The reason for interfaces truly eludes me. From what I understand, it is kind of a work around for the non-existent multi-inheritance which doesn't exist in C# (or so I was told).
All I see is, you predefine some members and functions, which then have to be re-defined in the class again. Thus making the interface redundant. It just feels like syntactic… well, junk to me (Please no offense meant. Junk as in useless stuff).
In the example given below taken from a different C# interfaces thread on stack overflow, I would just create a base class called Pizza instead of an interface.
easy example (taken from a different stack overflow contribution)
public interface IPizza
{
public void Order();
}
public class PepperoniPizza : IPizza
{
public void Order()
{
//Order Pepperoni pizza
}
}
public class HawaiiPizza : IPizza
{
public void Order()
{
//Order HawaiiPizza
}
}
No one has really explained in plain terms how interfaces are useful, so I'm going to give it a shot (and steal an idea from Shamim's answer a bit).
Lets take the idea of a pizza ordering service. You can have multiple types of pizzas and a common action for each pizza is preparing the order in the system. Each pizza has to be prepared but each pizza is prepared differently. For example, when a stuffed crust pizza is ordered the system probably has to verify certain ingredients are available at the restaurant and set those aside that aren't needed for deep dish pizzas.
When writing this in code, technically you could just do
public class Pizza
{
public void Prepare(PizzaType tp)
{
switch (tp)
{
case PizzaType.StuffedCrust:
// prepare stuffed crust ingredients in system
break;
case PizzaType.DeepDish:
// prepare deep dish ingredients in system
break;
//.... etc.
}
}
}
However, deep dish pizzas (in C# terms) may require different properties to be set in the Prepare() method than stuffed crust, and thus you end up with a lot of optional properties, and the class doesn't scale well (what if you add new pizza types).
The proper way to solve this is to use interface. The interface declares that all Pizzas can be prepared, but each pizza can be prepared differently. So if you have the following interfaces:
public interface IPizza
{
void Prepare();
}
public class StuffedCrustPizza : IPizza
{
public void Prepare()
{
// Set settings in system for stuffed crust preparations
}
}
public class DeepDishPizza : IPizza
{
public void Prepare()
{
// Set settings in system for deep dish preparations
}
}
Now your order handling code does not need to know exactly what types of pizzas were ordered in order to handle the ingredients. It just has:
public PreparePizzas(IList<IPizza> pizzas)
{
foreach (IPizza pizza in pizzas)
pizza.Prepare();
}
Even though each type of pizza is prepared differently, this part of the code doesn't have to care what type of pizza we are dealing with, it just knows that it's being called for pizzas and therefore each call to Prepare will automatically prepare each pizza correctly based on its type, even if the collection has multiple types of pizzas.
The point is that the interface represents a contract. A set of public methods any implementing class has to have. Technically, the interface only governs syntax, i.e. what methods are there, what arguments they get and what they return. Usually they encapsulate semantics as well, although that only by documentation.
You can then have different implementations of an interface and swap them out at will. In your example, since every pizza instance is an IPizza you can use IPizza wherever you handle an instance of an unknown pizza type. Any instance whose type inherits from IPizza is guaranteed to be orderable, as it has an Order() method.
Python is not statically-typed, therefore types are kept and looked up at runtime. So you can try calling an Order() method on any object. The runtime is happy as long as the object has such a method and probably just shrugs and says »Meh.« if it doesn't. Not so in C#. The compiler is responsible for making the correct calls and if it just has some random object the compiler doesn't know yet whether the instance during runtime will have that method. From the compiler's point of view it's invalid since it cannot verify it. (You can do such things with reflection or the dynamic keyword, but that's going a bit far right now, I guess.)
Also note that an interface in the usual sense does not necessarily have to be a C# interface, it could be an abstract class as well or even a normal class (which can come in handy if all subclasses need to share some common code – in most cases, however, interface suffices).
For me, when starting out, the point to these only became clear when you stop looking at them as things to make your code easier/faster to write - this is not their purpose. They have a number of uses:
(This is going to lose the pizza analogy, as it's not very easy to visualise a use of this)
Say you are making a simple game on screen and It will have creatures with which you interact.
A: They can make your code easier to maintain in the future by introducing a loose coupling between your front end and your back end implementation.
You could write this to start with, as there are only going to be trolls:
// This is our back-end implementation of a troll
class Troll
{
void Walk(int distance)
{
//Implementation here
}
}
Front end:
function SpawnCreature()
{
Troll aTroll = new Troll();
aTroll.Walk(1);
}
Two weeks down the line, marketing decide you also need Orcs, as they read about them on twitter, so you would have to do something like:
class Orc
{
void Walk(int distance)
{
//Implementation (orcs are faster than trolls)
}
}
Front end:
void SpawnCreature(creatureType)
{
switch(creatureType)
{
case Orc:
Orc anOrc = new Orc();
anORc.Walk();
case Troll:
Troll aTroll = new Troll();
aTroll.Walk();
}
}
And you can see how this starts to get messy. You could use an interface here so that your front end would be written once and (here's the important bit) tested, and you can then plug in further back end items as required:
interface ICreature
{
void Walk(int distance)
}
public class Troll : ICreature
public class Orc : ICreature
//etc
Front end is then:
void SpawnCreature(creatureType)
{
ICreature creature;
switch(creatureType)
{
case Orc:
creature = new Orc();
case Troll:
creature = new Troll();
}
creature.Walk();
}
The front end now only cares about the interface ICreature - it's not bothered about the internal implementation of a troll or an orc, but only on the fact that they implement ICreature.
An important point to note when looking at this from this point of view is that you could also easily have used an abstract creature class, and from this perspective, this has the same effect.
