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Loading a 3.5 compiled C++/CLI application in .NET 4.0

Answer 1

A:

You should be able to run 3.5 and 4.0 in the same process. However forcing the app to use 4.0 instead of 3.5 doesn't look possible.

What are you trying to accomplish?

Ben Voigt 2010-03-10 22:35:56

CLR 4 does allow CLR 2 (.NET 3.5) and CLR 4 (.NET 4) applications to be run in the same process. Mixed mode assemblies require special care, however.

Reed Copsey 2010-03-10 22:42:33

I'd just love to be able to try our application on .NET 4.0 without having to modify a lot of .vcproj and .csproj files!In an ideal world, I could see us enabling .NET 4.0 in our app post-release by simply updating the .config file - and assuming nothing went wrong. But at this point I'm not even able to load the app at all, even if I 'opt-in'. Just wondering if anyone else had any luck.

cunningdave 2010-03-10 22:44:48

@Reed: Having a .NET 3.5 application, you can now load plugins that use 4. Or you can load plugins requiring both versions into the same native app. But that's definitely not the same as forcing any code to run under a different version of the CLR than it was written for, in fact it's the exact opposite.

Ben Voigt 2010-03-11 00:39:52

@Ben: Read up on useLegacyV2RuntimeActivationPolicy. This causes it to use the non-standard in clr4 policy, explicitly using the CLR 4 to load CLR2 assemblies (including mixed mode), which is required to use mixed mode assemblies in CLR 4.

Reed Copsey 2010-03-11 01:00:50

I'm glad you were able to help the poster. It just seemed to me that the entire point of supporting side-by-side was to avoid loading an assembly using a newer CLR version. And as a result I wouldn't expect great effort making CLR4 completely compatible with CLR2. So as long as it works, great, but if you run into weird failures, try using the CLR version the assembly was designed for, because that won't block use of the new CLR in addition.

Ben Voigt 2010-03-11 07:05:09

Answer 2

+2 A:

You need to set useLegacyV2RuntimeActivationPolicy if you want to load a CLR 2 (.NET 3.5) mixed mode assembly in a CLR 4 process:

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

Without this, you'll get a nasty exception.

Reed Copsey 2010-03-10 22:42:08

Awesome, this was my problem and this works for me. Thanks!

cunningdave 2010-03-10 23:12:18

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Loading a 3.5 compiled C++/CLI application in .NET 4.0

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