public class A
    {
      private int X;
      private int Y;
    }

    public class B
    {
      public int C;
      public int Z;
    }

    [StructLayout(LayoutKind.Explicit)]
    public struct TempStruct
    {
      [FieldOffset(0)] public A A;
      [FieldOffset(0)] public B B;
    }