Code

class CanvasEx : Canvas
{
	public CanvasEx()
	{
		button = new Button();
		button.Content = "hello";
		Children.Add(button);
	}

	public Button button;
}

class BehaviorEx : CustomTypesBehavior
{
	public BehaviorEx(Diagram diagram)
		: base(diagram)
	{
	}

	public override InteractionState StartDraw(Point point)
	{
		HitTestResult htr;
		DiagramNode node = Diagram.GetNodeAt(point, true, false);
		if (node != null)
		{
			if (node.AnchorPattern != null)
			{
				foreach (AnchorPoint p in node.AnchorPattern.Points)
				{
					Rect r = node.Bounds;
					Point appos = new Point(r.X + r.Width * p.X / 100, r.Y + r.Height * p.Y / 100);

					if (p.AllowOutgoing && Math.Abs(point.X - appos.X) <= 3 && Math.Abs(point.Y - appos.Y) <= 3)
						return base.StartDraw(point);
				}

				DiagramNodeAdapter adapter = node as DiagramNodeAdapter;
				if (adapter != null)
				{
					CanvasEx canvas = adapter.UIElement as CanvasEx;
					if (canvas != null)
					{
						GeneralTransform tr = Diagram.DocumentPlane.TransformToVisual(canvas.button);
						Point localPoint = tr.Transform(point);
						htr = VisualTreeHelper.HitTest(canvas.button, localPoint);
						if (htr != null && htr.VisualHit != null)
							return null;
						else
							return new InteractionState(node, 8, MindFusion.Diagramming.Wpf.Action.Modify);
					}
				}
			}
		}

		return base.StartDraw(point);
	}
}