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;
}
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.
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; }
}
I spent the better part of yesterday trying to solve this problem as a bigger part of the issue of "Representing union bitfields using c#'s StrucLayout and FieldOffset", which will not only answer you question (above), but can be found here:
Representing union bitfields using c#'s StrucLayout and FieldOffset
Essentially, you will need to define a struct (a value type) and use the BitVector32 object to define the bitfield section for each bitfield you wish to represent. You can skip the part about the union since that does not pertain to your question, but the large majority of the post still pertains to your question.
Just for the fun of it, I thought I would whip up the C# struct for your RGB16 example:
Note: BitVector32 object are 32 bits in length, so the '16' on the moniker is kind of misleading... please take note of that
[StructLayout(LayoutKind.Explicit, Size = 1, CharSet = CharSet.Ansi)]
public struct Rgb16
{
#region Lifetime
/// <summary>
/// Ctor
/// </summary>
/// <param name="foo"></param>
public Rgb16(int foo)
{
// allocate the bitfield
buffer = new BitVector32(0);
// initialize bitfield sections
r = BitVector32.CreateSection(0x0f); // 4
g = BitVector32.CreateSection(0x1f, r); // 5
b = BitVector32.CreateSection(0x0f, g); // 4
}
#endregion
#region Bifield
// Creates and initializes a BitVector32.
[FieldOffset(0)]
private BitVector32 buffer;
#endregion
#region Bitfield sections
/// <summary>
/// Section - Red
/// </summary>
private static BitVector32.Section r;
/// <summary>
/// Section - Green
/// </summary>
private static BitVector32.Section g;
/// <summary>
/// Section - Blue
/// </summary>
private static BitVector32.Section b;
#endregion
#region Properties
/// <summary>
/// Flag 1
/// </summary>
public byte R
{
get { return (byte)buffer[r]; }
set { buffer[r] = value; }
}
/// <summary>
/// Flag 2
/// </summary>
public byte G
{
get { return (byte)buffer[g]; }
set { buffer[g] = value; }
}
/// <summary>
/// Flag 1
/// </summary>
public byte B
{
get { return (byte)buffer[b]; }
set { buffer[b] = value; }
}
#endregion
#region ToString
/// <summary>
/// Allows us to represent this in human readable form
/// </summary>
/// <returns></returns>
public override string ToString()
{
return $"Name: {nameof(Rgb16)}{Environment.NewLine}Red: {R}: Green: {G} Blue: {B} {Environment.NewLine}BitVector32: {buffer}{Environment.NewLine}";
}
#endregion
}
To use this, you would allocate as follows:
internal static class Program
{
/// <summary>
/// Main entry point
/// </summary>
/// <param name="args"></param>
static void Main(string[] args)
{
var rgb16 = new Rgb16(0)
{
R = 24,
G = 16,
B = 42
};
Also, please note that there is a reference to this:
There are many other answers here, but they have many pitfalls to be aware of. Perhaps the best thing I can do here is just list what you might want to look for:
I've marshaled bitfields like:
public struct Rgb16 {
public ushort Value; // two byte value that internally contain structure R(4):G(5):B(4)
public Rgb16BitField GetBitField
{
get; set;
}
}
where the property creates new structure like you mentioned by dividing Value to bits.
Aren't the best way to do that, but haven't found anything else that worked for me. I can provide the code for GetBitField if you want (it isn't very compact)
Upd: The link provided by Tony in the comments to your question uses the same idea but seems more accurate then mine, so use his solution if you cannot find any better
