Serialcoder en Français Serialcoder in English
TEL : +33 (0)9 72 13 15 17

Windows Forms FAQ resources

20. Windows Forms ListBox

20.2 How do I implement Drag and Drop support between ListBoxes?


The code below minimally handles D&D for a single selection list box by handling four events. The D&D is initiated in MouseDown if the user mouses down on the current selection. The DragEnter event is used to set the dragging effect to copy if you are not over the drag source. The DragDrop event is used to do the drop. And finally, the SelectedIndexChanged event is used to track the current selection for use in the MouseDown event. You can download a working project (C#, VB). This project handles additional events to provide visual queues during the drop.

The samples referenced above do not allow dragging and dropping within the same listbox. Here is another set of samples that does allow you to drag and drop within the same listbox.
public class ListBoxDragNDrop : ListBox
{
     private int lastMouseUpItemIndex = -1;
     private bool isDropSource = false;

     public ListBoxDragNDrop()
     {
          this.AllowDrop = true;     //allow D&D
          this.SelectionMode = SelectionMode.One;      //single selection

          DragDrop += new System.Windows.Forms.DragEventHandler(OnDragDrop);
          DragEnter += new System.Windows.Forms.DragEventHandler(OnDragEnter);
          MouseDown += new System.Windows.Forms.MouseEventHandler(OnMouseDown);
          SelectedIndexChanged += new System.EventHandler(OnSelectedIndexChanged);
     }
               
     private void OnDragDrop(object sender, DragEventArgs e)
     {
          if(e.Effect == DragDropEffects.Copy)
          {
               Point point = this.PointToClient(new Point(e.X, e.Y));
               int index = this.IndexFromPoint(point);
               if( index > -1 && index < this.Items.Count)
                    Items.Insert(index, e.Data.GetData(DataFormats.Text));
               else
                    Items.Add(e.Data.GetData(DataFormats.Text));
          }
     }

     private void OnDragEnter(object sender, System.Windows.Forms.DragEventArgs e)
     {
          if (e.Data.GetDataPresent(DataFormats.Text) && !isDropSource )
               e.Effect = DragDropEffects.Copy;
          else
               e.Effect = DragDropEffects.None;
     }

     private void OnMouseDown(object sender, System.Windows.Forms.MouseEventArgs e)
     {
          if(MouseButtons == MouseButtons.Left && SelectedIndex == lastMouseUpItemIndex)
          {
               isDropSource = true;
               DoDragDrop(SelectedItem, DragDropEffects.Copy);
               isDropSource = false;
               lastMouseUpItemIndex = this.SelectedIndex;
          }
     }

     private void OnSelectedIndexChanged(object sender, System.EventArgs e)
     {
          lastMouseUpItemIndex = this.SelectedIndex;
     }
}

One more note. If your listboxes contain full file pathnames, you can support dropping these paths onto Windows Explorer by supporting the FileDrop dataformat. In the OnMouseDown override, if you replace the DoDragDrop line with the following code block, you will be able to drop files.

     //put the file path is a string array
     string[] files = new String[1];
     files[0] = SelectedItem.ToString();

     //create a dataobject holding this array as a filedrop
     DataObject data = new DataObject(DataFormats.FileDrop, files);

     //also add the selection as textdata
     data.SetData(DataFormats.StringFormat, SelectedItem.ToString());

     //do the dragdrop
     DoDragDrop(data, DragDropEffects.Copy);