The technology used to do this is called P/Invoke; you can search for articles on the subject. Note that it is for calling C from C#, not C++ so much. So you'll need to wrap your C++ code in a C wrapper that your DLL exports.
P/Invoke is a nice technology, and it works fairly well, except for issues in loading the target DLL file. We've found that the best way to do things is to create a static library of native functions and link that into a Managed C++ (or C++/CLI) project that depends upon it.
One easy way to call into C++ is to create a wrapper assembly in C++/CLI. In C++/CLI you can call into unmanaged code as if you were writing native code, but you can call into C++/CLI code from C# as if it were written in C#. The language was basically designed with interop into existing libraries as its "killer app".