The Global.asax file, also known as the ASP.NET application file, is an optional file that contains code for responding to application-level and session-level events raised by ASP.NET or by HTTPModule.If you remember it's same as global.asa in ASP. The Global.asax file resides in the root directory of an ASP.NET application. At run time, Global.asax is parsed and compiled into a dynamically generated .NET Framework class derived from the HttpApplication base class. ASP.NET is configured so that any direct URL request for the Global.asax file is automatically rejected.
The HTTPApplication class exposes the following events for you to handle.