Unable to run NUnit test

Question

I installed NUnit 2.6.1 and trying to run a simple test on Windows 7 x64. It causes an exception

An attempt was made to load a program with an incorrect format. You may be attempting to load an assembly built with a later version of the CLR than the version under which NUnit is currently running (2.0.50727) or trying to load a 64-bit assembly into a 32-bit process.

It is pretty weird because nunit.exe.config looks like below

  <?xml version="1.0" encoding="utf-8" ?> 
- <configuration>
- <!-- 
   The GUI only runs under .NET 2.0 or higher. The
   useLegacyV2RuntimeActivationPolicy setting only
   applies under .NET 4.0 and permits use of mixed 
   mode assemblies, which would otherwise not load 
   correctly.


  --> 
- <startup useLegacyV2RuntimeActivationPolicy="true">
- <!--  Comment out the next line to force use of .NET 4.0 
  --> 
- <!--  <supportedRuntime version="v2.0.50727" /> 
  --> 
  <supportedRuntime version="v4.0.30319" /> 
  </startup>
- <runtime>
- <!--  Ensure that test exceptions don't crash NUnit 
  --> 
  <legacyUnhandledExceptionPolicy enabled="1" /> 
- <!--  Run partial trust V2 assemblies in full trust under .NET 4.0 
  --> 
  <loadFromRemoteSources enabled="true" /> 
- <!--  Look for addins in the addins directory for now 
  --> 
- <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
  <probing privatePath="lib;addins" /> 
  </assemblyBinding>
  </runtime>
  </configuration>

Answer 1

This doesn't sound like a problem with NUnit. This sounds like your unit test assemblies are not built for a 32 bit process. Are you sure your unit test assemblies are built for 32 bit? If NUnit is running 32 bit and your assemblies are built at 64 bit (or not built as Any CPU) you'll get this problem. The calling application determines the bit depth required of the assembly. You can't use 64 bit dll's with a 32 bit process and vice versa.

The only reason I mention this is because your question says you are trying to RUN the test. If NUnit was configured incorrectly it wouldn't even start.