This behaviour has been bugging lots of people. ASP.Net will restart its application for various reasons. All these reasons are legitimate, but the side effect is that the application will reset all its cache, application state, and InProc session states.

You can detect the restart of application and worker process by using the performance monitor. The counters to monitor are "ASP.NET\Application Restarts" and "ASP.NET\Worker Process Restarts".

For worker process restart, ASP.NET will write an entry in the eventlog to explain why (logLevel in <processModel> controls how often eventlog is written).

For application restart, however, in v1 and v1.1 there is no easy way for a customer to figure out why a restart happened.

So by listing all reasons for app restart, I hope I can help some customers to understand and avoid the restart.

For v1
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Things that causes application restart:

- The max compilation limit is reached (look for numRecompilesBeforeApprestart in machine.config)

- Physical application path of the web application has changed.

- Change in global.asax or one of the included files

- Change in machine.config

- Change in web.config in the app root

- Change inside bin directory or its subdirs

- A directory is renamed to bin, or bin is created

- Overwhelming change notifications – too many files are changed too fast in one of content directories – could happen if, for example, files are generated on the fly per request

- Change in the code-access security policy file

- The settings of various attributes in <processModel> in machine.config affect the restart/shutdown of the worker process itself. However, if you use Windows 2003, and (by default) you are NOT using IIS5 isolation mode, <processModel> is ignored. Instead, the settings in "Application Pools" in IIS manager is used.

For v1.1
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The list for v1.1 is the same as v1, with the following new conditions:

- User code called HttpRuntime.UnloadAppDomain

- Any directory under the web application folder is renamed

- Any directory under the web application folder is deleted

IIS 6.0
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If you're using IIS 6.0, and you're not using IIS 5 compatible mode, then aspnet_wp.exe will be replaced by w3wp.exe. You may want to go to IIS Manager, go to Application Pools/DefaultAppPool, and see if any of the parameters on the Recycling and Performance tabs are causing the IIS worker process (w3wp.exe) to shutdown.