//Random Uppercase letter:
Random rnd = new Random();            
int ascii_index = rnd.Next(65, 91); //ASCII character codes 65-90
char myRandomUpperCase = Convert.ToChar(ascii_index); //produces any char A-Z

//random lowercase letter:
Random rnd2 = new Random();            
int ascii_index2 = rnd2.Next(97, 123); //ASCII character codes 97-123
char myRandomLowerCase = Convert.ToChar(ascii_index2); //produces any char a-z

//Random letter irrespective of case: (search up "random number from 2 different ranges" for explanation)
int i = rnd.Next(1, 3) == 1 ? rnd.Next(65, 91) : rnd.Next(97, 123);
char anyLetter = Convert.ToChar(i);