What does 'useLegacyV2RuntimeActivationPolicy' do in the .NET 4 config?

Question

While converting a project that used SlimDX, and therefore has unmanaged code, to .NET 4.0 I ran into the following error:

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.

Googling around gave me the solution, which is to add this to the applications config:

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

My question is, what is the useLegacyV2RuntimeActivationPolicy doing? I can't find any documentation about it.

Answer 1

After a bit of time (and more searching), I found this blog entry by Jomo Fisher.

One of the recent problems we’ve seen is that, because of the support for side-by-side runtimes, .NET 4.0 has changed the way that it binds to older mixed-mode assemblies. These assemblies are, for example, those that are compiled from C++\CLI. Currently available DirectX assemblies are mixed mode. If you see a message like this then you know you have run into the issue:

Mixed mode assembly is built against version 'v1.1.4322' of the runtime and cannot be loaded in the 4.0 runtime without additional configuration information.

[Snip]

The good news for applications is that you have the option of falling back to .NET 2.0 era binding for these assemblies by setting an app.config flag like so:

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

So it looks like the way the runtime loads mixed-mode assemblies has changed. I can't find any details about this change, or why it was done. But the useLegacyV2RuntimeActivationPolicy attribute reverts back to CLR 2.0 loading.