What are the different ways to combine C ++ and C #?

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Writing a DLL in C / C ++ for .Net Compatibility

I am writing a school project in C ++ where I would like to have a graphical interface written in C # with WPF.

This leads me to my question:

What are the different ways to combine unmanaged C ++ and C #, and what are the benefits of each?

It is advisable that I have standard C ++ code in a separate project that I will wrap (if necessary), so I can build C ++ with a shell interface in Linux.

I am using VS2010 Ultimate.

Note: Writing C # code does not matter. There are some details in the algorithm that make STL more suitable than equivalent classes in .NET. I also need unmanaged code, so I can test performance without GC pauses.