Marshalling stucts with bit-fields in C#

Question

Is it possible to marshal a C-style struct containing bit-fields to a C# struct, or would you have to marshal it to a basic type and then do bit-masks?

E.g. I would like to marshal from a C style structure like this:

struct rgb16 {
    unsigned int R : 4;
    unsigned int G : 5;
    unsigned int B : 4;
}

And marshal it onto something like this:

[StructLayout(LayoutKind.Sequential)]
public struct Rgb16 {
    public byte R;
    public byte G;
    public byte B;
}

Answer 1

There are no bit-fields in C#. So I'd go with properties that encapsulate the bit fiddling:

[StructLayout(LayoutKind.Sequential)]
public struct Rgb16 {
    private readonly UInt16 raw;
    public byte R{get{return (byte)((raw>>0)&0x1F);}}
    public byte G{get{return (byte)((raw>>5)&0x3F);}}
    public byte B{get{return (byte)((raw>>11)&0x1F);}}

    public Rgb16(byte r, byte g, byte b)
    {
      Contract.Requires(r<0x20);
      Contract.Requires(g<0x40);
      Contract.Requires(b<0x20);
      raw=r|g<<5|b<<11;
    }
}

I've avoided adding setters, because I like immutable structs, but in principle you can add them too.

Answer 2

Thats my "safe c#" port of a rgb16 struct.

[StructLayout(LayoutKind.Explicit, Size = 2, Pack = 1)]
public class Color16
{
    // Btifield: 5
    [FieldOffset(0)]
    private ushort b_i;

    public ushort b
    {
        get { return (ushort)((b_i >> 11) & 0x1F); }
        set { b_i = (ushort)((b_i & ~(0x1F << 11)) | (value & 0x3F) << 11); }
    }

    // Bitfield: 6
    [FieldOffset(0)]
    private ushort g_i;

    public ushort g
    {
        get { return (ushort)((g_i >> 5) & 0x7F); }
        set { g_i = (ushort)((g_i & ~(0x7F << 5)) | (value & 0x7F) << 5); }
    }

    // Bitfield: 5
    [FieldOffset(0)]
    private ushort r_i;

    public ushort r
    {
        get { return (ushort) (r_i & 0x1F); }
        set { r_i = (ushort) ((r_i & ~0x1F) | (value & 0x1F)); }
    }

    [FieldOffset(0)]
    public ushort u;

    public Color16() { }
    public Color16(Color16 c) { u = c.u; }
    public Color16(ushort U) { u = U; }

}