As much as I'm all for advocating educating people about mental illness and mental health issues in order to reduce stigma - asking someone point blank about their diagnosis is not only a bit invasive (some people are willing to talk about their diagnosis, but normally on their own terms!), but also, more often than not, they may not actually know the specifics of their diagnosis.
But, more to the point, when dealing with an individual, the specifics of their diagnosis is not of any importance, unless they are being placed under your care (in which case one wouldn't be required to specifically ask that person about their diagnosis). If you want to get to know someone, you get to know them - not their mental illness.
A person's personality; morals; intelligence; appearance; class; wealth; social status; friends; hobbies; actions; and so on, are not defined by their mental illness.
In other words; a person cannot be defined by their mental illness.
Sometimes when I tell people I have a mental illness they express surprise; some even go as far to say they do not believe me. I have questioned these people who say they don't believe me as to why they think that; the response, "You seem too normal", never fails to surprise me!
Unfortunately, stigma towards mental health issues means people hold views that anyone with a mental illness should stick out from a mile away and are distinguishable from "normal folk" as they are busy "acting crazy".
An ignorant view which is supported by most of the media: as soon as anyone does anything out of the ordinary they're labelled "crazy" or "mad"; even in the cases of murderers, etc. words like "psycho" or "nutter" are used - and then these same adjectives are attributed to those with mental illnesses!
Yes some really horrible people are mentally ill. But so are some of the most honest, decent, beautiful (not in the aesthetic sense) individuals you could ever hope to meet!
Having a mental illness does not define a person. Mental illness does not pertain to whether someone is a good person or not. Knowing a person's diagnosis is not largely useful when getting to know an individual; however, I do believe that educating yourself on mental health is a highly commendable idea. But a person will always be a whole lot more than their mental illness!