You can only do that when using pointers or references:

    class B;
    class A {
      B* b;
    }
    class B {
      A* a;
    }

because the compiler can (obviously) not tell how large the instances of the other class are when you have a field of that type. Remember that the class layout has to be fixed at compile time and both instance sizes depend on one another. That's not solvable.

C# does the same, actually:

    struct A {
      B b;
    }
    struct B {
      A a;
    }

will cause the following error:

    test.cs(2,5): error CS0523: Struct member 'A.b' of type 'B' causes a cycle in the struct layout
    test.cs(6,5): error CS0523: Struct member 'B.a' of type 'A' causes a cycle in the struct layout

with classes and references it works, of course:

    class A {
      B b;
    }
    class B {
      A a;
    }