Monday, February 27, 2012

Methods in Global.asax

Methods in Global.asax

This blog is intended to spread some light on the various methods which are available in global.asax file in ASP.NET. It’s very important to understand the methods in global.asax so that we as programmers can handle some application level events very efficiently. I said application level events and reason for using application word is that global.asax is an application level file and methods in it are used to handle application level events and these methods are not at all specific to any aspx page. Some of the common methods in the order in which they are executed are listed below
  • Application_Start
  • Application_BeginRequest
  • Application_AuthenticateRequest
  • Session_Start
  • Application_EndRequest
  • Session_End
  • Application_End
  • Application_Error
Now let’s see what is the major difference between these methods or events. Oh I forgot to say, these are actually events not methods which get raised when a particular event gets triggered. Before we see the various methods in Global.asax I would like to tell you that Global.asax is actually derived from a class called “HttpApplication”. The above listed methods are only a few methods which I am gonna talk about. The listing of other methods can be found at the end of the blog. Now lets see the above mentioned events one by one.

Application_Start event gets triggered only once during the life cycle of the application. This once happens when the first request for any resource in the application comes. Resource can be a page or an image in the application. When the very first request for a resource, say a web page, is made by a user “Application_Start” is triggered after which this event is not at all executed. If by any chance the server where the application is hosted is restarted then this event is fired once again i.e. when the very first request for any resource in the application is made after the server is reset.

“Application_BeginRequest” is the second event which gets fired after “Application_Start”. Unlike the “Application_Start”, “Application_BeginRequest” is triggered for each and every request which comes to the application. Since this method is fired for any request made to the application you can use this method to keep track of what and all resources are accessed through this method.

“Application_AuthenticateRequest” is the next event in line which is triggered after “Application_BeginRequest” is triggered. “Application_AuthenticateRequest” is also fired for each and every request. This event can be used to write code in scenarios where you want to do something when the user is getting authenticated.

The next event in line which gets triggered after “Application_AuthenticateRequest” is “Session_Start”. Session start event is fired only when a new session for a user starts. Once “Session_Start” for a user is fired then if the user makes subsequent request to any resource within the application this event is not at all triggered. The event is triggered only when the user’s session expires and then the user tries to access any resource in the application again.
This event can be used when you want to do something when the user visits you site/application for the first time or when his session starts. This event doesn’t get triggered if you are not using sessions which can be disabled in the web.config.

The next event in line which gets fired once the request for the user is processed is “Applicatin_EndRequest”. This event is the closing event of “Applicatin_BeginRequest”. This event is also fired for each and every request which comes for the application.

The closing event of “Session_Start” event. Whenever a user’s session in the application expires this event gets fired. So anything you want to do when the user’s session expires you can write codes here. The session expiration time can be set in web.config file. By default session time out is set to 20 mins.

The same as “Application_Start”, “Application_End” is executed only once, when the application is unloaded. This event is the end event of “Application_Start”. This event is normally fired when the application is taken offline or when the server is stopped.

Now we come to the last event mentioned in this blog and that is “Application_Error”. This event gets fired when any unhandled exception/error occurs anywhere in the application. Any unhandled here means exception which are not caught using try catch block. Also if you have custom errors enabled in your application i.e. in web.config file then the configuration in web.config takes precedence and all errors will be directed to the file mentioned in the tag.
Lets see with an e.g. how these events get fired.
Suppose “A”, “B” and “C” are users who are going to access a site named “My Site”. “A” is the very first user to visit “My Site” and he/she is accessing “productlist.aspx” page. At this time the flow of the request is as follows. The “Application_Start” event is triggered, since “A” is the very first user to visit the application, after this “Application_BeginRequest”, then “Application_AuthenticateRequest”, then “Session_Start”, “productlist.aspx” page level events are processed and then “Application_EndRequest” event is triggered. After accessing “productlist.aspx” if “A” access some other page then for those page request the flow will be first “Application_BeginRequest”, “Application_AuthenticateRequest” then the page processing (page level events) and then “Application_EndRequest”. For every subsequent request this pattern is followed.

When “B” accesses some resource in the site, say “default.aspx”, then first “Applicatin_BeginRequest”, second “Application_AuthenticateRequest”, third “Session_Start” then “default.aspx” page level events are executed and after that “Application_EndRequest” is executed. After accessing “default.aspx” “B” access “productlist.aspx” then first “Application_BeginRequest”, second “Application_AuthenticateRequest” then “productlist.aspx” and then “Application_EndRequest” event is triggered. He refreshes the page the same events are executed in the same order.
The above same process is repeated for “C” also.
Suppose you have an unhandled exception and you don’t have custom errors enabled in web.config then when a user accesses a resource the flow will be first “Application_BeginRequest”, “Application_AuthenticateRequest”, page level event and an error occurs in the page then it goes to “Application_Error” after that “Application_EndRequest”.
The order mentioned above is how the events are triggered. So with this I hope you would have got a clear idea on how these events are triggered.
Some other events which are part of the HttpApplication class are as follows
  • PostAuthenticateRequest
  • AuthorizeRequest
  • PostAuthorizeRequest
  • ResolveRequestCache
  • PostResolveRequestCache
  • PostMapRequestHandler
  • AcquireRequestState
  • PostAcquireRequestState
  • PreRequestHandlerExecute
  • PostRequestHandlerExecute
  • ReleaseRequestState
  • PostReleaseRequestState
  • UpdateRequestCache
  • PostUpdateRequestCache
  • LogRequest. (Supported in IIS 7.0 only.)
  • PostLogRequest (Supported in IIS 7.0 only.)

No comments:

Post a Comment