Can't get data from DragEventArgs. getData() returns null

Question

I'm working on this wpf application where you can click on a StackPanel in a TreeView list and drag it onto another area of the window (in this case a map).

When I'm creating the StackPanel, I store the necessary data in the Tag attribute:

private StackPanel makeStackPanel(messageIconType itemType, 
                                   string filePath, CustomClass message)
    {
        StackPanel stack = new StackPanel();
        stack.Orientation = Orientation.Horizontal;

        stack.Tag = new List<Object>{message, itemType};

        stack.MouseDown += new MouseButtonEventHandler(stack_MouseDown);

        return stack;

    }

When I click on the StackPanel, the event is fired for MouseDown. I make a DataObject so that I can pass the data to the method that catches the "DragDrop" event firing.

private void stack_MouseDown(object sender, System.Windows.Input.MouseEventArgs e)
    {
        StackPanel sp = (StackPanel)sender;
        DataObject dataObj = new DataObject();
        dataObj.SetData(typeof(List<Object>), (List<Object>)sp.Tag);
        DragDrop.DoDragDrop(sp, dataObj, DragDropEffects.Copy);
    }

When we get to the DragDrop method, e.Data.GetData(typeof(List)) returns null.

1    private void Map_DragDrop(object sender, System.Windows.Forms.DragEventArgs e)
2    {
3        string[] data = e.Data.GetFormats();
4        int i=0;
5        if(e.Data.GetDataPresent(data[0]))
6        {
7            i++;
8        }
9        if (e.Data.GetDataPresent(typeof(List<Object>)))
10       {
11           i++;
12       }
13       List<Object> obj = (List<Object>)e.Data.GetData(typeof(List<Object>));
14   }

The lines before that are sanity checks. e.Data.GetFormats() returns a string saying that the format is a List. Lines 5 and 9 both evaluate to true, so there is Data of type List. When I'm in debugging, if I dig deep enough in the DragEventArgs e, I can find my data. It's just that in line 14, it always returns null and I can't find any other way of getting that data.

Answer 1

Instead of passing an instance of Object List, create a class with Property Data of type object. Create an instance of the class and set the Data Property with the object you want to Drag In the Map_DragDrop event call GetData(), passing the typeof(MyDraggedData) as argument and cast back to the original object

lookup DataFormats class and IDataObject implementation

public class MyDraggedData
{
    public object Data { get; set; }
}

private void stack_MouseDown(object sender, System.Windows.Input.MouseEventArgs e)
{
    StackPanel sp = (StackPanel)sender;
    MyDraggedData data = new MyDraggedData();
    data.Data = sp.Tag;
    DragDrop.DoDragDrop(sp, data, DragDropEffects.Copy);
}


private void Map_DragDrop(object sender, System.Windows.Forms.DragEventArgs e)
{
  MyDraggedData data = (MyDraggedData)e.Data.GetData(typeof(MyDraggedData));
  List<Object> obj = (List<Object>)data.Data;
}