Can I use threads to carry out long-running jobs on IIS?

You can accomplish what you want, but it is typically a bad idea. Several ASP.NET blog and CMS engines take this approach, because they want to be installable on a shared hosting system and not take a dependency on a windows service that needs to be installed. Typically they kick off a long running thread in Global.asax when the app starts, and have that thread process queued up tasks.

In addition to reducing resources available to IIS/ASP.NET to process requests, you also have issues with the thread being killed when the AppDomain is recycled, and then you have to deal with persistence of the task while it is in-flight, as well as starting the work back up when the AppDomain comes back up.

Keep in mind that in many cases the AppDomain is recycled automatically at a default interval, as well as if you update the web.config, etc.

If you can handle the persistence and transactional aspects of your thread being killed at any time, then you can get around the AppDomain recycling by having some external process that makes a request on your site at some interval - so that if the site is recycled you are guaranteed to have it start back up again automatically within X minutes.

Again, this is typically a bad idea.

EDIT: Here are some examples of this technique in action:

Community Server: Using Windows Services vs. Background Thread to Run Code at Scheduled Intervals Creating a Background Thread When Website First Starts

EDIT (from the far distant future) - These days I would use Hangfire.

I disagree with the accepted answer.

Using a background thread (or a task, started with Task.Factory.StartNew) is fine in ASP.NET. As with all hosting environments, you may want to understand and cooperate with the facilities governing shutdown.

In ASP.NET, you can register work needing to stop gracefully on shutdown using the HostingEnvironment.RegisterObject method. See this article and the comments for a discussion.

(As Gerard points out in his comment, there's now also HostingEnvironment.QueueBackgroundWorkItem that calls down to RegisterObject to register a scheduler for the background item to work on. Overall the new method is nicer since it's task-based.)

As for the general theme that you often hear of it being a bad idea, consider the alternative of deploying a windows service (or another kind of extra-process application):

• No more trivial deployment with web deploy
• Not deployable purely on Azure Websites
• Depending on the nature of the background task, the processes will likely have to communicate. That means either some form of IPC or the service will have to access a common database.

Note also that some advanced scenarios might even need the background thread to be running in the same address space as the requests. I see the fact that ASP.NET can do this as a great advantage that has become possible through .NET.