We produce Instant C++, which converts from C# to C++/CLI ("C++ Edition") and VB.NET to C++/CLI ("VB Edition"). Download the demo to try it out (100 line limit on snippets and 1000 line limit on projects).
The conversion accuracy is high, but you do have to assembly the resulting header and cpp files into a new C++/CLI project since we dont convert the C# project file to C++/CLI (due to the lack of correspondence between C# and C++ project files and types).