My first name is Mitchell. That's my legal name, and what I wish to be called. Not Mitch. I've never used "Mitch" in my life. As a result, I often find myself correcting individuals who shorten my name.
I'm not alone in having to deal with assumptions about my first name. For example, take someone given the name "Robert" at birth. That person may wish to be called "Robert." Or he may prefer to be called "Bob" or "Bobby" or "Robbie." And that doesn't even take into consideration different spellings or pronunciations.
The bottom line is that you can never assume someone's given first name is what they prefer to use. More often than not, it isn't. So you'd best never assume. Ask them what they prefer to be called. I've found that such an effort is always appreciated.