what is global.asax in asp.net

The Global.asax file, also known as the ASP.NET application file, is an optional file that contains code for responding to application-level events raised by ASP.NET or by HttpModules. The Global.asax file resides in the root directory of an ASP.NET-based application. At run time, Global.asax is parsed and compiled into a dynamically generated .NET Framework class derived from the HttpApplicationbase class. The Global.asax file itself is configured so that any direct URL request for it is automatically rejected; external users cannot download or view the code written within it.

The ASP.NET Global.asax file can coexist with the ASP Global.asax file. You can create a Global.asax file either in a WYSIWYG designer, in Notepad, or as a compiled class that you deploy in your application's \Bin directory as an assembly. However, in the latter case, you still need a Global.asax file that refers to the assembly.

The Global.asax file is optional. If you do not define the file, the ASP.NET page framework assumes that you have not defined any application or session event handlers.

The global.asax file is used to add application level logic & processing. Note that the global.asax does not handle any UI related processing, nor does it process individual page level requests. It basically controls the following events...
Application_Start
Application_End
Session_Start
Session_End

The Global.asax file contains the following events:

• Application_Init: Fired when an application initializes or is first called. It's invoked for all HttpApplication object instances.
• Application_Disposed: Fired just before an application is destroyed. This is the ideal location for cleaning up previously used resources.
• Application_Error: Fired when an unhandled exception is encountered within the application.
• Application_Start: Fired when the first instance of the HttpApplication class is created. It allows you to create objects that are accessible by all HttpApplication instances.
• Application_End: Fired when the last instance of an HttpApplication class is destroyed. It's fired only once during an application's lifetime.
• Application_BeginRequest: Fired when an application request is received. It's the first event fired for a request, which is often a page request (URL) that a user enters.
• Application_EndRequest: The last event fired for an application request.
• Application_PreRequestHandlerExecute: Fired before the ASP.NET page framework begins executing an event handler like a page or Web service.
• Application_PostRequestHandlerExecute: Fired when the ASP.NET page framework is finished executing an event handler.
• Applcation_PreSendRequestHeaders: Fired before the ASP.NET page framework sends HTTP headers to a requesting client (browser).
• Application_PreSendContent: Fired before the ASP.NET page framework sends content to a requesting client (browser).
• Application_AcquireRequestState: Fired when the ASP.NET page framework gets the current state (Session state) related to the current request.
• Application_ReleaseRequestState: Fired when the ASP.NET page framework completes execution of all event handlers. This results in all state modules to save their current state data.
• Application_ResolveRequestCache: Fired when the ASP.NET page framework completes an authorization request. It allows caching modules to serve the request from the cache, thus bypassing handler execution.
• Application_UpdateRequestCache: Fired when the ASP.NET page framework completes handler execution to allow caching modules to store responses to be used to handle subsequent requests.
• Application_AuthenticateRequest: Fired when the security module has established the current user's identity as valid. At this point, the user's credentials have been validated.
• Application_AuthorizeRequest: Fired when the security module has verified that a user can access resources.
• Session_Start: Fired when a new user visits the application Web site.
• Session_End: Fired when a user's session times out, ends, or they leave the application Web site.