And you could extract the creation out to a factory:
public class CreatureFactory {
public ICreature GetCreature(creatureType)
{
ICreature creature;
switch(creatureType)
{
case Orc:
creature = new Orc();
case Troll:
creature = new Troll();
}
return creature;
}
}
And our front end would then become:
CreatureFactory _factory;
void SpawnCreature(creatureType)
{
ICreature creature = _factory.GetCreature(creatureType);
creature.Walk();
}
The front end now does not even have to have a reference to the library where Troll and Orc are implemented (providing the factory is in a separate library) - it need know nothing about them whatsoever.
B: Say you have functionality that only some creatures will have in your otherwise homogenous data structure, e.g.
interface ICanTurnToStone
{
void TurnToStone();
}
public class Troll: ICreature, ICanTurnToStone
Front end could then be:
void SpawnCreatureInSunlight(creatureType)
{
ICreature creature = _factory.GetCreature(creatureType);
creature.Walk();
if (creature is ICanTurnToStone)
{
(ICanTurnToStone)creature.TurnToStone();
}
}
C: Usage for dependency injection
Most dependency injection frameworks work when there is a very loose coupling between the front end code and the back end implementation. If we take our factory example above and have our factory implement an interface:
public interface ICreatureFactory {
ICreature GetCreature(string creatureType);
}
Our front end could then have this injected (e.g an MVC API controller) through the constructor (typically):
public class CreatureController : Controller {
private readonly ICreatureFactory _factory;
public CreatureController(ICreatureFactory factory) {
_factory = factory;
}
public HttpResponseMessage TurnToStone(string creatureType) {
ICreature creature = _factory.GetCreature(creatureType);
creature.TurnToStone();
return Request.CreateResponse(HttpStatusCode.OK);
}
}
With our DI framework (e.g. Ninject or Autofac), we can set them up so that at runtime a instance of CreatureFactory will be created whenever an ICreatureFactory is needed in an constructor - this makes our code nice and simple.
It also means that when we write a unit test for our controller, we can provide a mocked ICreatureFactory (e.g. if the concrete implementation required DB access, we don't want our unit tests dependent on that) and easily test the code in our controller.
D: There are other uses e.g. you have two projects A and B that for 'legacy' reasons are not well structured, and A has a reference to B.
You then find functionality in B that needs to call a method already in A. You can't do it using concrete implementations as you get a circular reference.
You can have an interface declared in B that the class in A then implements. Your method in B can be passed an instance of a class that implements the interface with no problem, even though the concrete object is of a type in A.
Examples above don't make much sense. You could accomplish all above examples using classes (abstract class if you want it to behave only as a contract):
public abstract class Food {
public abstract void Prepare();
}
public class Pizza : Food {
public override void Prepare() { /* Prepare pizza */ }
}
public class Burger : Food {
public override void Prepare() { /* Prepare Burger */ }
}
You get the same behavior as with interface. You can create a List<Food> and iterate that w/o knowing what class sits on top.
More adequate example would be multiple inheritance:
public abstract class MenuItem {
public string Name { get; set; }
public abstract void BringToTable();
}
// Notice Soda only inherits from MenuItem
public class Soda : MenuItem {
public override void BringToTable() { /* Bring soda to table */ }
}
// All food needs to be cooked (real food) so we add this
// feature to all food menu items
public interface IFood {
void Cook();
}
public class Pizza : MenuItem, IFood {
public override void BringToTable() { /* Bring pizza to table */ }
public void Cook() { /* Cook Pizza */ }
}
public class Burger : MenuItem, IFood {
public override void BringToTable() { /* Bring burger to table */ }
public void Cook() { /* Cook Burger */ }
}
Then you can use all of them as MenuItem and don't care about how they handle each method call.
public class Waiter {
public void TakeOrder(IEnumerable<MenuItem> order)
{
// Cook first
// (all except soda because soda is not IFood)
foreach (var food in order.OfType<IFood>())
food.Cook();
// Bring them all to the table
// (everything, including soda, pizza and burger because they're all menu items)
foreach (var menuItem in order)
menuItem.BringToTable();
}
}
Simple Explanation with analogy
No interface (Example 1):
No interface (Example 2):
With an interface:
The Problem to Solve: What is the purpose of polymorphism?
Analogy: So I'm a foreperson on a construction site. I don't know which tradesperson is going to walk in. But I tell them what to do.
If it's a carpenter I say: build wooden scaffolding.
If it's a plumber, I say: Set up the pipes
If it's a BJP government bureaucrat, I say, three bags full of cash, sir.
The problem with the above approach is that I have to: (i) know who's walking in that door, and depending on who it is, I have to tell them what to do. This typically makes code harder to maintain or more error prone.
The implications of knowing what to do:
This means if the carpenter's code changes from: BuildScaffolding() to BuildScaffold() (i.e. a slight name change) then I will have to also change the calling class (i.e. the Foreperson class) as well - you'll have to make two changes to the code instead of (basically) just one. With polymorphism you (basically) only need to make one change to achieve the same result.
Secondly you won't have to constantly ask: who are you? ok do this...who are you? ok do that.....polymorphism - it DRYs that code, and is very effective in certain situations:
with polymorphism you can easily add additional classes of tradespeople without changing any existing code. (i.e. the second of the SOLID design principles: Open-close principle).
The solution
Imagine a scenario where, no matter who walks in the door, I can say: "Work()" and they do their respect jobs that they specialise in: the plumber would deal with pipes, and the electrician would deal with wires, and a bureaucrat could specialise in extracting bribes and making double work for everyone else.
The benefit of this approach is that: (i) I don't need to know exactly who is walking in through that door - all i need to know is that they will be a type of tradie and that they can do work, and secondly, (ii) i don't need to know anything about that particular trade. The tradie will take care of that.
So instead of this:
if(electrician) then electrician.FixCablesAndElectricity()
if(plumber) then plumber.IncreaseWaterPressureAndFixLeaks()
if(keralaCustoms) then keralaCustoms.askForBribes()
I can do something like this:
ITradesman tradie = Tradesman.Factory(); // in reality i know it's a plumber, but in the real world you won't know who's on the other side of the tradie assignment.
tradie.Work(); // and then tradie will do the work of a plumber, or electrician etc. depending on what type of tradesman he is. The foreman doesn't need to know anything, apart from telling the anonymous tradie to get to Work()!!
