The type or namespace name 'xxx' could not be found (are you missing a using directive or an assembly reference?)

I had a Asp.Net 4.0 Web Forms web application.  It has been working fine.  I created a class Library project.  I created the assembly.

I added the assembly to the bin directory of the web forms project.  I added a reference in the web forms project to the assembly, pointing at the file in the bin directory.  I am doing this is VS 2012.

I added a couple of "using"s for this assembly.  Now the "using"s are throwing the error "The type or namespace name 'xxx' could not be found (are you missing a using directive or an assembly reference?)"  when I try to compile.  This is a process I've done a thousand times without this issue.  What could be going on now?
Edward JoellAsked:
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ShareD_PointCommented:
Hi,

From the project properties, check whether it is targeting .NET Framework Client Profile.
If yes, you may need to change it to appropriate .Net framework

Hope this may helps.
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Rikin ShahMicrosoft Dynamics CRM ConsultantCommented:
Hi,

Check with above solution. And also try deleting the assembly references from the project and re-add them and then try building the project.
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Edward JoellAuthor Commented:
I checked the properties of the class library project.  Turns out that it was compiled as a .Net 4.5 project whereas the Web App was compiled as a .Net 4.0 project.  So I changed the class library to 4.0 and re-compiled it.  Then copied the new assembly to the the bin directory of the Web app.  I then opened the web app and it compiled quite nicely.

Thank You ShareD_Point for the quick and accurate assessment.
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Edward JoellAuthor Commented:
I am glad to finally be able to award some points.
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