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Exploring the Global.asax file in ASP.NET
Have you ever felt the need of writing logic at the application level;
precisely a location or a file where you could handle events or errors
at the application level? Well if yes, then enter the Global.asax. Using
this file, you can define event handlers with application-wide or
session-wide scope. In this article, we will explore the application
and session level events exposed in the Global.asax file and how we can
utilize these events in our applications.
The Global.asax, also known as the ASP.NET application file, is located in
the root directory of an ASP.NET application. This file contains code that
is executed in response to application-level and session-level events raised
by ASP.NET or by HTTP modules. You can also define ‘objects’ with
application-wide or session-wide scope in the Global.asax file.
These events and objects declared in the Global.asax are applied to
all resources in that web application.
Note 1: The Global.asax is an optional file. Use it only when there
is a need for it.
Note 2: If a user requests the Global.asax file, the request is rejected.
External users cannot view the file.
The Global.asax file is parsed and dynamically compiled by ASP.NET.
You can deploy this file as an assembly in the \bin directory of an
ASP.NET application.
How to create Global.asax
Adding a Global.asax to your web project is quiet simple.
Open Visual Studio 2005 or 2008 > Create a new website > Go to the
Solution Explorer > Add New Item > Global Application Class > Add.
Examining the methods related to the events in Global.asax
There are 2 ‘set’ of methods that fire corresponding to the events.
The first set which gets invoked on each request and the second set
which does not get invoked on each request. Let us explore these methods.
Methods corresponding to events that fire on each request
Application Request to trigger events in the following order:
1. Application_BeginRequest
2. Application_AuthenticateRequest
3. Application_AuthorizeRequest
4. Application_ResolveRequestCache
5. Application_AcquireRequestState
6. Application_PreRequestHandlerExecute
7. Application_PreSendRequestHeaders
8. Application_PreSendRequestContent
9. Application_PostRequestHandlerExecute
10. Application_ReleaseRequestState
11. Application_UpdateRequestCache
12. Application_EndRequest
Application_BeginRequest() – fired when a request for the
web application comes in.
Application_AuthenticateRequest –fired just before
the user credentials are authenticated. You can specify
your own authentication logic over here.
Application_AuthorizeRequest() – fired on successful
authentication of user’s credentials. You can use this
method to give authorization rights to user.
Application_ResolveRequestCache() – fired on successful
completion of an authorization request.
Application_AcquireRequestState() – fired just before the
session state is retrieved for the current request.
Application_PreRequestHandlerExecute() - fired before the page framework
begins before executing an event handler to handle the request.
Application_PostRequestHandlerExecute() – fired after HTTP handler has
executed the request.
Application_ReleaseRequestState() – fired before current state data kept
in the session collection is serialized.
Application_UpdateRequestCache() – fired before information is added to
output cache of the page.
Application_EndRequest() – fired at the end of each request
Methods corresponding to events that do not fire on each request
Application_Start() – fired when the first resource is requested from
the web server and the web application starts.
Session_Start() – fired when session starts on each new user
requesting a page.
Application_Error() – fired when an error occurs.
Session_End() – fired when the session of a user ends.
Application_End() – fired when the web application ends.
Application_Disposed() - fired when the web application is destroyed.
Show me an example!!
Let us see an example of how to use the Global.asax to catch
unhandled errors that occur at the application level.
To catch unhandled errors, do the following. Add a Global.asax file
(Right click project > Add New Item > Global.asax).
In the Application_Error() method, add the following code:
C#
void Application_Error(object sender, EventArgs e)
{
// Code that runs when an unhandled error occurs
Exception objErr = Server.GetLastError().GetBaseException();
string err = "Error in: " + Request.Url.ToString() +
". Error Message:" + objErr.Message.ToString();
}
VB.NET
Sub Application_Error(ByVal sender As Object, ByVal e As EventArgs)
' Code that runs when an unhandled error occurs
Dim objErr As Exception = Server.GetLastError().GetBaseException()
Dim err As String = "Error in: " & Request.Url.ToString() & ".
Error Message:" & objErr.Message.ToString()
End Sub
Here we make use of the Application_Error() method to capture the error
using the Server.GetLastError().
Conclusion:
In this article, we learnt that Global.asax is a file used to declare
application-level events and objects. The file is responsible for handling
higher-level application events such as Application_Start, Application_End,
Session_Start, Session_End, and so on. I would encourage you to explore the
methods corresponding to the events and analyze the best possible methods
to use them in your application, if needed.