What's the benefit?
The benefit is that if the specific job requirements of the carpenter etc change, then the foreperson won't need to change his code - he doesn't need to know or care. All that matters is that the carpenter knows what is meant by Work(). Secondly, if a new type of construction worker comes onto the job site, then the foreman doesn't need to know anything about the trade - all the foreman cares is if the construction worker (.e.g Welder, Glazier, Tiler etc.) can get some Work() done.
Summary
An interface allows you to get the person to do the work they are assigned to, without you having the knowledge of exactly who they are or the specifics of what they can do. This allows you to easily add new types (of trade) without changing your existing code (well technically you do change it a tiny tiny bit), and that's the real benefit of an OOP approach vs. a more functional programming methodology.
If you don't understand any of the above or if it isn't clear ask in a comment and i'll try to make the answer better.
Here are your examples reexplained:
public interface IFood // not Pizza
{
public void Prepare();
}
public class Pizza : IFood
{
public void Prepare() // Not order for explanations sake
{
//Prepare Pizza
}
}
public class Burger : IFood
{
public void Prepare()
{
//Prepare Burger
}
}
In the absence of duck typing as you can use it in Python, C# relies on interfaces to provide abstractions. If the dependencies of a class were all concrete types, you could not pass in any other type - using interfaces you can pass in any type that implements the interface.
The Pizza example is bad because you should be using an abstract class that handles the ordering, and the pizzas should just override the pizza type, for example.
You use interfaces when you have a shared property, but your classes inherit from different places, or when you don't have any common code you could use. For instance, this is used things that can be disposed IDisposable, you know it will be disposed, you just don't know what will happen when it's disposed.
An interface is just a contract that tells you some things an object can do, what parameters and what return types to expect.
Consider the case where you don't control or own the base classes.
Take visual controls for instance, in .NET for Winforms they all inherit from the base class Control, that is completely defined in the .NET framework.
Let's assume you're in the business of creating custom controls. You want to build new buttons, textboxes, listviews, grids, whatnot and you'd like them all to have certain features unique to your set of controls.
For instance you might want a common way to handle theming, or a common way to handle localization.
In this case you can't "just create a base class" because if you do that, you have to reimplement everything that relates to controls.
Instead you will descend from Button, TextBox, ListView, GridView, etc. and add your code.
But this poses a problem, how can you now identify which controls are "yours", how can you build some code that says "for all the controls on the form that are mine, set the theme to X".
Enter interfaces.
Interfaces are a way to look at an object, to determine that the object adheres to a certain contract.
You would create "YourButton", descend from Button, and add support for all the interfaces you need.
This would allow you to write code like the following:
foreach (Control ctrl in Controls)
{
if (ctrl is IMyThemableControl)
((IMyThemableControl)ctrl).SetTheme(newTheme);
}
This would not be possible without interfaces, instead you would have to write code like this:
foreach (Control ctrl in Controls)
{
if (ctrl is MyThemableButton)
((MyThemableButton)ctrl).SetTheme(newTheme);
else if (ctrl is MyThemableTextBox)
((MyThemableTextBox)ctrl).SetTheme(newTheme);
else if (ctrl is MyThemableGridView)
((MyThemableGridView)ctrl).SetTheme(newTheme);
else ....
}
In this case, you could ( and probably would ) just define a Pizza base class and inherit from them. However, there are two reasons where Interfaces allow you to do things that cannot be achieved in other ways:
A class can implement multiple interfaces. It just defines features that the class must have. Implementing a range of interfaces means that a class can fulfil multiple functions in different places.
An interface can be defined in a hogher scope than the class or the caller. This means that you can separate the functionality, separate the project dependency, and keep the functionality in one project or class, and the implementation of this elsewhere.
One implication of 2 is that you can change the class that is being used, just requiring that it implements the appropriate interface.
Consider you can't use multiple inheritance in C#, and then look at your question again.
I did a search for the word "composition" on this page and didn't see it once. This answer is very much in addition to the answers aforementioned.
One of the absolutely crucial reasons for using interfaces in an Object Oriented Project is that they allow you to favour composition over inheritance. By implementing interfaces you can decouple your implementations from the various algorithms you are applying to them.
This superb "Decorator Pattern" tutorial by Derek Banas (which - funnily enough - also uses pizza as an example) is a worthwhile illustration:
https://www.youtube.com/watch?v=j40kRwSm4VE
Interface = contract, used for loose coupling (see GRASP).
If I am working on an API to draw shapes, I may want to use DirectX or graphic calls, or OpenGL. So, I will create an interface, which will abstract my implementation from what you call.
So you call a factory method: MyInterface i = MyGraphics.getInstance(). Then, you have a contract, so you know what functions you can expect in MyInterface. So, you can call i.drawRectangle or i.drawCube and know that if you swap one library out for another, that the functions are supported.
This becomes more important if you are using Dependency Injection, as then you can, in an XML file, swap implementations out.
So, you may have one crypto library that can be exported that is for general use, and another that is for sale only to American companies, and the difference is in that you change a config file, and the rest of the program isn't changed.
This is used a great deal with collections in .NET, as you should just use, for example, List variables, and don't worry whether it was an ArrayList or LinkedList.
As long as you code to the interface then the developer can change the actual implementation and the rest of the program is left unchanged.
This is also useful when unit testing, as you can mock out entire interfaces, so, I don't have to go to a database, but to a mocked out implementation that just returns static data, so I can test my method without worrying if the database is down for maintenance or not.
Interfaces are for applying connection between different classes. for example, you have a class for car and a tree;
public class Car { ... }
public class Tree { ... }
you want to add a burnable functionality for both classes. But each class have their own ways to burn. so you simply make;
public class Car : IBurnable
{
public void Burn() { ... }
}
public class Tree : IBurnable
{
public void Burn() { ... }
}
You will get interfaces, when you will need them :) You can study examples, but you need the Aha! effect to really get them.
