Question regarding Target Framework Selection under VS 2010

After running into some issues with one of my projects not linking with a dll from a 3rd party company that we are using, one of their senior developers told me that my process was *likely* not using the 4.0 version of the framework even though I had selected that as the target framework in my project's properties. He explained that although I had selected 4.0 as the target framework for the build, this was interpreted only as a "Suggestion", not a "Demand" and that I cannot be assured that my application would necessarily use the framework I select under the project's properties.

I verified in Process Explorer that it was indeed running under 4.0.

Can anyone verify the broader point, though, of whether or not selecting a particular target framework is a "demand" or a "suggestion"?

I think this guy was wrong but I cannot find any info to corroborate or contradict this statement.
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I think he is wrong. cause when you change the framework to 4.0 you get more features in Visual Studio (for example support in Parallel).

You should also see that the references to the Microsoft DLLs are changed  to the 4.0 version.

Furthermore, if you have a .Net 3.5 project that reference a class library and you change the framework of the class library, the code will most likely not complied.
Furthermore, if you take a .Net 4.0 compiled program and try to run it on a computer that only has .Net 3.5 then it will crash.
I agree with @saragani.  Maybe the issue is with the CPU type of your project (32 vs 64 bits) and the CPU type of the 3rd party DLL.
I agree with the other experts. If you have choosen .NET 4.0 then you will be coding in that specific version.

If the dll is not working or you get any compile errors please post them here.
_Wade_Author Commented:
My thoughts were the same as Saragani's. It absolutely makes no sense that this could be a suggestiong. If you build for Framework 1, your linq assemblies become invalid.

I was just hoping to find an article reference or something confirming from microsoft. But, it's not necessary. The guy saying was sticking with his theory of what was going wrong and I think he just improvised his answer.

Actually, the CPU build property isn't the issue either. But thanks everyone for the response.
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