Updating a tooltip in Silverlight 2

Question

I have a control in Silverlight 2 that changes state during the application. When its state changes, I want to update the tooltip that goes with it.

I'm using the ToolTipService:

ToolTipService.SetToolTip(ent.LayoutRoot, "FooBar");

which works fine the first time I set a tooltip, but fails the second time time around I get:

"Object reference not set to an instance of an object" from somewhere inside Silverlight where I can't get a useful stack trace / break point.

Trawling the net pops up a few similar problems but no solutions.

Can anyone shed some light on this?

Answer 1

Found the problem - it looks like you can't set a ToolTip after the control is created. So now, I set the ToolTip in the controls constructor, then when it needs updating, the call to SetToolTop() works just fine.