Now that you know what interfaces are, just code without them. Sooner or later you will run into a problem, where the use of interfaces will be the most natural thing to do.
An interface is really a contract that the implementing classes must follow, it is in fact the base for pretty much every design pattern I know.
In your example, the interface is created because then anything that IS A Pizza, which means implements the Pizza interface, is guaranteed to have implemented
public void Order();
After your mentioned code you could have something like this:
public void orderMyPizza(IPizza myPizza) {
//This will always work, because everyone MUST implement order
myPizza.order();
}
This way you are using polymorphism and all you care is that your objects respond to order().
I'm surprised that not many posts contain the one most important reason for an interface: Design Patterns. It's the bigger picture into using contracts, and although it's a syntax decoration to machine code (to be honest, the compiler probably just ignores them), abstraction and interfaces are pivotal for OOP, human understanding, and complex system architectures.
Let's expand the pizza analogy to say a full fledge 3 course meal. We'll still have the core Prepare() interface for all our food categories, but we'd also have abstract declarations for course selections (starter, main, dessert), and differing properties for food types (savoury/sweet, vegetarian/non-vegetarian, gluten free etc).
Based on these specifications, we could implement the Abstract Factory pattern to conceptualise the whole process, but use interfaces to ensure that only the foundations were concrete. Everything else could become flexible or encourage polymorphism, yet maintain encapsulation between the different classes of Course that implement the ICourse interface.
If I had more time, I'd like to draw up a full example of this, or someone can extend this for me, but in summary, a C# interface would be the best tool in designing this type of system.
Here's an interface for objects that have a rectangular shape:
interface IRectangular
{
Int32 Width();
Int32 Height();
}
All it demands is that you implement ways to access the width and height of the object.
Now let's define a method that will work on any object that is IRectangular:
static class Utils
{
public static Int32 Area(IRectangular rect)
{
return rect.Width() * rect.Height();
}
}
That will return the area of any rectangular object.
Let's implement a class SwimmingPool that is rectangular:
class SwimmingPool : IRectangular
{
int width;
int height;
public SwimmingPool(int w, int h)
{ width = w; height = h; }
public int Width() { return width; }
public int Height() { return height; }
}
And another class House that is also rectangular:
class House : IRectangular
{
int width;
int height;
public House(int w, int h)
{ width = w; height = h; }
public int Width() { return width; }
public int Height() { return height; }
}
Given that, you can call the Area method on houses or swimming-pools:
var house = new House(2, 3);
var pool = new SwimmingPool(3, 4);
Console.WriteLine(Utils.Area(house));
Console.WriteLine(Utils.Area(pool));
In this way, your classes can "inherit" behavior (static-methods) from any number of interfaces.
What ?
Interfaces are basically a contract that all the classes implementing the Interface should follow. They looks like a class but has no implementation.
In C# Interface names by convention is defined by Prefixing an 'I' so if you want to have an interface called shapes, you would declare it as IShapes
Now Why ?
Improves code re-usability
Lets say you want to draw Circle, Triangle.
You can group them together and call them Shapesand have methods to draw Circle and Triangle
But having concrete implementation would be a bad idea because tomorrow you might decide to have 2 more Shapes Rectangle & Square. Now when you add them there is a great chance that you might break other parts of your code.
With Interface you isolate the different implementation from the Contract
Live Scenario Day 1
You were asked to create an App to Draw Circle and Triangle
interface IShapes
{
void DrawShape();
}
class Circle : IShapes
{
public void DrawShape()
{
Console.WriteLine("Implementation to Draw a Circle");
}
}
Class Triangle: IShapes
{
public void DrawShape()
{
Console.WriteLine("Implementation to draw a Triangle");
}
}
static void Main()
{
List <IShapes> shapes = new List<IShapes>();
shapes.Add(new Circle());
shapes.Add(new Triangle());
foreach(var shape in shapes)
{
shape.DrawShape();
}
}
Live Scenario Day 2
If you were asked add Square and Rectangle to it, all you have to do is create the implentation for it in class Square: IShapes and in Main add to list shapes.Add(new Square());
An interface defines a contract between the provider of a certain functionality and the correspondig consumers. It decouples the implementation from the contract (interface). You should have a look at object oriented architecture and design. You may want to start with wikipedia: http://en.wikipedia.org/wiki/Interface_(computing)
There are a lot of good answers here but I would like to try from a slightlt different perspective.
You may be familiar with the SOLID principles of object oriented design. In summary:
S - Single Responsibility Principle
O - Open/Closed Principle
L - Liskov Substitution Principle
I - Interface Segregation Principle
D - Dependency Inversion Principle
Following the SOLID principles helps to produce code that is clean, well factored, cohesive and loosely coupled. Given that:
"Dependency management is the key challenge in software at every scale" (Donald Knuth)
then anything that helps with dependency management is a big win. Interfaces and the Dependency Inversion Principle really help to decouple code from dependencies on concrete classes, so code can be written and reasoned about in terms of behaviours rather than implementations. This helps to break the code into components which can be composed at runtime rather than compile time and also means those components can be quite easily plugged in and out without having to alter the rest of the code.
Interfaces help in particular with the Dependency Inversion Principle, where code can be componentized into a collection of services, with each service being described by an interface. Services can then be "injected" into classes at runtime by passing them in as a constructor parameter. This technique really becomes critical if you start to write unit tests and use test driven development. Try it! You will quickly understand how interfaces help to break apart the code into manageable chunks that can be individually tested in isolation.
Soo many answers!
Giving my best shot. hehe.
So to begin, yes you could have used a concrete base and derived class here. In that case, you would have to do an empty or useless implementation for the Prepare method in the base class also making this method virtual and then the derived classes would override this Prepare method for themselves. This case, the implementation of Prepare in Base class is useless.
The reason why you chose to use an Interface is because you had to define a contract, not an implementation.
