Example.cs

using System;
using System.Runtime.InteropServices;
   
    [StructLayout(LayoutKind.Explicit)]
    public class CustomStructWithLayout
    {
        [FieldOffset(0)]
        public Test1 Test1;
        [FieldOffset(0)]
        public Test2 Test2;
    }

    public class Test1
    {
        public virtual int Useless(int param)
        {
            Console.WriteLine(param);
            return param;
        }
    }

    public class Test2
    {
        public virtual int Useless()
        {
            return 888;
        }
    }

    public class Stub
    {
        public void Foo(int stub) { }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Test2 fake = new CustomStructWithLayout
            {
                Test2 = new Test2(),
                Test1 = new Test1()
            }.Test2;
            Stub bar = new Stub();
            int param = 55555;
            bar.Foo(param);
            fake.Useless();
            Console.Read();
        }
    }