c# - Drag and Drop between Instances of the same Windows Forms Application

Question

I have created a small Windows Forms test application to try out some drag/drop code. The form consists of three PictureBoxes. My intention was to grab a picture from one PictureBox, display it as a custom cursor during the drag operation, then drop it on another PictureBox target.

This works fine from one PictureBox to another as long as they are on the same form.

If I open two instances of the same application and attempt to drag/drop between them, I get the following cryptic error:

This remoting proxy has no channel sink which means either the server has no registered server channels that are listening, or this application has no suitable client channel to talk to the server.

For some reason, however, it does work to drag/drop to Wordpad (but not MS Word or Paintbrush).

The three PictureBoxes get their events hooked up like this:

foreach (Control pbx in this.Controls) {
    if (pbx is PictureBox) {
        pbx.AllowDrop = true;
        pbx.MouseDown    += new MouseEventHandler(pictureBox_MouseDown);
        pbx.GiveFeedback += new GiveFeedbackEventHandler(pictureBox_GiveFeedback);
        pbx.DragEnter    += new DragEventHandler(pictureBox_DragEnter);
        pbx.DragDrop     += new DragEventHandler(pictureBox_DragDrop);
    }
}

Then there are the four events like this:

void pictureBox_MouseDown(object sender, MouseEventArgs e) {
    int width = (sender as PictureBox).Image.Width;
    int height = (sender as PictureBox).Image.Height;

    Bitmap bmp = new Bitmap(width, height);
    Graphics g = Graphics.FromImage(bmp);
    g.DrawImage((sender as PictureBox).Image, 0, 0, width, height);
    g.Dispose();
    cursorCreatedFromControlBitmap = CustomCursors.CreateFormCursor(bmp, transparencyType);
    bmp.Dispose();

    Cursor.Current = this.cursorCreatedFromControlBitmap;

    (sender as PictureBox).DoDragDrop((sender as PictureBox).Image, DragDropEffects.All);
}

---

void pictureBox_GiveFeedback(object sender, GiveFeedbackEventArgs gfea) {
    gfea.UseDefaultCursors = false;
}

---

void pictureBox_DragEnter(object sender, DragEventArgs dea) {
    if ((dea.KeyState & 32) == 32) { // ALT is pressed
        dea.Effect = DragDropEffects.Link;
    }
    else if ((dea.KeyState & 8) == 8) { // CTRL is pressed
        dea.Effect = DragDropEffects.Copy;
    }
    else if ((dea.KeyState & 4) == 4) { // SHIFT is pressed
        dea.Effect = DragDropEffects.None;
    }
    else {
        dea.Effect = DragDropEffects.Move;
    }
}

---

void pictureBox_DragDrop(object sender, DragEventArgs dea) {
    if (((IDataObject)dea.Data).GetDataPresent(DataFormats.Bitmap))
        (sender as PictureBox).Image = (Image)((IDataObject)dea.Data).GetData(DataFormats.Bitmap);
}

Any help would be greatly appreciated!

Answer 1

After much gnashing of teeth and pulling of hair, I was able to come up with a workable solution. It seems there is some undocumented strangeness going on under the covers with .NET and its OLE drag and drop support. It appears to be trying to use .NET remoting when performing drag and drop between .NET applications, but is this documented anywhere? No, I don't think it is.

So the solution I came up with involves a helper class to help marshal the bitmap data between processes. First, here is the class.

[Serializable]
public class BitmapTransfer
{
    private byte[] buffer;
    private PixelFormat pixelFormat;
    private Size size;
    private float dpiX;
    private float dpiY;

    public BitmapTransfer(Bitmap source)
    {
        this.pixelFormat = source.PixelFormat;
        this.size = source.Size;
        this.dpiX = source.HorizontalResolution;
        this.dpiY = source.VerticalResolution;
        BitmapData bitmapData = source.LockBits(
            new Rectangle(new Point(0, 0), source.Size),
            ImageLockMode.ReadOnly, 
            source.PixelFormat);
        IntPtr ptr = bitmapData.Scan0;
        int bufferSize = bitmapData.Stride * bitmapData.Height;
        this.buffer = new byte[bufferSize];
        System.Runtime.InteropServices.Marshal.Copy(ptr, buffer, 0, bufferSize);
        source.UnlockBits(bitmapData);
    }

    public Bitmap ToBitmap()
    {
        Bitmap bitmap = new Bitmap(
            this.size.Width,
            this.size.Height,
            this.pixelFormat);
        bitmap.SetResolution(this.dpiX, this.dpiY);
        BitmapData bitmapData = bitmap.LockBits(
            new Rectangle(new Point(0, 0), bitmap.Size),
            ImageLockMode.WriteOnly, bitmap.PixelFormat);
        IntPtr ptr = bitmapData.Scan0;
        int bufferSize = bitmapData.Stride * bitmapData.Height;
        System.Runtime.InteropServices.Marshal.Copy(this.buffer, 0, ptr, bufferSize);
        bitmap.UnlockBits(bitmapData);
        return bitmap;
    }
}

To use the class in a manner that will support both .NET and unmanaged recipients of the bitmap, a DataObject class is used for the drag and drop operation as follows.

To start the drag operation:

DataObject dataObject = new DataObject();
dataObject.SetData(typeof(BitmapTransfer), 
  new BitmapTransfer((sender as PictureBox).Image as Bitmap));
dataObject.SetData(DataFormats.Bitmap, 
  (sender as PictureBox).Image as Bitmap);
(sender as PictureBox).DoDragDrop(dataObject, DragDropEffects.All);

To complete the operation:

if (dea.Data.GetDataPresent(typeof(BitmapTransfer)))
{
    BitmapTransfer bitmapTransfer = 
       (BitmapTransfer)dea.Data.GetData(typeof(BitmapTransfer));
    (sender as PictureBox).Image = bitmapTransfer.ToBitmap();
}
else if(dea.Data.GetDataPresent(DataFormats.Bitmap))
{
    Bitmap b = (Bitmap)dea.Data.GetData(DataFormats.Bitmap);
    (sender as PictureBox).Image = b;
}

The check for the customer BitmapTransfer is performed first so it takes precedence over the existence of a regular Bitmap in the data object. The BitmapTransfer class could be placed in a shared library for use with multiple applications. It must be marked serializable as shown for drag and drop between applications. I tested it with drag and drop of bitmaps within an application, between applications, and from a .NET application to Wordpad.

Hope this helps you out.