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

Question

I have a project in which I'd like to use some of the .NET 4.0 features but a core requirement is that I can use the System.Data.SQLite framework which is compiled against 2.X. I see mention of this being possible such as the accepted answer here but I don't see how to actually achieve this.

When I just try and run my 4.0 project while referencing the 2.X assembly I get:

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.

What "additional configuration" is necessary?

Answer 1

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.

Answer 2

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>