title: WPF/C# can't find valid assembly reference
body: <p>I've downloaded an open source third-party assembly for use in my WPF/.NET 4/VS2010 project.  I've added a reference to the project's DLL and verified that VS2010 can see it by walking through the exposed classes in the object browser.  When I add a "using" directive to the appropriate class in my project, Intellisense offers the new assembly as a suggestion, so VS2010 is aware of it at some level.  Intellisense also generates tips for the exposed classes that I'm trying to use.
     Unfortunately, my project fails to compile because of a missing assembly reference for this assembly.  The classes are highlighted as valid, Intellisense lets me navigate to the methods, etc, but the compiler won't acknowledge them.
     In case it was a C# version mismatch, I recompiled the open source project, targeting .NET 4.  This didn't change a thing.</p>
  
  <p>What am I missing?  This seems like simple developer error, but I'm not sure how to proceed.</p>