using System;

public class Program

{

    public static void Main()

    {

        //  Initialize D with a new A: No problem

        var c1 = new C {

            D = new A { B = 3 }

        };

        //  Just initialize D's properties: Exception, because C doesn't create its own A

        var c2 = new C { 

            D = { B = 3 } 

        };

    }

    public class A

    {

        public int B;

    }

    public class C

    {

        public A D;

    }

}