As with any other ASP.NET application, sometimes you have actions that only need to occur once at application startup. The standard .NET way to do this would be to modify the global.asax file and add your logic in the Application_Start event handler

public class Global : System.Web.HttpApplication
{
    protected void Application_Start(object sender, EventArgs e)
    {
        // Insert your startup logic here
    }
}

Unfortunately in Umbraco, all sites are required to use the built-in umbraco.Global class.

So how DO you register an Application_Start event handler in Umbraco?

The trick is to create a class that inherits from umbraco.BusinessLogic.ApplicationBase, wich is a special base class  built-in to Umbraco.

So what make ApplicationBase so Special?

Well, when Umbraco first starts up, it will search the bin directory for any class that inherits from ApplicationBase and automatically instantiate that class.

The main purpose of ApplicationBase is generally for hooking into all of the Umbraco events, but as classes are only instantiated at the time of startup,  it’s also a great alternative to the Application_Start event handler.

public class MyApplicationBase : umbraco.BusinessLogic.ApplicationBase
{
    public MyApplicationBase()
    {
        // Insert your startup logic here
    }
}

So there you have it. Registering an Application_Start event handler in Umbraco.

UPDATE – July 11, 2010 19:59

Thanks to a tip off by Ismail, it turns out that in Umbraco v4.1 upwards, you can now simply inherit the umbraco.Global class and override the Application_Start event handler (and any other global.asax event handler for that matter).

public class MyGlobal : umbraco.Global
{
    protected override void Application_Start(object sender, EventArgs e)
    {
        base.Application_Start(sender, e);

        // Insert your startup logic here
    }
}

Knowing this, I would probably now say, if you are working on Umbraco sites in version v4.1 or greater, to go with Ismails suggestion, but if you are working on older versions, or want to maintain backwards compatability, then I’d stick with the ApplicationBase method.

UPDATE – July 12, 2010 22:12

Thanks to another tip off, this time by Aaron Powel (aka Slace), it seems there is a way in versions prior to 4.1, to inherit from umbraco.Global. All you need to do is create your own global.asax file, inheriting from umbraco.Global, and delete the App_global.asax.dll from within your bin folder (if it exists). I can’t say I’ve tested this myself, but definitely another one to try, especially if you need to hook in to other global.asax event handlers.

One thing I would bear in mind though with this method, is if you were to upgrade from one version to another, you may get a new App_global.asax.dll file in you bin folder, so don’t forget to delete it (unless you upgrade to v4.5, in which case I’d just use Ismails method).

Well there you have it, we started off with no standard way to hook into the Application_Start event handler, and have ended up with 3. So, depending on your circumstances, there should be at least one method that suites your needs.