Question

I’ve got a TreeView and Canvas in my WPF application. I’m trying to implement functionality whereby users can drag a TreeViewItem and a method should be called when the user drops on the canvas, passing the TreeViewItem header as a parameter to this method.

This is what I’ve done so far:

private void TreeViewItem_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
     if (e.Source.GetType().Name.Equals("TreeViewItem"))
     {
         TreeViewItem item = (TreeViewItem)e.Source;

         if (item != null)
         {
              DataObject dataObject = new DataObject();
              dataObject.SetData(DataFormats.StringFormat, item.Header.ToString());
              DragDrop.DoDragDrop(item, dataObject, DragDropEffects.Copy);
         }
     }
 }

When I drag and drop to the canvas nothing happens. I am thus unsure of what I should do next. I feel that it’s something really small, but I’m at a loss. How can I call the method and detect that the header has been dropped?

Any ideas?

Answer 1

You need to set AllowDrop to true on your target element and then handle DragOver and Drop events on the target element.

Example:

    private void myElement_DragOver(object sender, DragEventArgs e)
    {
        if (!e.Data.GetDataPresent(typeof(MyDataType)))
        {
            e.Effects = DragDropEffects.None;
            e.Handled = true;
        }
    }

    private void myElement_Drop(object sender, DragEventArgs e)
    {
        if (e.Data.GetDataPresent(typeof(MyDataType)))
        {
            // do whatever you want do with the dropped element
            MyDataType droppedThingie = e.Data.GetData(typeof(MyDataType)) as MyDataType;
        }
    }