When you first create a .NET 4.0 project inside VS 2010 and add reference to a .NET 3.5 DLL (say log4net or Rhino.Mocks), the project tends to not build. You can get errors as shown below.
It might appear to be a CLR version issue – unable to run 2.0 DLL inside a 4.0 App Domain. And i thought that is what it was until now.
You can get over this :)
By default, VS 2010 creates the projects with Target Framework on the properties set to “.NET Framework 4 Client Profile”. You would have to change that to “.NET Framework 4” by going to Project Properties –> Application –> Target Framework. And every thing begins to compile.
So I guess, one has to be aware of this when migrating old solutions from Visual Studio 2008 to Visual Studio 2010.
Proof that it works :) Notice the .NET Framework 4.0 features as well Log4Net and Rhino.Mocks used in all the same example. (It is a stupid example, but the intention was to show it works).
If for some reason, it does not work for you, try to add
<?xml version ="1.0"?>
<supportedRuntime version="v2.0.5727" />
to your csc.exe.config/msbuild.exe.config/VCSExpress.exe.config/devenv.exe.config …
I initially thought it has something to do with not enabling Side-By-Side Execution of the compiler and stuff but it turns out that it is not the case. For your information, i have added supported runtime as .NET 2.0 but then commented it to be sure that its ONLY the Target Framework that has to be changed.