I got trouble with IActiveViewEvent.SelectionChanged and don't know how to solve this. Maybe, this is a bug of ArcObject
I registered an event of IActiveViewEvent.SelectionChanged. It's working ok. When user selects an objetc on ActiveView, my applicaiton will show information about that feature on a form. On the form, there is a button which will show Crystal Report document if it's clicked. If user clicks on this button, CrystaL Report is shown ok, but then IActiveViewEvent.SelectionChanged will be lost. This event will not fired any more.