FizzBuzz in accordance with standards
This is how you end up with projects with forms with 25,000 lines of code.  It's also how you end up with duplicated code.

Your (poorly named) Form1 class should only be responsible for stuff directly related to the form it's displaying.  Otherwise, complicated forms end up with even more complicated code.

And what happens if you have another form that wants to do some of the same FizzBuzz things?  Well, now you have to copy &amp; paste all of your code.  As soon as you start to copy &amp; paste anything while programming, that should be a massive red flag that you're absolutely doing something wrong.  You need to take a step back and figure out how to do it without copy &amp; pasting.

In this case, we need a separate class to hold all of our FizzBuzzing logic.  Now, any form or anything else, that wants to use our FizzBuzz code instantiates our FizzBuzz class and uses its method (although perhaps, we've implemented as a static class and we just call its methods without instantiation).