What 'additional configuration' is necessary to reference a .NET 2.0 mixed mode assembly in a .NET 4.0 project?

In order to use a CLR 2.0 mixed mode assembly, you need to modify your App.Config file to include:

<?xml version="1.0"?>
<configuration>
  <startup useLegacyV2RuntimeActivationPolicy="true">
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/>
  </startup>
</configuration>

The key is the useLegacyV2RuntimeActivationPolicy flag. This causes the CLR to use the latest version (4.0) to load your mixed mode assembly. Without this, it will not work.

Note that this only matters for mixed mode (C++/CLI) assemblies. You can load all managed CLR 2 assemblies without specifying this in app.config.

This forum post on the .NET Framework Developer Center. It might provide some insight.

(Add to the app's config file.)

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