I'm planning to research & learn from brute force methods, but all the popular ones I found like John the Ripper or hashcat are written in C & OpenGL which I don't have much knowledge about so far. Can anybody recommend a decent brute force program written in C#, if it includes source code that would be nice.
I don't know any particular brute force in C#, but having worked in both C and C# (and inferring that you know C# from your question) I think with a good reference you should be able to jump into C. You will of course need to be very careful with garbage collection (malloc, etc.) and it will require learning procedural vs. object oriented coding but it's doable.
Here's a good thread: https://stackoverflow.com/questions/4002442/learning-c-from-a-background-in-c-sharp