HttpModule Init method is called several times - why?

1. Inspect the HttpContext.Current.Request to see, for what request the module's init is fired. Could be browser sending multiple request.

2. If you are connected to IIS, do check IIS logs to know whether any request is received for the time you are staying at the break point.

It's normal for the Init() method to be called multiple times. When an application starts up, the ASP.NET Worker process will instantiate as many HttpApplication objects as it thinks it needs, then it'll pool them (e.g. reuse them for new requests, similar to database connection pooling).

Now for each HttpApplication object, it will also instantiate one copy of each IHttpModule that is registered and call the Init method that many times. So if 5 HttpApplication objects are created, 5 copies of your IHttpModule will be created, and your Init method called 5 times. Make sense?

Now why is it instantiating 5 HttpApplication objects say? Well maybe your ASPX page has links to other resources which your browser will try to download, css, javascript, WebResource.aspx, maybe an iframe somewhere. Or maybe the ASP.NET Worker Process 'is in the mood' for starting more than 1 HttpApplication object, that's really an internal detail/optimisation of the ASP.NET process running under IIS (or the VS built in webserver).

If you want code that's guaranteed to run just once (and don't want to use the Application_StartUp event in the Global.asax), you could try the following in your IHttpModule:

private static bool HasAppStarted = false;
private readonly static object _syncObject = new object();

public void Init(HttpApplication context)
{
    if (!HasAppStarted)
    {
        lock (_syncObject)
        {
            if (!HasAppStarted)
            {
                // Run application StartUp code here

                HasAppStarted = true;
            }
        }
    }
}

I've done something similar and it seems to work, though I'd welcome critiques of my work in case I've missed something.