Global.asax is a global event handling file in App_Start folder. This file is read first, and you can specify events and actions at app level.
When you save changes to an active Global.asax file, the ASP.NET page framework detects that the file has been changed. It completes all current requests for the application, sends the Application_OnEnd event to any listeners, and restarts the application domain. In effect, this reboots the application, closing all browser sessions and flushing all state information. When the next incoming request from a browser arrives, the ASP.NET page framework reparses and recompiles the Global.asax file and raises the Application_OnStart event.