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copied!<p>We found the actual problem, with MS ASP.NET support's help. It's pretty subtle. I think MS has said they will fix the issue in a follow on to the App Fabric release (which is now RTM). Fingers crossed.</p> <p>The problem consistently occurs in this scenario:</p> <p>1) ASP.NET web application not yet running. It includes WCF Net.Pipe and / or Net.Tcp bindings. I think the same would occur for NetMsmq but did not try it.</p> <p>2) An inbound NetPipe or NetTcp WCF Windows Activation Service request is the initial request that starts the App Domain.</p> <p>3) Application uses an 'Integrated' IIS App Pool (IIS7 or IIS 7.5)</p> <p>4) The application uses HttpServerUtility.Execute during that 1st request.</p> <p>It turns out that our application was firing an ASP.NET Health Monitoring event during the very 1st WCF operation -- the very operation that caused Windows Activation Service (WAS) to start our application. Our Health Monitoring configuration includes the TemplatedMailWebEventProvider.</p> <p>Our application is using an 'Integrated' IIS App Pool. </p> <p>The <code>TemplatedMailWebEventProvider</code> is implemented to create an email message body as HTML. It uses the <code>System.Web.HttpServerUtility.Execute(string, TextWriter, Boolean)</code> overload.</p> <p>For this use case that overload does the wrong thing -- it initializes a 'Classic' IIS App Pool based HTTP pipeline. Because that's the wrong pipeline for an 'Integrated' IIS App Pool the pipeline gets corrupted with the next HTTP request -- which is actually the first inbound HTTP request.</p> <p>So you get the 500.21 error for all future HTTP requests until the application is re-cycled. You don't need to perform the relatively drastic steps of IISRESET, clearing Temporary ASP.NET cache to clear up the error -- just restart the app via saving web.config and avoid the particular startup path that causes the error.</p> <p>MS suggested a workaround for us -- use the SimpleMailWebEventProvider instead of the TemplatedMailWebEventProvider. That does work, since it takes HttpServerUtility.Execute out of the code path for the first request.</p> <p>I'd suggested that MS introduce a new web.config <code><system.web></code> boolean setting -- <code>UseIntegrated</code> -- that let's the application specify the typeof App Pool to initialize with. Evidently IIS does not forward the App Pool type to ASP.NET, so my sugggestion is a work-around to <em>that</em>. </p> <p>The TemplatedMailWebEvent provider is much more user friendly than the SimpleMailWebEventProvider, and we do hope MS addresses the issue.</p> <p>Thanks all for reading,</p> <p>Howard Hoffman</p>

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