Using Interface variables

Question

I\'m still trying to get a better understanding of Interfaces. I know about what they are and how to implement them in classes.

What I don\'t understand is when you

Answer 1

This is a fundamental concept in object-oriented programming -- polymorphism. (wikipedia)

The short answer is that by using the interface in Class A, you can give Class A any implementation of IMyInterface.

This is also a form of loose coupling (wikipedia) -- where you have many classes, but they do not rely explicitly on one another -- only on an abstract notion of the set of properties and methods that they provide (the interface).

Answer 2

The purpose of the Interface is to define a contract between several objects, independent of specific implementation.

So you would usually use it when you have an Intrace ISomething, and a specific implementation

class Something : ISomething

So the Interface varialbe would come to use when you instantiate a contract:

ISomething myObj = new Something();
myObj.SomeFunc();

You should also read interface C#

Update:

I will explaing the logic of using an Interface for the variable and not the class itself by a (real life) example:

I have a generic repositor interace:

Interface IRepository {
    void Create();
    void Update();
}

And i have 2 seperate implementations:

class RepositoryFile : interface IRepository {}
class RepositoryDB : interface IRepository {}

Each class has an entirely different internal implementation.

Now i have another object, a Logger, that uses an already instansiated repository to do his writing. This object, doesn't care how the Repository is implemented, so he just implements:

void WriteLog(string Log, IRepository oRep);

BTW, this can also be implemented by using standard classes inheritance. But the difference between using interfaces and classes inheritance is another discussion.

For a slightly more details discussion on the difference between abstract classes and interfaces see here.

Answer 3

Because this:

public void ReadItemsList(List<string> items);
public void ReadItemsArray(string[] items);

can become this:

public void ReadItems(IEnumerable<string> items);

Edit

Think of it like this:

You have to be able to do this.

rather than:

You have to be this.

Essentially this is a contract between the method and it's callers.

Answer 4

You are not creating an instance of the interface - you are creating an instance of something that implements the interface.

The point of the interface is that it guarantees that what ever implements it will provide the methods declared within it.

So now, using your example, you could have:

MyNiftyClass : IMyInterface
{
    public void CallSomeMethod()
    {
        //Do something nifty
    }
}

MyOddClass : IMyInterface
{
    public void CallSomeMethod()
    {
        //Do something odd
    }
}

And now you have:

IMyInterface nifty = new MyNiftyClass()
IMyInterface odd = new MyOddClass()

Calling the CallSomeMethod method will now do either something nifty or something odd, and this becomes particulary useful when you are passing in using IMyInterface as the type.

public void ThisMethodShowsHowItWorks(IMyInterface someObject)
{
    someObject.CallSomeMethod();
}

Now, depending on whether you call the above method with a nifty or an odd class, you get different behaviour.

public void AnotherClass()
{
    IMyInterface nifty = new MyNiftyClass()
    IMyInterface odd = new MyOddClass()

    // Pass in the nifty class to do something nifty
    this.ThisMethodShowsHowItWorks(nifty);

    // Pass in the odd class to do something odd
    this.ThisMethodShowsHowItWorks(odd);

}

EDIT

This addresses what I think your intended question is - Why would you declare a variable to be of an interface type?

That is, why use:

IMyInterface foo = new MyConcreteClass();

in preference to:

MyConcreteClass foo = new MyConcreteClass();

Hopefully it is clear why you would use the interface when declaring a method signature, but that leaves the question about locally scoped variables:

public void AMethod()
{
    // Why use this?
    IMyInterface foo = new MyConcreteClass();

    // Why not use this?
    MyConcreteClass bar = new MyConcreteClass();
}

Usually there is no technical reason why the interface is preferred. I usually use the interface because:

• I typically inject dependencies so the polymorphism is needed
• Using the interface clearly states my intent to only use members of the interface

The one place where you would technically need the interface is where you are utilising the polymorphism, such as creating your variable using a factory or (as I say above) using dependency injection.

Borrowing an example from itowlson, using concrete declaration you could not do this:

public void AMethod(string input)
{               
    IMyInterface foo;

    if (input == "nifty")
    {
        foo = new MyNiftyClass();
    }
    else
    {
        foo = new MyOddClass();
    }
    foo.CallSomeMethod();
}

Answer 5

An interface is used so you do not need to worry about what class implements the interface. An example of this being useful is when you have a factory method that returns a concrete implementation that may be different depending on the environment you are running in. It also allows an API designer to define the API while allowing 3rd parties to implement the API in any way they see fit. Sun does this with it's cryptographic API's for Java.

public interface Foo {

}

public class FooFactory {
    public static Foo getInstance() {
        if(os == 'Windows') return new WinFoo();
        else if(os == 'OS X') return new MacFoo();
        else return new GenricFoo();
    }
}

Your code that uses the factory only needs to know about Foo, not any of the specific implementations.

Answer 6

Using interface variables is the ONLY way to allow handler methods to be written which can accept data from objects that have different base classes.

This is about as clear as anyone is going to get.