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

This is not a C# thing as much as it is a compiler thing. One of the jobs of a compiler is to lay out a class in memory, that is a bunch of basic data types, pointers, function pointers and other classes.

It can't construct the layout for class A until it knows what the layout of class B is. It can't know what the layout of class B is until it finished with the layout of class A. Circular dependency.