Immutable collections in .NET

http://blogs.msdn.com/b/dotnet/archive/2013/09/25/immutable-collections-ready-for-prime-time.aspx
http://msdn.microsoft.com/en-us/library/dn467185(v=vs.110).aspx
http://blogs.msdn.com/b/ericlippert/archive/2007/11/13/immutability-in-c-part-one-kinds-of-immutability.aspx