Code

public class MyBehavior : LinkShapesBehavior
{
	public MyBehavior(DiagramView view) : base(view)
	{
	}

	public override InteractionState StartDraw(PointF point)
	{
		int handle = -1;

		DiagramNode node = Diagram.GetNodeAt(point);
		if (node != null &&
			(node == DiagramView.AutoHandlesItem ||
			!node.HitTestHandle(point, ref handle)))
		{
			Diagram.Selection.Change(node);
			return new InteractionState(node, 8, Action.Modify);
		}

		return base.StartDraw(point);
	}
}

// in form_load
diagramView.CustomBehavior = new MyBehavior(diagramView);