Application_Start method called by ASP.NET once for the lifetime of the application domain, not for each HttpApplication instance. It's called when the first resource (such as a page) in an ASP.NET application is requested.
During asp.net application life cycle, the application raises events that you can handle and calls particular methods that you can override. To handle application events or methods, you can create a file named Global.asax in the root directory of your application.
If you create a Global.asax file, ASP.NET compiles it into a class derived from the HttpApplication class, and then uses the derived class to represent the application.
An instance of HttpApplication processes only one request at a time. This simplifies application event handling because you do not need to lock non-static members in the application class when you access them. This also allows you to store request-specific data in non-static members of the application class. For example, you can define a property in the Global.asax file and assign it a request-specific value.
ASP.NET automatically binds application events to handlers in the Global.asax file using the naming convention Application_event, such as Application_BeginRequest. This is similar to the way that ASP.NET page methods are automatically bound to events, such as the page's Page_Load event. For details, see ASP.NET Page Life Cycle Overview.
The Application_Start and Application_End methods are special methods that do not represent HttpApplication events. ASP.NET calls them once for the lifetime of the application domain, not for each HttpApplication instance.
The following table lists some of the events and methods that are used during the application life cycle. There are many more events than those listed, but they are not commonly used.
