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

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.