There is a IPizza type and it provides a functionality to Prepare. This is contract. How it is prepared is the implementation and it is not your lookout. It is responsibility of the various Pizza implementations.
An interface or an abstract class is preferred here over a concrete base class because you had to create an abstraction, i.e., the Prepare method. You cannot create an abstract method in a concrete base class.
Now you could say, why not use an abstract class?
So, when you need to achieve 100% abstraction, you need to go with Interface. But when you need some abstraction along with a concrete implementation, go with abstract class. It means.
Example: Lets say all your pizzas will have a base and base preparation will be the same process. However, all pizza types and toppings will vary. In this case you could create an Abstract class with an abstract method Prepare and a concrete method PreparePizzaBase.
public abstract class Pizza{
// concrete method which is common to all pizzas.
public PizzaBase PreparePizzaBase(){
// code for pizza base preparation.
}
public abstract void Prepare();
}
public class DeluxePizza: Pizza{
public void Prepare(){
var base=PreparePizzaBase();
// prepare deluxe pizza on pizza base.
}
}
The main purpose of the interfaces is that it makes a contract between you and any other class that implement that interface which makes your code decoupled and allows expandability.
Therese are ask really great examples.
Another, in the case of a switch statement, you no longer have the need to maintain and switch every time you want rio perform a task in a specific way.
In your pizza example, if want to make a pizza, the interface is all you need, from there each pizza takes care of it's own logic.
This helps to reduce coupling and cyclomatic complexity. You have to still implement the logic but there will be less you have to keep track of in the broader picture.
For each pizza you can then keep track of information specific to that pizza. What other pizzas have doesn't matter because only the other pizzas need to know.
The simplest way to think about interfaces is to recognize what inheritance means. If class CC inherits class C, it means both that:
Class CC can use any public or protected members of class C as though they were its own, and thus only needs to implement things which do not exist in the parent class.
A reference to a CC can be passed or assigned to a routine or variable that expects a reference to a C.
Those two function of inheritance are in some sense independent; although inheritance applies both simultaneously, it is also possible to apply the second without the first. This is useful because allowing an object to inherit members from two or more unrelated classes is much more complicated than allowing one type of thing to be substitutable for multiple types.
An interface is somewhat like an abstract base class, but with a key difference: an object which inherits a base class cannot inherit any other class. By contrast, an object may implement an interface without affecting its ability to inherit any desired class or implement any other interfaces.
One nice feature of this (underutilized in the .net framework, IMHO) is that they make it possible to indicate declaratively the things an object can do. Some objects, for example, will want data-source object from which they can retrieve things by index (as is possible with a List), but they won't need to store anything there. Other routines will need a data-depository object where they can store things not by index (as with Collection.Add), but they won't need to read anything back. Some data types will allow access by index, but won't allow writing; others will allow writing, but won't allow access by index. Some, of course, will allow both.
If ReadableByIndex and Appendable were unrelated base classes, it would be impossible to define a type which could be passed both to things expecting a ReadableByIndex and things expecting an Appendable. One could try to mitigate this by having ReadableByIndex or Appendable derive from the other; the derived class would have to make available public members for both purposes, but warn that some public members might not actually work. Some of Microsoft's classes and interfaces do that, but that's rather icky. A cleaner approach is to have interfaces for the different purposes, and then have objects implement interfaces for the things they can actually do. If one had an interface IReadableByIndex and another interface IAppendable, classes which could do one or the other could implement the appropriate interfaces for the things they can do.
Interfaces can also be daisy chained to create yet another interface. This ability to implement multiple Interfaces give the developer the advantage of adding functionality to their classes without having to change current class functionality (SOLID Principles)
O = "Classes should be open for extension but closed for modification"
To me an advantage/benefit of an interface is that it is more flexible than an abstract class. Since you can only inherit 1 abstract class but you can implement multiple interfaces, changes to a system that inherits an abstract class in many places becomes problematic. If it is inherited in 100 places, a change requires changes to all 100. But, with the interface, you can place the new change in a new interface and just use that interface where its needed (Interface Seq. from SOLID). Additionally, the memory usage seems like it would be less with the interface as an object in the interface example is used just once in memory despite how many places implement the interface.
Interfaces are used to drive consistency,in a manner that is loosely coupled which makes it different to abstract class which is tightly coupled.That's why its also commonly defined as a contract.Whichever classes that implements the interface has abide to "rules/syntax" defined by the interface and there is no concrete elements within it.
I'll just give an example supported by the graphic below.
Imagine in a factory there are 3 types of machines.A rectangle machine,a triangle machine and a polygon machine.Times are competitive and you want to streamline operator training.You just want to train them in one methodology of starting and stopping machines so you have a green start button and red stop button.So now across 3 different machines you have a consistent way of starting and stopping 3 different types of machines.Now imagine these machines are classes and the classes need to have start and stop methods,how you going to drive consistency across these classes which can be very different? Interface is the answer.
A simple example to help you visualize,one might ask why not use abstract class? With an interface the objects don't have to be directly related or inherited and you can still drive consistency across different classes.
public interface IMachine
{
bool Start();
bool Stop();
}
public class Car : IMachine
{
public bool Start()
{
Console.WriteLine("Car started");
return true;
}
public bool Stop()
{
Console.WriteLine("Car stopped");
return false;
}
}
public class Tank : IMachine
{
public bool Start()
{
Console.WriteLine("Tank started");
return true;
}
public bool Stop()
{
Console.WriteLine("Tank stopped");
return false;
}
}
class Program
{
static void Main(string[] args)
{
var car = new Car();
car.Start();
car.Stop();
var tank = new Tank();
tank.Start();
tank.Stop();
}
}
class Program {
static void Main(string[] args) {
IMachine machine = new Machine();
machine.Run();
Console.ReadKey();
}
}
class Machine : IMachine {
private void Run() {
Console.WriteLine("Running...");
}
void IMachine.Run() => Run();
}
interface IMachine
{
void Run();
}
Let me describe this by a different perspective. Let’s create a story according to the example which i have shown above;
Program, Machine and IMachine are the actors of our story. Program wants to run but it has not that ability and Machine knows how to run. Machine and IMachine are best friends but Program is not on speaking terms with Machine. So Program and IMachine make a deal and decided that IMachine will tell to Program how to run by looking Machine(like a reflector).
