string getString(int count) 
{ 
int number; 
string checkCode = String.Empty; //存放随机码的字符串 
System.Random random = new Random(); 
for (int i = 0; i < count; i++) //产生4位校验码 
{ 
    number = random.Next(); 
    number = number % 36; 
    if (number < 10) 
        { number += 48; //数字0-9编码在48-57 } 
    else 
        { number += 55; //字母A-Z编码在65-90 } 
    checkCode += ((char)number).ToString(); 
} 
return checkCode； 
}