When my colleague Niklas Melinder applied patch 2 for EPiServer 7 he ran into trouble when the site was deployed to our acceptance server. The site started to throw “System.InvalidOperationException: The SessionStateTempDataProvider class requires session state to be enabled.”. Since our acceptance environment runs with sessions state switched off it was pretty clear what caused the error but why it happened was not.
After some reflection the following in PropertyExtensions seemed to be the culprit
Using TempData requires session state if the value is not used in the same request (see the answer to this q on SO). This code is used for the PropertyFor extensions methods so it seems webform pages are not affected by this but we haven’t really looked into that.