Code

if (Keyboard.Modifiers == ModifierKeys.Control)
{
	var node = new ShapeNode(Diagram);
	node.Brush = Brushes.Transparent;
	return new InteractionState(node, null, Action.Create);
} 

Code

class RectanglesBehavior : DrawShapesBehavior
{
	public RectanglesBehavior(Diagram diagram) : base(diagram)
	{
		diagram.DoubleClicked += OnDiagramDoubleClicked;
	}

	void OnDiagramDoubleClicked(object sender, DiagramEventArgs e)
	{
		// hold down the Ctrl key + double-click
		if (Keyboard.Modifiers == ModifierKeys.Control)
		{
			var cx = e.MousePosition.X;
			var cy = e.MousePosition.Y;

			var rectangle1 = Diagram.Factory.CreateShapeNode(
				cx - 50, cy - 50, 100, 100, Shapes.Rectangle);

			rectangle1.Brush = Brushes.Transparent;
			rectangle1.Stroke = Brushes.Green;
		}
	}
	...