And Program learns how to run by help of IMachine.
Interface provides communication and developing loosely coupled projects.
PS: I’ve the method of concrete class as private. My aim in here is to achieve loosely coupled by preventing accessing concrete class properties and methods, and left only allowing way to reach them via interfaces. (So i defined interfaces’ methods explicitily).
This question came to mind while I was writing a class that iterates over a list, with methods next() and previous() that will continuously loop (e.g. if at the last object, return it, and then reset index to 0)
In the constructor I was pondering adding a boolean variable, which if true would just act like a regular iterator with only next() methods and no looping. In this case, having the method previous() would make no sense. So I'm curious, is it possible to hide the previous() method in this case. Is it possible to achieve this somehow in Java or C#?.
What about other languages?
C#
It is possible by making the two methods part of two different interfaces, and casting the object to one of the two interfaces. For example:
interface ILoopingIterator
{
void Next();
void Previous();
}
interface INonLoopingIterator
{
void Next();
}
class PlaysItBothWays : ILoopingIterator, INonLoopingIterator
{
void ILoopingIterator.Next()
{
this.NextCore();
}
void ILoopingIterator.Previous()
{
// since this code will never be shared anyway, put it here
}
void INonLoopingIterator.Next()
{
this.NextCore();
}
private void NextCore()
{
// do stuff here; this method only exists so that code can be shared
}
}
Note that I have made the class implement both interfaces explicitly; this way, users of instances are forced to select which "mode" they want to use the class in. You could implement only one interface explicitly instead (providing a "default" mode that can be changed).
and now:
var looping = (ILoopingIterator) new PlaysItBothWays(); // selects mode A
var nonLooping = (INonLoopingIterator) new PlaysItBothWays(); // selects mode B
Of course this does not stop anyone from casting the instance to the "other" interface if they want to, but if the programmer wants to subvert their own code they can also use reflection which allows much more than that.
Java
In Java, the above is not possible. You can come close by having the class expose methods that return instances of one of the two interfaces, and using the returned value. Of course then the object is really a factory and not a service provider, so that's feels like cheating on the problem.
class PlaysItBothWays
{
public ILoopingIterator asLooping() { return /* something */ }
public INonLoopingIterator asNonLooping() { return /* something else */ }
}
Rather than passing a boolean to a constructor, you should simply use inheritance.
Suppose you have a base iterator that supports only next(). If that's the only functionality you need, instantiate it.
To provide more functionality, inherit from this base iterator, make a class called TwoWayIterator or something like that, and provide a previous() method.
Both of these classes will share a common super class, so you can treat them as one, and you can hide the previous() method by treating an instance as its base class.
It is not possible to hide a method like that in a statically typed language. The best you can do is implement the method to throw an exception (or equivalent) if the method is called.
There are tricks that you can do to make it appear like the methods are not there. For instance, having the class implement two interfaces, and using different factory methods to create them. However, they don't work if the constructor is used directly, or if you want the choice to be determined by the value of a constructor or factory method parameter.
Not in Java. You can't "Hide" methods at runtime. I'd suggest you to create Two interfaces
, one with the next method and the other one extending the first one and adding the "previous" method. Then, you can have 2 factories methods to create an instance of one of these classes.
Please take a look to the Java "Iterator" class
interface Iterator<T> {
T next();
}
interface LoopingIterator<T> extends Iterator<T>{
T previous();
}
Then you can cast them. Similar to the previous C# answer
You can't hide class members at run time (well, not in C# anyway - not sure about Java). If you so worried about Previous() method being used in the context where it is not doing anything useful, then simply have it throw InvalidOperationException in that case.
It is also worth noting that .NET already has standard "iterator" interface. It is called IEnumerable (and generic version IEnumerable<T>) and is forward-only.
I am still having trouble understanding what interfaces are good for. I read a few tutorials and I still don't know what they really are for other then "they make your classes keep promises" and "they help with multiple inheritance".
Thats about it. I still don't know when I would even use an interface in a real work example or even when to identify when to use it.
From my limited knowledge of interfaces they can help because if something implements it then you can just pass the interface in allowing to pass in like different classes without worrying about it not being the right parameter.
But I never know what the real point of this since they usually stop short at this point from showing what the code would do after it passes the interface and if they sort of do it it seems like they don't do anything useful that I could look at and go "wow they would help in a real world example".
So what I guess I am saying is I am trying to find a real world example where I can see interfaces in action.
I also don't understand that you can do like a reference to an object like this:
ICalculator myInterface = new JustSomeClass();
So now if I would go myInterface dot and intellisense would pull up I would only see the interface methods and not the other methods in JustSomeClass. So I don't see a point to this yet.
Also I started to do unit testing where they seem to love to use interfaces but I still don't understand why.
Like for instance this example:
public AuthenticationController(IFormsAuthentication formsAuth)
{
FormsAuth = formsAuth ?? new FormsAuthenticationWrapper();
}
public class FormsAuthenticationWrapper : IFormsAuthentication
{
public void SetAuthCookie(string userName, bool createPersistentCookie)
{
FormsAuthentication.SetAuthCookie(userName, createPersistentCookie);
}
public void SignOut()
{
FormsAuthentication.SignOut();
}
}
public IFormsAuthentication FormsAuth
{
get;
set;
}
Like why bother making this interface? Why not just make FormsAuthenticationWrapper with the methods in it and call it a day? Why First make an interface then have the Wrapper implement the interface and then finally write the methods?
Then I don't get what the statement is really saying.
