using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;

namespace Union
{
    class Program
    {
        class TestObj1
        {
            ~TestObj1() { Console.WriteLine("TestObj1 collected"); }
        }

        class TestObj2
        {
            public virtual void SomeDummyMethodTomakeVTableDifferenced() { }
            ~TestObj2() { Console.WriteLine("TestObj2 collected"); }
        }

        struct A
        {
            public int i;
            public TestObj1 s;

            public A(int i, TestObj1 s)
            {
                this.i = i;
                this.s = s;
            }
        }

        struct B
        {
            public double d;
            public List<TestObj2> objs;

            public B(double d, List<TestObj2> objs)
            {
                this.d = d;
                this.objs = objs;
            }
        }

        [StructLayout(LayoutKind.Explicit)]
        struct Union
        {
            public enum Tag
            {
                A, B, C
            }

            [FieldOffsetAttribute(4)]
            Tag _tag;

            // A fields
            [FieldOffsetAttribute(sizeof(Tag) + 4)]
            int i;
            [FieldOffsetAttribute(sizeof(Tag) + sizeof(double) + 4)]
            TestObj1 s;

            // B fields
            [FieldOffsetAttribute(sizeof(Tag) + 4)]
            double d;
            [FieldOffsetAttribute(sizeof(Tag) + sizeof(double) + 4)]
            List<TestObj2> objs;

            public A A
            {
                get
                {
                    if (_tag != Tag.A)
                        throw new InvalidCastException();

                    return new A(i, s);
                }
            }

            public B B
            {
                get
                {
                    if (_tag != Tag.B)
                        throw new InvalidCastException();

                    return new B(d, objs);
                }
            }

            public Union(A a)
                : this()
            {
                _tag = Tag.A;
                i = a.i;
                s = a.s;
            }

            public Union(B b)
                : this()
            {
                _tag = Tag.B;
                d = b.d;
                objs = b.objs;
            }
        }

        static void Main(string[] args)
        {
            A a = new A(1, new TestObj1());
            B b = new B(1.0, new List<TestObj2> { new TestObj2(), new TestObj2() });

            Union u1 = new Union(a);
            Union u2 = new Union(b);
        }
    }
}