does that make sense?
As interface does not has an Access Modifier, if you still want your method private in the class which is implementing that interface, you can Implement that interface EXPLICITLY.
In that way your class methods will be Private.
You have to fully understand what interfaces are. In fact there are just descriptions of the expectations that outside world could have about the class members. It do not creates the member, it just informs that specified class have specified method to use in public scope. So, as you can see by interface you could only describe public members.
On the other hand if you want to declare some private members that are fixed or virtual you can use classic inheritance with the abstract base class. In this case you will make all methods that you want to implement in subclasses as abstract, and implement methods that you want to be defined in base class.
Hope this helps.. Regards
Interfaces are only good for public access. Internally, it would be strange for an object to refer to itself through an interface.
If you want to have private variables that you force an implementation of, you want to use an abstract class, and mark them as protected.
Think a little about this - and you understand that this can not be done:
Interfaces are like a contact. all the public fields of the interface are parts of the contact.
So, you can't hide them in a subclass... What would happen if someone were to upcast your class object to the interface's type ?
You'd probably want to change your design - may be split your interface in to two interfaces?
or and interface and an abstract class? we need more details to know...