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I'd be looking at the size and configuration of the &quot;application pool&quot;<br />
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You didn't mention which version of IIS you're using so I can't give you an exact link to look at. &nbsp;However, you should be able to google it with something like this:<br />
<br />
&quot;IIS 7 application pool&quot;
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I'd be looking at the size and configuration of the "application pool"

You didn't mention which version of IIS you're using so I can't give you an exact link to look at.  However, you should be able to google it with something like this:

"IIS 7 application pool"

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I've checked the app pool - it is unlimited and there's &gt;&nbsp;1GB of main memory available
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I've checked the app pool - it is unlimited and there's > 1GB of main memory available

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Now working with Micorsoft support on this
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Now working with Micorsoft support on this

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I trap the Application close event in Global.asax, and it reports that it keeps being closed with <br />
ApplicationShutdownReason.<wbr />HostingEnv<wbr />ironment (which I understand to be normal &quot;idle&quot; practice) or BinDirChangeOrDirectoryRen<wbr />ame<br />
Needless to say, I don't change the bin directory.<br />
Both of these happen whens sessions are active and so the application and session variables are reset, with the effect that active users crash.<br />
<br />
This happens randomly 10 times a day or more<br />
<br />
Do you have any idea why this is happening, or can you suggest how I might diagnose the problem?<br />
<br />
&nbsp;have looked at the machine Application and Windows event logs but I can't see anything happening at the same timeI 
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I trap the Application close event in Global.asax, and it reports that it keeps being closed with 
ApplicationShutdownReason.HostingEnvironment (which I understand to be normal "idle" practice) or BinDirChangeOrDirectoryRename
Needless to say, I don't change the bin directory.
Both of these happen whens sessions are active and so the application and session variables are reset, with the effect that active users crash.

This happens randomly 10 times a day or more

Do you have any idea why this is happening, or can you suggest how I might diagnose the problem?

 have looked at the machine Application and Windows event logs but I can't see anything happening at the same timeI 

IIS random(?) application shutdown

IIS is Internet Information Services, the web server included with Windows Server operating systems. All current versions are built on a modular architecture; modules can be added or removed individually so that those required for specific functionality are installed. The full installation of IIS includes HTTP, security, content, compression, caching, logging and diagnostics.

Microsoft IIS Web Server

The successor to Active Server Pages, ASP.NET websites utilize the .NET framework to produce dynamic, data and content-driven web applications and services. ASP.NET code can be written using any .NET supported language. As of 2009, ASP.NET can also apply the Model-View-Controller (MVC) pattern to web applications

ASP.NET

.NET application servers, a Windows Server role in an application role, includes Internet Information Services (IIS) to provide web server support, the .NET Framework to provide application support, ASP.NET to provide server side scripting, COM+ for application component communication, Message Queuing for multithreaded processing, and the Windows Communication Foundation (WCF) for application communication. There are also third party server systems that provide .NET support, including Mono, Base4, TNAPS and cc Devnet Data Access Server.