Anders Hejlsberg and Charles Torre talked about a future version of C#, version 6.0, at Windows Build 2013. http://channel9.msdn.com/Events/Build/2013/9-006 (34:30)
Visual Studio 2013 is the minor release version in an annual major/minor release cycle. The version of .NET included in VS 2013 is 4.5.1 which is an in-place upgrade of version 4.5. Major changes are expected for Visual Studio 2014 and .NET 5.0.
C# 6.0 will feature an all new C#-based compiler based on Roslyn, which will be released with VS 2014. Roslyn is currently available as a community technology preview, which opens up new compiler services for third parties. The C# team aims to make Roslyn as close to the performance of the native compiler as possible (within a factor of 2 during development and better on release), and, in some cases, Roslyn already outperforms the native compiler due to a more modern immutable AST architecture that supports parallel compilation. The immutable AST architecture is detailed in a recently filed patent.
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Read more: ANDERS ON C# 6.0 AT BUILD 2013