How to differentiate between button press and button long press in c# on .Net Framework 3.5

Easy. Except for button press, there are lower-level events, button down and then button up. You can detect the time between these events. Further detail depends on UI library you want to use and application type, but the code would be very similar.
 
[EDIT]
 
In most applications, I would not recommend you to operate on timing between down and up events, as the users are not used to it, this is not a common UI style. But sometimes we want to go out of common UI styles, then your idea might work for you. But even then, don't assume the user will get it if you don't care about explaining this unusual feature somehow.
 

—SA