How to detect if the current application pool is winding up in IIS7.5 and Asp.Net 3.5+

I recently stumbled across this article on Brain.Save() which talks about exactly this issue from the point of view of hosting WCF (he's Steve Maine - A program manager at Redmond on the Connected Servies Division).

They need to be able to do this when a WCF service is hosted inside Asp.Net since they need to be able to shutdown any open listeners so that the WCF engine in the new app domain will be able to open them all up again.

As the article demonstrates, the answer is to implement the IRegisteredObject interface, call ApplicationManager.CreateObject to create an instance of your object and then register it with HostingEnvironment.RegisterObject (all detailed in the MSDN documentation for the interface).

When this object's IRegisteredObject.Stop(bool) implementation is called with false as the parameter, this is notification that the app domain is being shut down and that the object should be unregistered (kind of like a global dispose) with a call to HostingEnvironment.UnregisterObject.

When it's called with true it means you've not unregistered in good time, and that if you don't Unregister immediately, it'll be done for you.

I can certainly use this mechanism to find out, when an exception occurs, if the AppDomain is being killed or not. The nature of the object in question that throws the exception means that if it's not at shutdown, it must be during initial startup.

Equally, however, I may well start looking at this persistence mechanism for some of my other more complicated static information!

The History

The article also explains some of the history, and rationale, of why you would want to use IRegisteredObject rather than Application_Start and Application_End methods in global.asax:

Traditional ASP.NET applications can hook application lifecycle events (application startup/shutdown) by implementing methods like Application_Start and Application_Stop in global.asax. However, global.asax is for application code. Infrastructure pieces (of which the WCF hosting system is one) need a mechanism of hooking AppDomain lifecycle events that do not involve dumping infrastructure code in your global.asax file. That space is reserved for you, the user, and it would be rude of use to pollute that with a bunch of hosting goo we need to make the whole thing work. Instead, the ASP.NET folks did some great work during the Whidbey release to open up the hosting API’s and make it easy for people like WCF to come along and hook these lifecycle events in a way that’s invisible to application code.

You can check the value of System.Web.Hosting.HostingEnvironment.ShutdownReason, when the app pool is not in the process of closing / recycling it will have the ShutdownReason of None.

Adding the actual code to do this:

public class RecycleWatcher : IRegisteredObject
{
    public static bool IsRecycling { get; private set; }

    public void Register()
    {
        HostingEnvironment.RegisterObject(this);
    }
    public void Stop(bool immediate)
    {
        IsRecycling = true;
    }
}

Then enable it by running

new RecycleWatcher().Register();

After that just check that property for IsRecycling to know if you are recyling or not.

if (RecycleWatcher.IsRecycling) DoSomething();