INFO: Application Instances, Application Events, and Application State in ASP.NET

The ASP.NET HttpApplication object hides many complex concepts to simplify the programming model. This article describes some of these complexities. In addition, this article describes how ASP.NET handles compatibility issues with the classic Microsoft Active Server Pages (ASP) Application object.

This article is divided into the following sections:

• Application Class and Application Instances
• Application Events
• Application_OnStart and Application_OnEnd
• Application State

Application Class and Application Instances

An application class is defined in the Global.asax file. The code in Global.asax defines a new class that is derived from System.Web.HttpApplication. In the absence of a Global.asax file, the base class, HttpApplication, is used as the application class.

The ASP.NET runtime creates as many instances of application classes as needed to process requests simultaneously. For most applications, this number is limited to the number of threads and remains in the range of 1 through 100, depending on the hardware, server load, configuration, and so on. Many requests reuse application instances, and a free list of application instances is kept during periods of reduced load. Application instances are used in a thread-safe manner, that is, one request at a time. This has important implications:

• You do not have to worry about locking when you access non-static members of the application class.
• Application code can store request data for each request in non-static members of the application class (but not after the EndRequest event because this event may maintain the request for a long time).

Because static members of any class, including an application class, are not thread-safe, the user code must provide appropriate locking for access to static members. This applies to any static member that you add to the application class.

Use the following guidelines to access the application instance that is associated with the current request:

• From the Global.asax, use the this or Me object.
• From a page, every page includes a strongly-typed ApplicationInstance property.
• From the HttpContext object, use the HttpContext.ApplicationInstance property (which you type as HttpApplication).

Note Because Application refers to the global application state dictionary in classic ASP, ASP.NET uses ApplicationInstance and not Application as a property name to refer to the application instance that processes the current request.

Application Events

The lifetime of a request consists of a series of the application events (and some implicit steps that ASP.NET implements). These events are listed below in the order in which they are executed:

1. BeginRequest
2. AuthenticateRequest event
3. DefaultAuthentication internal event
4. AuthorizeRequest event
5. ResolveRequestCache event
6. Internal step to "map handler" (when compilation takes place, a page instance is created)
7. AcquireRequestState event
8. PreRequestHandlerExecute event
9. Internal step to "execute handler" (when the page code is executed)
10. PostRequestHandlerExecute event
11. ReleaseRequestState event
12. Internal step to filter responses UpdateRequestCache event
13. UpdateRequestCache event
14. EndRequest event

Note If you use Server.Transfer or Response.Redirect(string), the current request is effectively interrupted and some of the events that are listed above will not be raised. However, the EndRequest event will be raised in this scenario.

The following items can handle these events:

• Internal ASP.NET page framework (for example, steps 6, 9, and 12 in the preceding list).
• HTTP modules that are configured for the application. The default list of HTTP modules is defined in the Machine.config file.
• Code in Global.asax that is hooked through the Application_[On]EventName method or that is hooked explicitly when you add event handlers for an alternative handler name.

Each event can have synchronous and asynchronous subscribers. Asynchronous subscribers are executed first. Not all events are always executed; the only event that is always executed is EndRequest. As a result, perform all after-request cleanup in the EndRequest event.

Note In most cases, the actual response is sent to the client after the application instance is finished with the response (which is after EndRequest).

Application_OnStart and Application_OnEnd

ASP.NET introduces the unique Application_OnStart and Application_OnEnd "events" for compatibility with classic ASP. These "events" are executed only once in the lifetime of an application and not for every application instance. Therefore, if you change non-static members in these methods, you affect only one application instance and not all instances. You can initialize one application instance either in the constructor or by overriding the Init method.

Application_OnStart is a logical equivalent to the class constructor for the application class, but it offers one advantage: the code has access to the HttpContext for the first request to the application.

Application State

Application state is a global dictionary of late-bound objects, which classic ASP introduces to compensate for the absence of global variables in Microsoft Visual Basic Scripting Edition (VBScript). In ASP.NET, you can access application state through one of the following:

• Application property (which is defined in both HttpApplication and Page classes)
• HttpContext.Application

ASP.NET includes application state primarily for compatibility with classic ASP so that it is easier to migrate existing applications to ASP.NET. It is recommended that you store data in static members of the application class instead of in the Application object. This increases performance because you can access a static variable faster than you can access an item in the Application dictionary.

To access static application members from pages in Microsoft Visual C# .NET and Microsoft Visual Basic .NET, you must use the ClassName attribute in Global.asax to name your application class. For example: If a static member is named MyStaticMember in your Global.asax file, you can use MyClass.MyStaticMember to access it from your page.