Question

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Interfaces: Why can't I seem to grasp them?

to work with a library I just found I need to implement a few interfaces first. But some methods seem to be asking for objects that have the type of some interfaces…
And if I have an interface called MyInterface I can write things like :

    MyInterface shoe;

It does not really make sense to me. Can somebody teach me this concept ?
I read this : http://www.dotnetperls.com/interface but it did not really help, I think this concept is a bit more complex than what is presented here.

Thanks !

edit :

For those who wonder, I am not new to Interfaces but it is the first time I ran into such a use of them. And for those downgrading my question, I did search but was unlucky apparently.

Answer 1

You can declare an Interface, like in your example. However you can not instantiate one.

MyInterface shoe = new MyInterface ();

The above is not legal code. Since an Interface just describes a contract, it has no implementation details, this is left to the client code (you). Therefore it makes no sense to be able to create actual instances of MyInterface.

What you can do, is have a class SomeClass, implement the MyInterface contract:

SomeClass: MyInterface
{
  //implement the methods of MyInterface. All of them, to fulfill the contract.
}

Then you can do things like:

MyInterface shoe = new SomeClass();

Since SomeClass implements the MyInterface contract, the above is legal. You can create an instance of SomeClass because it contains implementation details.

Then you can build on this and create more classes which implement MyInterface.

The beauty of this is that you can have a method for example:

void someMethod (MyInterface test)
{

}

You can pass this method the SomeClass object or any other class you created which implements MyInterface.

Then inside this method, you can call methods that the contract contains without knowing the exact object which has been passed to you. This makes writing future code easier. You can create new objects and so long as they implement MyInterface, it is valid to pass this object to someMethod without changing the declaration of the method.