Like I now know that the statement is saying this
FormsAuth = formsAuth ?? new FormsAuthenticationWrapper();
if formsAuth is null then make a new FormsAuthenticationWrapper and then assign it to the property that is an Interface.
I guess it goes back to the whole point of why the reference thing. Especially in this case since all the methods are exactly the same. The Wrapper does not have any new methods that the interface does not have and I am not sure but when you do this the methods are filled right(ie they have a body) they don't get converted to stubs because that would really seem pointless to me(it it would be converted back to an interface).
Then in the testing file they have:
var formsAuthenticationMock = new Mock<AuthenticationController.IFormsAuthentication>();
So they just pass in the FormsAuthentication what I am guessing makes all the fake stubs. I am guessing the wrapper class is used when the program is actually running since it has real methods that do something(like sign a person out).
But looking at new Mock(from moq) it accepts a class or an interface. Why not just again made the wrapper class put those methods in and then in the new Mock call that?
Would that not just make the stubs for you?
Ok, I had a hard time understanding too at first, so don't worry about it.
Think about this, if you have a class, that lets say is a video game character.
public class Character
{
}
Now say I want to have the Character have a weapon. I could use an interface to determin the methods required by a weapon:
interface IWeapon
{
public Use();
}
So lets give the Character a weapon:
public class Character
{
IWeapon weapon;
public void GiveWeapon(IWeapon weapon)
{
this.weapon = weapon;
}
public void UseWeapon()
{
weapon.Use();
}
}
Now we can create weapons that use the IWeapon interface and we can give them to any character class and that class can use the item.
public class Gun : IWeapon
{
public void Use()
{
Console.Writeline("Weapon Fired");
}
}
Then you can stick it together:
Character bob = new character();
Gun pistol = new Gun();
bob.GiveWeapon(pistol);
bob.UseWeapon();
Now this is a general example, but it gives a lot of power. You can read about this more if you look up the Strategy Pattern.
Interfaces define contracts.
In the example you provide, the ?? operator just provides a default value if you pass null to the constructor and doesn't really have anything to do with interfaces.
What is more relevant is that you might use an actual FormsAuthenticationWrapper object, but you can also implement your own IFormsAuthentication type that has nothing to do with the wrapper class at all. The interface tells you what methods and properties you need to implement to fulfill the contract, and allows the compiler to verify that your object really does honor that contract (to some extent - it's simple to honor a contract in name, but not in spirit), and so you don't have to use the pre-built FormsAuthenticationWrapper if you don't want to. You can build a different class that works completely differently but still honors the required contract.
In this respect interfaces are much like normal inheritance, with one important difference. In C# a class can only inherit from one type but can implement many interfaces. So interfaces allow you to fulfill multiple contracts in one class. An object can be an IFormsAuthentication object and also be something else, like IEnumerable.
Interfaces are even more useful when you look at it from the other direction: they allow you to treat many different types as if they were all the same. A good example of this is with the various collections classes. Take this code sample:
void OutputValues(string[] values)
{
foreach (string value in values)
{
Console.Writeline(value);
}
}
This accepts an array and outputs it to the console. Now apply this simple change to use an interface:
void OutputValues(IEnumerable<string> values)
{
foreach (string value in values)
{
Console.Writeline(value);
}
}
This code still takes an array and outputs it to the console. But it also takes a List<string> or anything else you care to give it that implements IEnumerable<string>. So we've taken an interface and used it to make a simple block of code much more powerful.
Another good example is the ASP.Net membership provider. You tell ASP.Net that you honor the membership contract by implementing the required interfaces. Now you can easily customize the built-in ASP.Net authentication to use any source, and all thanks to interfaces. The data providers in the System.Data namespace work in a similar fashion.
One final note: when I see an interface with a "default" wrapper implementation like that, I consider it a bit of an anit-pattern, or at least a code smell. It indicates to me that maybe the interface is too complicated, and you either need to split it apart or consider using some combination of composition + events + delegates rather than derivation to accomplish the same thing.
Perhaps the best way to get a good understanding of interfaces is to use an example from the .NET framework.
Consider the following function:
void printValues(IEnumerable sequence)
{
foreach (var value in sequence)
Console.WriteLine(value);
}
Now I could have written this function to accept a List<T>, object[], or any other type of concrete sequence. But since I have written this function to accept a parameter of type IEnumerable that means that I can pass any concrete type into this function that implements the IEnumerable interface.
The reason this is desirable is that by using the interface type your function is more flexible than it would otherwise be. Also you are increasing the utility of this function as many different callers will be able to make use of it without requiring modification.
By using an interface type you are able to declare the type of your parameter as a contract of what you need from whatever concrete type is passed in. In my example I don't care what type you pass me, I just care that I can iterate it.
All of the answers here have been helpful and I doubt I can add anything new to the mix but in reading the answers here, two of the concepts mentioned in two different answers really meshed well in my head so I will compose my understanding here in the hopes that it might help you.
A class has methods and properties and each of the methods and properties of a class has a signature and a body
public int Add(int x, int y)
{
return x + y;
}
The signature of the Add method is everything before the first curly brace character
public int Add(int x, int y)
The purpose of the method signature is to assign a name to a method and also to describe it's protection level (public, protected, internal, private and / or virtual) which defines where a method can be accessed from in code
The signature also defines the type of the value returned by the method, the Add method above returns an int, and the arguments a method expects to have passed to it by callers
Methods are generally considered to be something an object can do, the example above implies that the class the method is defined in works with numbers
The method body describes precisly (in code) how it is that an object performs the action described by the method name. In the example above the add method works by applying the addition operator to it's parameters and returing the result.
One of the primary differences between an interface and a class in terms of language syntax is that an interface can only define the signature of a methd, never the method body.
Put another way, an interface can describe in a the actions (methods) of a class, but it must never describe how an action is to be performed.
Now that you hopefully have a better understanding of what an interface is, we can move on to the second and third parts of your question when, and why would we use an interface in a real program.
