Some events don't fire with every request: : Global.asax « ASP.Net Instroduction « ASP.NET Tutorial

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Start() is invoked when the application first starts up.
Start() is invoked each time a new session begins. 
Error() is invoked whenever an unhandled exception occurs in the application.
End() is invoked whenever the user's session ends. 
End() is invoked just before an application ends. 
Disposed() is invoked after the application has been shut down.