Drag canvas boundary

Hi,

The website given below is the coding I used for dragging of canvas:
http://blogs.msdn.com/marcelolr/arch...02/542641.aspx

I want to set the boundary for the canvas. I know how to set boundary of a rectangle. But how can I set boundary for a circle or some other shapes like a 'T', 'L' shape, etc ...? Meaning that the user can drag the canvas within the circle, T shape, L shape, etc using VB code.

I'm using WPF.

Thanks