Accessing C++ code(unmanaged & if possible, managed as well) in C#

I want to write a managed C++ class that I can then access in C# code.  The C++ class will contain unmanaged code as well.  The C++ class has to keep things in memory as well.  It's a class that deals with image manipulation, so I gotta keep a USHORT* in memory (it has to be this type or some other type that easily converts into USHORT* because we use 3rd party code for image enhancement and it requires USHORT*). So, it has to be something that can be long-lived.  When I use a function that causes the buffer to change and then use another function to access that buffer later on, i want to access the newly changed one, not a new instance.  I've tried creating an MFC DLL and just started trying to create a C++ Class library instead.  I can't get anything to work.  Can anybody give me any pointers on how to deal with this? I"ve never done cross-platform stuff before AND its also my first time working in C#.  Any help would be appreciated.  Any websites or examples you could point me towards would also be appreciated.