Nope. No #include functionality in C#.

One possible solution would be partial classes. You could collect the common method implementations for a class into one file. Then have the product-specific method implementations in two separate files. Specify file 'A' in product 'A's project and file 'B' in project 'B's project. Tying the files together with the partial keyword at least allows you to break out the product-specific implementations into separate files.

Your mileage may vary.

You can use #if/#endif in C# for using preprocessor directives.

You can also add files as a link, so you keep the file in a common directory and any number of projects can include the file as a link which means they'll all share the same C# file, but it will be compiled into all the projects. As long as the projects don't reference each other you should be fine (if the projects reference each other, you're likely to get an error for having the same type compiled twice).

I wouldn't recommend it, but you can use the C/C++ preprocessor to process C# files before sending them to the C# compiler. Maybe mark them with a different extension, say .cspp, and then send each through a pre-compiler step that runs them through the preprocessor which spits out a temp file that you send to the C# compiler in its place.

This will give you all the C/C++ preprocessor abilities including #define macros and #includes.

Note this applies to source files in any language, not just C#.

But again, I would not recommend this. Unless you have a very good reason, like macro tables.

C# doesn't need includes becuase shared source files can be added as Linked Files in visual studio, or if rellay needed, compile them as libraries and add the project reference.