Question

i defined my own 24 bit data type like this

class _24bit{
        public byte[] _value = new byte[3]; 
   }

i have data in binary in a _24bit data array

 _24bit  []data = new _24bit

i declared my bitmap as

Bitmap b = new Bitmap(columns / 3, rows, PixelFormat.Format24bppRgb);

but when i declare this to get a row in the bitmap

_24bit *row = (_24bit *)bmd.Scan0 + (j * bmd.Stride);

i get this compile error

Cannot take the address of, get the size of, or declare a pointer to a managed type (‘mynamespace._24bit’)

How can i proceed with my own 24bit data type ?

Answer 1

You really can’t declare pointers to managed reference types (and your _24bit class is managed reference type). Even if you declare _24bit as a struct (making it a value type), it still contains a reference to a byte array (not byte array itself, only a reference, which makes size of this struct larger than 24 bits). You can declare you array as fixed and unsafe to fix that. You also might need to specify StructLayout attribute:

[StructLayout(LayoutKind.Sequential, Pack=1, Size=3)]
struct _24bpp
{
    public unsafe fixed byte _value[3];
}

Note, that you can only access content of this struct in unsafe context. Or drop array declaration at all:

[StructLayout(LayoutKind.Sequential, Pack=1, Size=3)]
struct _24bpp
{
    public byte B;
    public byte G;
    public byte R;
}

Alternatively, you can recreate bitmap in terms of your data structures and copy data from unmanaged memory to your data structures. Something like

public static _24bit[][] GetBitmapPixelsFrom24bpp(BitmapData data)
{
    if(data == null) throw new ArgumentNullException("data");

    var bitmapRows = new _24bit[rows][];
    for(int row = 0; row < data.Height; ++r)
    {
        bitmapRows[row] = new _24bit[data.Width];
        for(int pixel = 0; pixel < data.Width; ++pixel)
        {
            byte b = Marshal.ReadByte(data.Scan0, data.Stride * row + pixel * 3 + 0);
            byte g = Marshal.ReadByte(data.Scan0, data.Stride * row + pixel * 3 + 1);
            byte r = Marshal.ReadByte(data.Scan0, data.Stride * row + pixel * 3 + 2);
            var bitmapPixel = new _24bit();
            bitmapPixel._value[0] = b;
            bitmapPixel._value[1] = g;
            bitmapPixel._value[2] = r;
            bitmapRows[row][pixel] = bitmapPixel;
        }
    }
    return bitmapRows;
}