ConstantTaggedUnion

using System;
using System.Diagnostics;
using System.Runtime.InteropServices;

using CT1 = Int32;
using CT2 = System.Collections.Generic.List<Int32>;

public struct ConstantTaggedUnion
{
    // System.TypeLoadException: Could not load type '_Union' because it contains an object field at offset 0 that is incorrectly aligned or overlapped by a non-object field.
    public static void test()
    {
        ConstantTaggedUnion foo = 1;

        Debug.Assert(foo.GetUnionType() == typeof(int));
        { int bar = (int) foo; }

        foo = new CT2();
    }

    public ConstantTaggedUnion(CT1 value) { _union=new _Union{Type1=value}; _id=1; }
    public ConstantTaggedUnion(CT2 value) { _union=new _Union{Type2=value}; _id=2; }

    public CT1 Type1 { get{ if(_id!=1)_TypeError(1); return _union.Type1; } set{ _union.Type1=value; _id=1; } }
    public CT2 Type2 { get{ if(_id!=2)_TypeError(2); return _union.Type2; } set{ _union.Type2=value; _id=2; } }

    public static explicit operator CT1(ConstantTaggedUnion value) { return value.Type1; }
    public static explicit operator CT2(ConstantTaggedUnion value) { return value.Type2; }
    public static implicit operator ConstantTaggedUnion(CT1 value) { return new ConstantTaggedUnion(value); }
    public static implicit operator ConstantTaggedUnion(CT2 value) { return new ConstantTaggedUnion(value); }

    public byte Tag {get{ return _id; }}

    public Type GetUnionType()
    {switch(_id){
        case 1:  return typeof(CT1);
        case 2:  return typeof(CT2);
        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 CT1 Type1;
        [FieldOffset(0)] public CT2 Type2;
    }
}