Sudoku 4.0

This is not my first Sudoku Program. But this is my first program created by C#. Please give me some feedback. In this version, I remove so many tools to help you solve the puzzle. You do like what you did with pencil and eraser on the real Sudoku problem. Why? After a long time playing with Sudoku, I realize that, solving without tools is more fun. Btw about Sudoku, no matter how good or bad you are, the puzzle will give you a good mental workout. Have fun.