Mixed mode assembly error

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Why are my tests failing due to a "Mixed Mode Assembly" error?

If a test fails with the exception: "Mixed mode assembly is built against version 'v2.0.50727' of the runtime and cannot be loaded in the 4.0 runtime without additional configuration information", then a change to the vstest.executionengine.x86.exe.config file will be needed.

1.   Close Microsoft Visual Studio 2012 (and IIS express)

2.   Launch notepad.exe as an administrator

3.   Open the file "C:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE\CommonExtensions\Microsoft\TestWindow\vstest.executionengine.x86.exe.config".

4.   Add and save the following lines in bold:

<?xml version="1.0" encoding="utf-8" ?>



<legacyUnhandledExceptionPolicy enabled="1" />

<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1" >

<probing privatePath="Extensions" />





<add name="TpTraceLevel" value="0" />





<!--<add key="ExecutionThreadApartmentState" value ="MTA"/>-->

<!--<add key="TraceLogMaxFileSizeInKb" value ="10240"/>-->

<!-- MsTest Adapter Specific AppSettings -->

<add key="TestProjectRetargetTo35Allowed" value="true" />



<startup useLegacyV2RuntimeActivationPolicy="true" >

<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0" />