I had a requirement to use a .NET dll assembly that was built against version 'v2.0.50727'to reference to it in my .Net Framework 4.0 windows forms application.

After referencing the dll, compilation error occurred in the application - "are you missing a using directive or an assembly reference."

Reason: As the DLL is a CLR 2.0 runtime assembly, it cannot be loaded in the 4.0 runtime.


Solution:

You will need to set useLegacyV2RuntimeActivationPolicy to true in app.config file. This is required to use mixed mode CLR 2 assemblies(targeting version 2.0.50727 of the CLR), in a .NET 4 application.

<startup useLegacyV2RuntimeActivationPolicy="true">
   <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/>
</startup>


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