C# Drag-and-Drop: Show the dragged item while dragging

Question

I\'m building a desktop app in C# with Windows Forms. I have a custom Control, and I\'d like to be able to drag and drop it within my application (not outside). Right now I\

Answer 1

Drag an ImageList onto your form and use the Image List functions to move it around:

1. Create a Bitmap object of the size of your control (but not more than 256x256).
2. Copy the image of your control into the Bitmap: using (Graphics gfx = Graphics.FromImage(bmp)) { gfx.CopyFromScreen(...) }
3. Add it to your ImageList.
4. Call ImageList_BeginDrag().
5. Call DoDragDrop()
6. In your OnDragMove handler, call ImageList_DragMove() to move it around as the mouse moves.
7. When DoDragDrop returns, call ImageList_DragLeave().

I have been calling ImageList_DragEnter() and ImageList_DragLeave(), which seems to convert the coordinates used by ImageList_DragMove() to client coordinates, but my reading of the documentation suggests it is not necessary.

Answer 2

I thought I should come back and answer this myself, since I did get it working eventually.

I created a CursorUtil class with these functions:

public struct IconInfo {
    public bool fIcon;
    public int xHotspot;
    public int yHotspot;
    public IntPtr hbmMask;
    public IntPtr hbmColor;
}

public class CursorUtil {
    [DllImport("user32.dll")]
    public static extern IntPtr CreateIconIndirect(ref IconInfo icon);

    [DllImport("user32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool GetIconInfo(IntPtr hIcon, ref IconInfo pIconInfo);

    [DllImport("gdi32.dll")]
    public static extern bool DeleteObject(IntPtr handle);

    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    extern static bool DestroyIcon(IntPtr handle);

    // Based on the article and comments here:
    // http://www.switchonthecode.com/tutorials/csharp-tutorial-how-to-use-custom-cursors
    // Note that the returned Cursor must be disposed of after use, or you'll leak memory!
    public static Cursor CreateCursor(Bitmap bm, int xHotspot, int yHotspot) {
        IntPtr cursorPtr;
        IntPtr ptr = bm.GetHicon();
        IconInfo tmp = new IconInfo();
        GetIconInfo(ptr, ref tmp);
        tmp.xHotspot = xHotspot;
        tmp.yHotspot = yHotspot;
        tmp.fIcon = false;
        cursorPtr = CreateIconIndirect(ref tmp);

        if (tmp.hbmColor != IntPtr.Zero) DeleteObject(tmp.hbmColor);
        if (tmp.hbmMask != IntPtr.Zero) DeleteObject(tmp.hbmMask);
        if (ptr != IntPtr.Zero) DestroyIcon(ptr);

        return new Cursor(cursorPtr);
    }

    public static Bitmap AsBitmap(Control c) {
        Bitmap bm = new Bitmap(c.Width, c.Height);
        c.DrawToBitmap(bm, new Rectangle(0, 0, c.Width, c.Height));
        return bm;
    }

Then I wrote a Drag class (also not object-oriented, alas, but I figured you can only drag one thing at a time in a desktop app). Here is a bit of that code:

    public static void StartDragging(Control c) {
        Dragged = c;
        DisposeOldCursors();
        Bitmap bm = CursorUtil.AsBitmap(c);
        DragCursorMove = CursorUtil.CreateCursor((Bitmap)bm.Clone(), DragStart.X, DragStart.Y);      
        DragCursorLink = CursorUtil.CreateCursor((Bitmap)bm.Clone(), DragStart.X, DragStart.Y);      
        DragCursorCopy = CursorUtil.CreateCursor(CursorUtil.AddCopySymbol(bm), DragStart.X, DragStart.Y);
        DragCursorNo = CursorUtil.CreateCursor(CursorUtil.AddNoSymbol(bm), DragStart.X, DragStart.Y);
        //Debug.WriteLine("Starting drag");
    }   

    // This gets called once when we move over a new control,
    // or continuously if that control supports dropping.
    public static void UpdateCursor(object sender, GiveFeedbackEventArgs fea) {
        //Debug.WriteLine(MainForm.MousePosition);
        fea.UseDefaultCursors = false;
        //Debug.WriteLine("effect = " + fea.Effect);
        if (fea.Effect == DragDropEffects.Move) {
            Cursor.Current = DragCursorMove;

        } else if (fea.Effect == DragDropEffects.Copy) {
            Cursor.Current = DragCursorCopy;

        } else if (fea.Effect == DragDropEffects.None) {
            Cursor.Current = DragCursorNo;

        } else if (fea.Effect == DragDropEffects.Link) {
            Cursor.Current = DragCursorLink;

        } else {
            Cursor.Current = DragCursorMove;
        }
    }

You can use these methods when you set up your controls, for example in the constructor:

GiveFeedback += new GiveFeedbackEventHandler(Drag.UpdateCursor);

and in this method:

    protected override void OnMouseMove(MouseEventArgs mea) {
        if (Drag.IsDragging(mea)) {
            Drag.StartDragging(this);
            DragDropEffects dde = DoDragDrop(Plan, DragDropEffects.Move | DragDropEffects.Copy);
            Drag.StopDragging();
        }
    }

Answer 3

Based on a previous answer:

First define the method MouseDown (in my case i use a button, but is aplicable at other controls).

        private void btn_MouseDown(object sender, MouseEventArgs e)
    {
        //Cast the sender to control type youre using
        Button send = (Button)sender;
        //Copy the control in a bitmap
        Bitmap bmp = new Bitmap(send.Width, send.Height);
        send.DrawToBitmap(bmp, new Rectangle(Point.Empty, bmp.Size));            
        //In a variable save the cursor with the image of your controler
        this.BitMapCursor = new Cursor(bmp.GetHicon());             
        send.DoDragDrop(send.Text, DragDropEffects.Move);
    }

i am casting the sender because in my aplicattion the buttón is be generated in real time.

Now define the method GiveFeedBack(this method occurs during a drag operation.)

    private void btn_GiveFeedback(object sender, GiveFeedbackEventArgs e)
    {
        //Deactivate the default cursor
        e.UseDefaultCursors = false;       
        //Use the cursor created from the bitmap
        Cursor.Current = this.BitMapCursor;

    }

To end dont forget suscribe the control to the methods

btn.Click += new EventHandler(ClickButton);
btn.MouseDown += new MouseEventHandler(btn_MouseDown);
btn.GiveFeedback += new GiveFeedbackEventHandler(btn_GiveFeedback);

Answer 4

this could be an option:

private void btntarget_MouseDown(object sender, MouseEventArgs e)
    {                                

        Bitmap bmp = new Bitmap(btntarget.Width, btntarget.Height);
        btntarget.DrawToBitmap(bmp, new Rectangle(Point.Empty, bmp.Size));
        //optionally define a transparent color
        bmp.MakeTransparent(Color.White);

        Cursor cur = new Cursor(bmp.GetHicon());                                
        Cursor.Current = cur;            

    }

the cursor's hotspot will be created at the middle of the image

Answer 5

That's usually handled by the GiveFeedback event.