How can I simulate a C++ union in C#?

Question

I have a small question about structures with the LayoutKind.Explicit attribute set. I declared the struct as you can see, with a fieldTotal with 64 bits, being fieldFirst the first 32 bytes and fieldSecond the last 32 bytes. After setting both fieldfirst and fieldSecond to Int32.MaxValue, I'd expect fieldTotal to be Int64.MaxValue, which actually doesn't happen. Why is this? I know C# does not really support C++ unions, maybe it will only read the values well when interoping, but when we try to set the values ourselves it simply won't handle it really well?

[StructLayout(LayoutKind.Explicit)]
struct STRUCT {
    [FieldOffset(0)]
    public Int64 fieldTotal;

    [FieldOffset(0)]
    public Int32 fieldFirst;

    [FieldOffset(32)]
    public Int32 fieldSecond;
}

        STRUCT str = new STRUCT();
        str.fieldFirst = Int32.MaxValue;
        str.fieldSecond = Int32.MaxValue;
        Console.WriteLine(str.fieldTotal); // <----- I'd expect both these values
        Console.WriteLine(Int64.MaxValue); // <----- to be the same.
        Console.ReadKey();

Answer 1

The reason is that FieldOffsetAttribute takes a number of bytes as parameter -- not number of bits. This works as expected:

[StructLayout(LayoutKind.Explicit)]
struct STRUCT
{
    [FieldOffset(0)]
    public Int64 fieldTotal;

    [FieldOffset(0)]
    public Int32 fieldFirst;

    [FieldOffset(4)]
    public Int32 fieldSecond;
}