The first thing I'd do would be to break up the MFC application into a "user interface" component and separate "business logic" components, which should be separate COM components.
You can then start by writing a new C# UI to call the old MFC business logic.
When this works, I'd think about what all the "business logic" functions do and write a unit test for them in C++/MFC to make sure I understood this.
I'd then write the same unit test in C#, using something like NUnit , port the code across, and debug it so the unit test passes.
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