Just came across this http://msdn.microsoft.com/en-us/library/dn385366(v=vs.110).aspx.
Note that immutable classes are not part of the core class library distributed with the .NET Framework; they are available via NuGet.
Concurrent (Thread-Safe) Collections
.NET Framework Class Library
The .NET Framework class library is a library of classes, interfaces, and value types that provide access to system functionality. It is the foundation on which .NET Framework applications, components, and controls are built. The namespaces and namespace categories in the class library are listed in the following table and documented in detail in this reference. The namespaces and categories are listed by usage, with the most frequently used namespaces appearing first.