How to cancel with button?

Question

Hi,

I have written a c# windowsapplication to convert an ASCII file.
First I select an input file-->with a button a method() reads and converts the content of the file (with for loops) and writes the new content to a new file.

How can I cancel this method() with a button?

When the methods() runs, I can't escape it or use the exit button of the form

Greetzzz,

G