Creating mask bitmap not working

public static Bitmap CreateMask (Bitmap bmp, int crTransparent)
{
    // get the desktop dc
    IntPtr hDC = Win32.GetWindowDC(IntPtr.Zero);
 
    // create the source and mask dcs
    IntPtr hSrcDC = Win32.CreateCompatibleDC(hDC);
    IntPtr hMaskDC = Win32.CreateCompatibleDC(hDC);
 
    // create the mask
    IntPtr hSrcBmp = bmp.GetHbitmap();
    IntPtr hMaskBmp = Win32.CreateBitmap(bmp.Width, bmp.Height, 1, 1, IntPtr.Zero);
 
    // release the desktop dc
    Win32.ReleaseDC(IntPtr.Zero, hDC);
 
    // select the source and mask bmps
    IntPtr hOldSrcBitmap = Win32.SelectObject(hSrcDC, hSrcBmp);
    IntPtr hOldMaskBitmap = Win32.SelectObject(hMaskDC, hMaskBmp);
 
    // create mask
    int nOldBkColor = Win32.SetBkColor(hSrcDC, crTransparent);
    Win32.BitBlt(hMaskDC, 0, 0, bmp.Width, bmp.Height, hSrcDC, 0, 0, (int)Win32.ROP.SRCCOPY);
    Win32.SetBkColor(hSrcDC, nOldBkColor);
 
    /*
    for (int y = 0; y < 32; y++)
        for (int x = 0; x < 32; x++)
            Win32.SetPixel(hMaskDC, x, y, Win32.GetPixel(hSrcDC, x, y) == crTransparent ? 0x00FFFFFF : 0);
    */
 
    // clean up the dcs
    Win32.SelectObject(hSrcDC, hOldSrcBitmap);
    Win32.SelectObject(hMaskDC, hOldMaskBitmap);
 
    // Delete the helper DC
    Win32.DeleteDC(hMaskDC);
    Win32.DeleteDC(hSrcDC);
 
    // delete the source handle we created
    Win32.DeleteObject(hSrcBmp);
 
    // create a bitmap object from the handle
    Bitmap b = Bitmap.FromHbitmap(hMaskBmp);
    
    // finally, delete the mask handle
    Win32.DeleteObject(hMaskBmp);
 
    return b;
}