Looking at http://msdn.microsoft.com/en-us/library/dd287191(v=vs.110).aspx, it seems that ConcurrentDictionary and all of its friends in the System.Collections.Concurrent namespace are available for use in a Portable Class Library.
However, when I create either an F# or C# Portable Class Library, even if I explicitly add a reference to mscorlib.dll, the compilation fails when using ConcurrentDictionary.
Why?
 
     
    