TaggedUnion

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) {}
    }
}