Question

I have been considering using the code example shown at Shell Style Drag and Drop in .NET – Part 3 within a WPF project. The sample project works fine, it is a great article so check it out!

However when moving the code to my project I receive an error when compiling “Unsafe code may only appear if compiling with /unsafe” I understand that I could just change the compile options, however I would prefer to not have the unsafe code in there. The unsafe code is as follows.

How can I make this code safe? I don’t really have experience in this field.

/// <summary>
/// Replaces any pixel with a zero alpha value with the specified transparency key.
/// </summary>
/// <param name="bmpData">The bitmap data in which to perform the operation.</param>
/// <param name="transKey">The transparency color. This color is rendered transparent
/// by the DragDropHelper.</param>
/// <remarks>
/// This function only supports 32-bit pixel formats for now.
/// </remarks>
private static void ReplaceTransparentPixelsWithTransparentKey(BitmapData bmpData, DrawingColor transKey)
{
    DrawingPixelFormat pxFormat = bmpData.PixelFormat;

    if (DrawingPixelFormat.Format32bppArgb == pxFormat
        || DrawingPixelFormat.Format32bppPArgb == pxFormat)
    {
        int transKeyArgb = transKey.ToArgb();

        // We will just iterate over the data... we don't care about pixel location,
        // just that every pixel is checked.
        unsafe
        {
            byte* pscan = (byte*)bmpData.Scan0.ToPointer();
            {
                for (int y = 0; y < bmpData.Height; ++y, pscan += bmpData.Stride)
                {
                    int* prgb = (int*)pscan;
                    for (int x = 0; x < bmpData.Width; ++x, ++prgb)
                    {
                        // If the alpha value is zero, replace this pixel's color
                        // with the transparency key.
                        if ((*prgb & 0xFF000000L) == 0L)
                            *prgb = transKeyArgb;
                    }
                }
            }
        }
    }
    else
    {
        // If it is anything else, we aren't supporting it, but we
        // won't throw, cause it isn't an error
        System.Diagnostics.Trace.TraceWarning("Not converting transparent colors to transparency key.");
        return;
    }
}

The function calling this code is as follows, maybe the ReplaceTransparentPixelsWithTransparentKey function could be removed entirely by another method. Any ideas?

/// <summary>
/// Gets a System.Drawing.Bitmap from a BitmapSource.
/// </summary>
/// <param name="source">The source image from which to create our Bitmap.</param>
/// <param name="transparencyKey">The transparency key. This is used by the DragDropHelper
/// in rendering transparent pixels.</param>
/// <returns>An instance of Bitmap which is a copy of the BitmapSource's image.</returns>
private static Bitmap GetBitmapFromBitmapSource(BitmapSource source, Color transparencyKey)
{
    // Copy at full size
    Int32Rect sourceRect = new Int32Rect(0, 0, source.PixelWidth, source.PixelHeight);

    // Convert to our destination pixel format
    DrawingPixelFormat pxFormat = ConvertPixelFormat(source.Format);

    // Create the Bitmap, full size, full rez
    Bitmap bmp = new Bitmap(sourceRect.Width, sourceRect.Height, pxFormat);
    // If the format is an indexed format, copy the color palette
    if ((pxFormat & DrawingPixelFormat.Indexed) == DrawingPixelFormat.Indexed)
        ConvertColorPalette(bmp.Palette, source.Palette);

    // Get the transparency key as a System.Drawing.Color
    DrawingColor transKey = transparencyKey.ToDrawingColor();

    // Lock our Bitmap bits, we need to write to it
    BitmapData bmpData = bmp.LockBits(
        sourceRect.ToDrawingRectangle(),
        ImageLockMode.ReadWrite,
        pxFormat);
    {
        // Copy the source bitmap data to our new Bitmap
        source.CopyPixels(sourceRect, bmpData.Scan0, bmpData.Stride * sourceRect.Height, bmpData.Stride);

        // The drag image seems to work in full 32-bit color, except when
        // alpha equals zero. Then it renders those pixels at black. So
        // we make a pass and set all those pixels to the transparency key
        // color. This is only implemented for 32-bit pixel colors for now.
        if ((pxFormat & DrawingPixelFormat.Alpha) == DrawingPixelFormat.Alpha)
            ReplaceTransparentPixelsWithTransparentKey(bmpData, transKey);
    }
    // Done, unlock the bits
    bmp.UnlockBits(bmpData);

    return bmp;
}

Answer 1

If you don’t want a significant performance hit, then you don’t really have any other options.

You really shouldn’t be scared of unsafe keyword and the associated compiler switch, either – I often see people trying to find some workaround that is still unsafe (e.g. using Marshal, or Win32 API), but doesn’t require unsafe keyword. There’s no point in that – if anything, it’s more harmful, because unsafe stands out.

In this case, for example, you could of course move the entire pointer arithmetic part into a C DLL or C++/CLI assembly, and invoke that from C# directly or via P/Invoke. But what would be the point?