Nested generic interfaces

Question

I have a schema of interfaces like the following (C# .NET4)

interface A 
{

}

interface B 
{
    List<A> a;
}

interface C 
{
    List<B> b;
}

and I implemented it in this way:

public interface A 
{

}

public interface B<T> where T : A 
{
    List<T> a { get; set; }
}

public interface C<T> where T : B
{
    List<T> b { get; set; } // << ERROR: Using the generic type 'B<T>' requires 1 type arguments
}

I don't know how to avoid the error Using the generic type 'B' requires 1 type arguments

Answer 1

Since interface B<T> is generic, you need to provide a formal type argument for it when declaring interface C<T>. In other words, the current problem is that you are not telling the compiler what type of interface B interface C "inherits" from.

The two Ts will not necessarily refer to the same type. They can be the same type, as in

public interface C<T> where T : B<T>, A { ... }

or they can be two distinct types:

public interface C<T, U> where T : B<U> where U : A { ... }

The restrictions on the type argument are of course tighter in the first case.

Answer 2

C looks like Generic Generic type (for lack of a better word).

Would this definition of C work instead?

public interface C<T,U> where T : B<U> where U : A
{
    List<T> b{ get; set; } 
}