addin problem

[Hin]

I'm writting an Addin for Office applications
(not only Visio, but i use Visio for testing)

In the OnConnection method of IDTExtensibility2 interface,
I do the following:

// app is the root object passed by the host application
PropertyInfo info = app.GetType().GetProperty("CommandBars");

However i always get a null here, anybody know why?
Thanks.