HMMM..well what you are doing there is trying to put a string value in to a char type...
use single quotation marks...like this...
char G = 'g';
char M = 'm';
if ((textbox.text == m) || (textbox.text == g))
messagebox.show( " iD is ok ");
messagebox.show( " Please input a valid iD ");
remember if an id does not have the m or g in the 7th (or 6th) position in your char array, your going to run in to problems. What you could do is use string values....check what position the m or g is at then decide whether to let it go or throw exception etc...
Hi i should use regulair expressions as stated earlier. This will make you life in these kinds of problems much easier. I undstand that the learning curve is a little bit steeper, but the investment is worthwhile. I can tell you that from experience.
Bing around, the G word is a little bit difficult on this website ;) for regulair expressions and there are tons of very helpfull sites.