Occasionally I get caught out by an assembly in .NET having been compiled in, say, .NET 2.0 and now wanting to use it in a newer version of .NET, for example .NET 4.0.

You’ll probably see an error like the following

System.IO.FileLoadException: Mixed mode assembly is built against version ‘v2.0.50727’ of the runtime and cannot be loaded in the 4.0

Obviously if I could recompile the offending 2.0 assembly I would, but this occasionally happens with libraries such as those imported into an application via NuGet and I really don’t need the headache of finding the original source and rebuilding.

So to fix this simple add the following to the App.Config in the configurations section

<startup useLegacyV2RuntimeActivationPolicy="true">
   <supportedRuntime version="v4.0"/>
</startup>