title: Multi select treeview that supports drag of multiple Items in WPF
body: <p>I was searching for possible solution for hours and could not find any.
  Hopefully someone can help me.</p>
  
  <p>I managed to implement Multi Select WPF TreeView using the following answer: <a href="https://stackoverflow.com/a/6681993/1679059">https://stackoverflow.com/a/6681993/1679059</a> </p>
  
  <p>It works nicely but I want to be able to drag selected items and drop them into the DataGrid. 
  In <code>PreviewMouseMove</code> event handler previously selected items get deselected so I can't prevent that from happening. </p>
  
  <p>I was trying to prevent deselecting items in <code>PreviewMouseLeftButtonDown</code> event handler but at that point I cannot know if a user intends to select an item or drag selected items.</p>
  
  <p>Can someone help me with that problem?</p>