How to keep ASP.NET assemblies in AppDomain alive?

In IIS 6, go to the Application Pools section, and right-click > Properties on the pool which hosts the ASP.NET application in question. Go to the Performance tab and uncheck "Shutdown worker processes after being idle for:"

In IIS 7, go to the Connections pane and find Application Pools, and select Advanced Settings for the pool which hosts your application. Find the "Idle Timeout" property and set it to "0" (this disables it).

The default is 20 minutes of inactivity. By unchecking the box, once your AppDomain is loaded by the worker process, it will never die (unless you kill the process or something of course). By default, IIS will recycle the process when it reaches some limit, such as a memory cap, but it will also start a new one and "phase over" all incoming requests until the old one is unused, so as to minimize disruption.

I've also written a small c# class which will keep your ASP.NET application alive (alternate archived version) under normal circumstances. Since it runs within the application, obviously it can't stop IIS or anything else from explicitly killing the process, but it will keep the application "hot", e.g. the app will never go idle long enough for IIS to decide to shut it off.

If you do not have direct control over your IIS configuration (shared host, for example) your best bet is to have a small application running on a separate system - say, an always-on workstation - which hits your site every x minutes to keep the application pool from timing out. Nothing fancy - a simple WebRequest and a while() loop in a console application will do.

One of the advantages of ASP .net is the posibility to create static (shared) instances of objects.

To avoid the necessity of an external process you can create an static timer in (per example) global.asax which calls for a page on the domain with a simple WebRequest. On this way the site keeps alive himself until a manual reset of the pool is done.