wpf - problem with DragAndDrop between two canvases

Question

Hi I have strange problem with drag and drop operation between two canvases. I wanted to drag my custom control from one canvas to another.

I started dragging operation with this code

 DataObject dragObject = new DataObject("CalendarBlock",block);

             DragDrop.DoDragDrop(this, dragObject, DragDropEffects.Move);

In second canvas I wrote this code

 void CalendarCanvas_PreviewDrop(object sender, DragEventArgs e)
 {
     if (e.Data.GetDataPresent("CalendarBlock"))
     {
         UcCalendarBlock calendarBlock = e.Data.GetData("CalendarBlock") as UcCalendarBlock;

         CalendarCanvas parent = calendarBlock.Parent as CalendarCanvas;
        // parent.Children.Remove(calendarBlock);


         (e.Source as CalendarCanvas).Children.Remove(calendarBlock);
         this.AddCalendarTask(calendarBlock);
     }
 }

The problem is that I get an error

"Specified element is already the logical child of another element. Disconnect it first."

It was strange because I already removed this element using that code

(e.Source as CalendarCanvas).Children.Remove(calendarBlock);

However it truned out that (e.Source as CalendarCanvas).Children collection is empty(I have no idea why), so I used that code

parent.Children.Remove(calendarBlock);

Now everything is working fine, however I think is not too elegant solution. Could somebody tells me why (e.Source as CalendarCanvas).Children is empty ??