One of the main times interfaces are used in a program is when one wants to perform an action, without wanting to know, or be tied to the specifics of how those actions are performed.
That is a very abstract concept to grapsp so perhaps an example might help to firm things up in your mind
Imagine you are the author of a very popular web browser that millions of people use every day and you have thousands of feature requests from people, some big, some little, some good and some like "bring back <maquee> and <blink> support".
Because you only have a relitivly small number of developers, and an even smaller number of hours in the day, you can't possibly implement every requested feature yourself, but you still want to satisfy your customers
So you decide to allow users to develop their own plugins, so they can <blink 'till the cows come home.
To implement this you might come up with a plugin class that looks like:
public class Plugin
{
public void Run (PluginHost browser)
{
//do stuff here....
}
}
But how could you reasonably implement that method? You can't possibly know precisly how every poossible future plugin is going to work
One possible way around this is to define Plugin as an interface and have the browser refer to each plugin using that, like this:
public interface IPlugin
{
void Run(PluginHost browser);
}
public class PluginHost
{
public void RunPlugins (IPlugin[] plugins)
{
foreach plugin in plugins
{
plugin.Run(this);
}
}
}
Note that as discussed earlier the IPlugin interface describes the Run method but does not specify how Run does it's job because this is specific to each plugin, we don't want the plugin host concerned with the specifics of each individual plugin.
To demonstrate the "can-be-a" aspect of the relationship between a class and an interface I will write a plugin for the plugin host below that implements the <blink> tag.
public class BlinkPlugin: IPlugin
{
private void MakeTextBlink(string text)
{
//code to make text blink.
}
public void Run(PluginHost browser)
{
MakeTextBlink(browser.CurrentPage.ParsedHtml);
}
}
From this perspective you can see that the plugin is defined in a class named BlinkPlugin but because it also implements the IPlugin interface it can also be refered to as an IPlugin object,as the PluginHost class above does, because it doesn't know or care what type the class actually is, just that it can be an IPlugin
I hope this has helped, I really didnt intend it to be quite this long.
I'll give you an example below but let me start with one of your statements. "I don't know how to identify when to use one". to put it on edge. You don't need to identify when to use it but when not to use it. Any parameter (at least to public methods), any (public) property (and personally I would actually extend the list to and anything else) should be declared as something of an interface not a specific class. The only time I would ever declare something of a specific type would be when there was no suitable interface.
I'd go
IEnumerable<T> sequence;
when ever I can and hardly ever (the only case I can think off is if I really needed the ForEach method)
List<T> sequence;
and now an example. Let's say you are building a sytem that can compare prices on cars and computers. Each is displayed in a list.
The car prices are datamined from a set of websites and the computer prices from a set of services.
a solution could be:
create one web page, say with a datagrid and Dependency Injection of a IDataRetriever
(where IDataRetriver is some interface making data fetching available with out you having to know where the data came from (DB,XML,web services or ...) or how they were fetched (data mined, SQL Quering in house data or read from file).
Since the two scenarios we have have nothing but the usage in common a super class will make little sense. but the page using our two classes (one for cars and one for computers) needs to perform the exact same operations in both cases to make that possible we need to tell the page (compiler) which operations are possible. We do that by means of an interface and then the two classes implement that interfcae.
using dependency injection has nothing to do with when or how to use interfaces but the reason why I included it is another common scenario where interfaces makes you life easier. Testing. if you use injection and interfaces you can easily substitute a production class for a testing class when testing. (This again could be to switch data stores or to enforce an error that might be very hard to produce in release code, say a race condition)
We use interfaces (or abstract base classes) to allow polymorphism, which is a very central concept in object-oriented programming. It allows us to compose behavior in very flexible ways. If you haven't already, you should read Design Patterns - it contains numerous examples of using interfaces.
In relation to Test Doubles (such as Mock objects), we use interfaces to be able to remove functionality that we currently don't want to test, or that can't work from within a unit testing framework.
Particularly when working with web development, a lot of the services that we rely on (such as the HTTP Context) isn't available when the code executes outside of the web context, but if we hide that functionality behind an interface, we can replace it with something else during testing.
The way I understood it was:
Derivation is 'is-a' relationship e.g., A Dog is an Animal, A Cow is an Animal but an interface is never derived, it is implemented.
So, interface is a 'can-be' relationship e.g., A Dog can be a Spy Dog, A Dog can be a Circus Dog etc. But to achieve this, a dog has to learn some specific things. Which in OO terminology means that your class has to able to do some specific things (contract as they call it) if it implements an interface. e.g., if your class implements IEnumerable, it clearly means that your class has (must have) such a functionality that it's objects can be Enumerated.
So, in essence, through Interface Implementation a Class exposes a functionality to its users that it can do something and it is NOT inheritance.
With almost everything written about interfaces, let me have a shot.
In simple terms, interface is something which will relate two or more , otherwise, non related classes.
Interfaces define contract which ensures that any two or more classes, even if not completely related, happens to implement a common interface, will contain a common set of operations.
Combined with the support of polymorphism , one can use interfaces to write cleaner and dynamic code.
eg.
Interface livingBeings
-- speak() // says anybody who IS a livingBeing need to define how they speak
class dog implements livingBeings
--speak(){bark;} // implementation of speak as a dog
class bird implements livingBeings
--speak(){chirp;}// implementation of speak as a bird
ICalculator myInterface = new JustSomeClass();
JustSomeClass myObject = (JustSomeClass) myInterface;
Now you have both "interfaces" to work with on the object.
I am pretty new to this too, but I like to think of interfaces as buttons on a remote control. When using the ICalculator interface, you only have access to the buttons (functionality) intended by the interface designer. When using the JustSomeClass object reference, you have another set of buttons. But they both point to the same object.
There are many reasons to do this. The one that has been most useful to me is communication between co-workers. If they can agree on an interface (buttons which will be pushed), then one developer can work on implementing the button's functionality and another can write code that uses the buttons.
Hope this helps.