The Dirty property is used by the http://msdn.microsoft.com/en-us/library/system.web.sessionstate.sessionstatemodule.aspx to determine whether the values in an http://msdn.microsoft.com/en-us/library/system.web.sessionstate.isessionstateitemcollection.aspx implementation have been modified.
In implementing the http://msdn.microsoft.com/en-us/library/system.web.sessionstate.isessionstateitemcollection.aspx interface, you should initialize the Dirty property as false and set the http://msdn.microsoft.com/en-us/library/system.web.sessionstate.sessionstateitemcollection.dirty.aspx property to true in the implementations of the http://msdn.microsoft.com/en-us/library/system.web.sessionstate.isessionstateitemcollection.remove.aspx, http://msdn.microsoft.com/en-us/library/system.web.sessionstate.isessionstateitemcollection.removeat.aspx, or http://msdn.microsoft.com/en-us/library/system.web.sessionstate.isessionstateitemcollection.clear.aspx methods.