The type or namespace name 'xxxx' does not exist in the namespace 'yyyy' (are you missing an assembly reference?)

Problem:

Error    1          The type or namespace name 'xxxx' does not exist in the namespace 'yyyy' (are you missing an assembly reference?)

Screenshot:

My environment: Visual Studio 2010, .Net 4.0.   Two projects involved, 1 is a dll, other is a console app.

You know your reference is correct, but you still get that reference error.  Your using clause fails, too.

Screenshot:

 

Solution:

In this case, my console app had the Target Framework in the project properties set to .NET Framework 4 Client Profile, the dll project had it set to .NET Framework 4.  I changed the console app to use .NET Framework 4, and everything built fine.

Console App Project properties before:

Console App Project properties after: