Declaring interface inheritance in C# [duplicate]

Question

I am a bit unclear on the syntax of inheritance for interfaces in C#.

For example:

public interface IFoo
{
}
public interface IBar : IFoo
{
}

What is the difference between this:

public interface IQux : IBar
{
}

and this:

public interface IQux : IBar, IFoo
{
}

Or, for a real world example, Why is ICollection<T> declared like this:

public interface ICollection<T> : IEnumerable<T>, IEnumerable

instead of this:

public interface ICollection<T> : IEnumerable<T>

given that IEnumerable<T> already inherits from IEnumerable?

Answer 1

Eric Lippert explains it very well in this article:

https://blogs.msdn.microsoft.com/ericlippert/2011/04/04/so-many-interfaces

If IBar inherits from IFoo, then, from the compiler's perspective, there is no difference between:

public interface IQux : IBar
{
}

and this:

public interface IQux : IBar, IFoo
{
}

You can choose to state that IQux incorporates IFoo if you think it makes the code more readable. Or you can choose not to.

Answer 2

Generics didn't exist from the start -- look at the C# 1.0 docs and you won't see IEnumerable<T>.

Regarding the first question: there is no difference (not even with explicit interface implementations as far as I can tell).

Conside this:

public interface IFoo
{
    void M();
}

public interface IBar : IFoo { }
public interface IQux : IBar, IFoo { }
public interface IQux2 : IBar { }

// Both work:
// class X : IQux
class X : IQux2
{
    void IFoo.M() { }
}