The problem is that when you click inside the empty rectangle no mouse event is fired(as far as I tested no doubleClick or mouseDown events) and that's why you'll have to use some thing else.
I think that the best way to do this is to handle the RadDiagram's MouseDoubleClick event and check if there's a connection around the mouse:
sender, MouseButtonEventArgs e)
var point =
var connection =
Rect(point.X - 2, point.Y - 2, 4, 4)).FirstOrDefault();
(Note: you could try other methods from the hitTestService if you want - GetItemsNearPoint, GetTopItemNearPoint and etc.)
I hope I was able to help you and if you need further assistance feel free to ask.
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