I have a C# application that sends serial data to the Arduino, and reads any responses.
Altering that application to send specific commands to the Arduino when specific buttons are pressed is easy.
Making the Arduino do something in response to a serial command is easy. That can include sending serial data back to the C# application.
I'm trying to control Arduino directly from a small code from C# but i can't get it to work.
Post your C# code and Arduino code. We can then help you figure out what the problem is.