Hello,

I've modified a couple of different applications' .config file like this:

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

When I did this for devenv.exe.config (VS 2005 - don't ask :) ), things work great - most of the Visual Studio used .NET 2.0 but I was able to make use of an assembly targeting .NET 4.0 framework.

I tried to do the same thing for a custom .exe, which happens to be based on MS CAB (slightly modified) and has a hybrid mix of WPF and WinForms content. As soon as I modified this application's app config file, I started getting this exception, sometime during application startup:

The Undo operation encountered a context that is different from what was applied in the corresponding Set operation. The possible cause is that a context was Set on the thread and not reverted(undone). System.InvalidOperationException: The Undo operation encountered a context that is different from what was applied in the corresponding Set operation. The possible cause is that a context was Set on the thread and not reverted(undone).

There's a big long stack trace that doesn't show anything in my application code directly (just a bunch of MS assemblies).

If I modify the application's .config file to this:

<startup useLegacyV2RuntimeActivationPolicy="true">
</startup>

i.e.I remove the supportedRuntime element, then the application doesn't throw this exception. But when I go to the point in my code where I try to load my .NET 4 assembly, if fails with this:

System.BadImageFormatException: Could not load file or assembly '' or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded.

I guess this is expected.

I have two questions:

1) Any idea why I'm getting the System.InvalidOperationException exception when I modify this application's configuration file to include the supportedRuntime element, adding .NET 4 support and any suggestions on what I can do about it?

2) If the answer is "no idea why/don't know what you can do about it", then is possible for my .NET 3.5 SP1 code (C#) to provide more fine grain support for conditionally adding .NET 4 runtime support for a certain assembly(ies) without converting my entire application to target .NET 4, or without using the declarative config file approach? At some point I would convert the entire application to target .NET 4, but for the short term this is daunting task and I'm hope for some short term solution/hack.

Thank you very much for any advice you can give!