View difference between Paste ID: ZSz37Cu6 and 1vzGR82f

 using System;
 using System.Diagnostics;
 using System.Runtime.InteropServices;
 
 public struct TaggedUnion<T1,T2>
 {
 	public TaggedUnion(T1 value) { _union=new _Union{Type1=value}; _id=1; }
 	public TaggedUnion(T2 value) { _union=new _Union{Type2=value}; _id=2; }
 
 	public T1 Type1 { get{ if(_id!=1)_TypeError(1); return _union.Type1; } set{ _union.Type1=value; _id=1; } }
 	public T2 Type2 { get{ if(_id!=2)_TypeError(2); return _union.Type2; } set{ _union.Type2=value; _id=2; } }
 
 	public static explicit operator T1(TaggedUnion<T1,T2> value) { return value.Type1; }
 	public static explicit operator T2(TaggedUnion<T1,T2> value) { return value.Type2; }
 	public static implicit operator TaggedUnion<T1,T2>(T1 value) { return new TaggedUnion<T1,T2>(value); }
 	public static implicit operator TaggedUnion<T1,T2>(T2 value) { return new TaggedUnion<T1,T2>(value); }
 
 	public byte Tag {get{ return _id; }}
 
 	public Type GetUnionType()
 	{switch(_id){
 		case 1:  return typeof(T1);
 		case 2:  return typeof(T2);
 		default: return typeof(void);
 	}}
 
 	public override string ToString()
 	{switch(_id){
 		case 1:  return _union.Type1.ToString();
 		case 2:  return _union.Type2.ToString();
 		default: return "void";
 	}}
 
 	_Union _union;
 	byte _id;
 	void _TypeError(byte id) { throw new InvalidCastException(/* todo */); }
 
 	[StructLayout(LayoutKind.Explicit)]
 	struct _Union
 	{
 		[FieldOffset(0)] public T1 Type1;
 		[FieldOffset(0)] public T2 Type2;
 	}
 }
 
 public static class TaggedUnion
 {
 	// System.TypeLoadException: Could not load type '_Union' because generic types cannot have explicit layout.
 	public static void test()
 	{
 		TaggedUnion<int, double> foo = 1;
 
 		Debug.Assert(foo.GetUnionType() == typeof(int));
 		{ int bar = (int) foo; }
 		try {
 			double bar = (double)foo;
 			Debug.Assert(false);
 		} catch (InvalidCastException) {}
 
 		foo = 1.0;
 
 		Debug.Assert(foo.GetUnionType() == typeof(double));
 		{ double bar = (double) foo; }
 
 		try { Console.WriteLine((int)foo); Debug.Assert(false); }
 		catch (InvalidCastException) {}
 	}
 }