private string GenerateCheckCode(int num)
{
int number;
char code;
string checkCode = String.Empty;
Random random = new Random();
for (int i = 0; i < num; i++)
{
number = random.Next();
if (number % 2 == 0)
code = (char)('0' + (char)(number % 10));
else
code = (char)('A' + (char)(number % 26));
checkCode += code.ToString();
}
return checkCode;
}