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WCF service not available

Last week, some of my integration tests suddenly start failing on our build environment. I noticed that these tests were using a WCF service, so I logged on to the web server and checked the event viewer. There I found the following error message:

It is not possible to run two different versions of ASP.NET in the same IIS process. Please use the IIS Administration Tool to reconfigure your server to run the application in a separate process.

What happened? Someone installed a newer version of this WCF service which was using .NET Framework 4.0 instead of 3.5. But he forgot to update the application pool to run under ASP.NET 4.0 with the error above as a result.

Solving this was easy, I created a new application pool running under ASP.NET 4.0 and configured the web service to run under this process.

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