I am writing an Excel 2007 Addin. using VS2008 and .net 3.5, C#. I catched Microsoft.Office.Interop.Excel.Application's WindowActivate and WindowDeActivate events. It was surprised to know that WindowActivate and Deactivate only triggers when i switch between two Excel Windows. if i switch to notepad, i expect Deactivate to be triggered, but its not happening. same way from notepad if i switch to excel window, i expect Activate to be triggered but its not happening. It looks like the behaviour indicates windows are MDI-Child windows. Now what i want to do is get HWnd of Excel's Mainwindow and hook Window Activate and Deactivates using dllimport features. Can anyone guide to me on this. Regards