Why can't a class extend its own nested class in C#?

Question

For example:

public class A : A.B
{
    public class B { }
}

Which generates this error from the compiler:

Circular

Answer 1

I was able to avoid this (at least with interfaces) by inheriting from a separate class containing the nested interfaces. (In my scenario I am also returning references to these interfaces.)

Instead of:

public class MyClass :
   MyClass.Interface
where T1 : ...
where T2 : ... 
where T3 : ... {
   public interface Interface { Interface SomeMethod(); }

   Interface Interface.SomeMethod() {
      ...
   }
}

// compile error: Circular base class dependency

Do something like this:

public sealed class MyClassInterfaces
where T1 : ...
where T2 : ... 
where T3 : ... {
   public interface Interface { Interface SomeMethod(); }
}

sealed class MyClass :
   MyClassInterfaces.Interface
where T1 : ...
where T2 : ... 
where T3 : ... {
   MyClassInterfaces.Interface
   MyClassInterfaces.Interface.SomeMethod() {
      ...
   }
}

To avoid the ugliness with explicit interface implementations, you can also inherit from the other class, though that wouldn't work if you were trying to inherit from a nested class, since you can't inherit from both classes.

public abstract class MyClassInterfaces
where T1 : ...
where T2 : ... 
where T3 : ... {
   public interface Interface { Interface SomeMethod(); }
}

sealed class MyClass :
   MyClassInterfaces,
   MyClassInterfaces.Interface
where T1 : ...
where T2 : ... 
where T3 : ... {
   Interface Interface.SomeMethod() {
      ...
   }
}