Drag&Drop a clone of an object

Question

Hi everyone.

I need to do some dragging and dropping and came across a little problem.

I'm doing pretty much the same as seen in this code example (not my code, I just googled this to quickly show roughly what I'm doing):

 public partial class Form1 : Form
 {
  private bool m_DragInProgress;

  public Form1()
  {
   InitializeComponent();
  }

  private void listView1_MouseMove(object sender, MouseEventArgs e)
  {
   if (e.Button == MouseButtons.Left)
   {
    if (!m_DragInProgress && listView1.GetItemAt(e.Location.X, e.Location.Y) != null)
    {
     m_DragInProgress = true;
     listView1.DoDragDrop(listView1.GetItemAt(e.Location.X, e.Location.Y), DragDropEffects.Copy);
    }
   }
   else
   {
    m_DragInProgress = false;
   }
  }

  private void listView1_DragOver(object sender, DragEventArgs e)
  {
   if (m_DragInProgress)
   {
    Point l_point = listView1.PointToClient(new Point(e.X, e.Y));
    ListViewItem l_item = listView1.GetItemAt(l_point.X, l_point.Y);

    if (l_item != null && l_item != listView1.SelectedItems[0])
    {
     e.Effect = DragDropEffects.Copy;
    }
    else
    {
     e.Effect = DragDropEffects.None;
    }
   }
  }

  private void listView1_DragDrop(object sender, DragEventArgs e)
  {
   if (m_DragInProgress)
   {
    Point l_point = listView1.PointToClient(new Point(e.X, e.Y));
    ListViewItem l_item = listView1.GetItemAt(l_point.X, l_point.Y);

    if (l_item != null && l_item != listView1.SelectedItems[0])
    {
     if (listView1.SelectedItems[0].ImageIndex != -1)
     {
      l_item.ImageIndex = listView1.SelectedItems[0].ImageIndex;
     }
     else
     {
      l_item.ImageKey = listView1.SelectedItems[0].ImageKey;
     }
    }

    m_DragInProgress = false;
   }
  }
 }

My problem is that I can't disconnect the UserControl from it's StackPanel before dropping it in another container, because I don't want it to disappear from its original location. DragDropEffects.Copy apparently doesn't do what it sounds like and so I tried a different approach.

I try to drag an exact clone of the UserControl from its StackPanel into the other container. For this I serialize the UserControl and deserialize it in a new instance of my UserControl using following method:

public static T Clone<T>(this T source)
		{

			// Don't serialize a null object, simply return the default for that object
			if (Object.ReferenceEquals(source, null))
			{
				return default(T);
			}

			Stream stream = new MemoryStream();
			using (stream)
			{
				Serializer.Serialize(stream, source);
				stream.Seek(0, SeekOrigin.Begin);
				return (T)Serializer.Deserialize<T>(stream);
			}
		}

Problem is apparently instead of a duplicate I only get a reference to the original and so I still end up with an object which has the StackPanel as a parent. And if I disconnect my 'clone', the original is being disconnected aswell.

I'm sure this can be done much easier and better. Any ideas?

Answer 1

First, DragDropEffects.Copy doesn't do anything. It's a value. Your code can change its behavior according to the value of DragEventArgs.Effect.

Cloning an object by serializing and deserializing is a bad idea. You should implement a specific Clone method.

Answer 2

For do dragdrop with copy-paste issue, see the following thread:

http://social.msdn.microsoft.com/Forums/en/winforms/thread/e886bff1-66e7-491f-b208-a4d2d4784183 

Best Regards

---

Neddy Ren[MSFT]
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