Q:

Library references keeps disappearing

I have a WCF library project that I reference from a console app.  The problem is every time I add the reference and it registers, it disappears in all of the following cases:

If I rebuild either project
If I try to run the console app

A:

Wrong target framework? 
Go to your application's properties, click on the Compile tab and set the "Target framework (all configurations):" dropdown from ".NET Framework 4 Client Profile" to ".NET Framework 4"
See also:
Visual Studio 2010 is Eating My DLLs! -  referenced third party dlls missing after build
and:
http://msdn.microsoft.com/en-us/library/cc656912.aspx

If you are targeting the .NET
  Framework 4 Client Profile, you cannot
  reference an assembly that is not in
  the .NET Framework 4 Client Profile. 
  Instead you must target the .NET
  Framework 4. 
  [...]
Features Not Included in the .NET Framework Client Profile
The .NET
  Framework 4 Client Profile does not
  include the following features.  You
  must install the .NET Framework 4 to
  use these features in your
  application:
ASP.NET
Advanced Windows Communication Foundation (WCF) functionality
.NET Framework Data Provider for
  Oracle
MSBuild for compiling

