private int rep=0;

///
/// 生成随机数字字符串
///
/// 待生成的位数
/// 生成的数字字符串
private string GenerateCheckCodeNum(int codeCount)
{
string str = string.Empty;
long num2 = DateTime.Now.Ticks + this.rep;
this.rep++;
Random random = new Random(((int)(((ulong)num2) & 0xffffffffL)) | ((int)(num2 >> this.rep)));
for (int i = 0; i < codeCount; i++)
{
int num = random.Next();
str = str + ((char)(0x30 + ((ushort)(num % 10)))).ToString();
}
return str;
}

方法二：随机生成字符串（数字和字母混和）

///
/// 生成随机字母字符串(数字字母混和)
///
/// 待生成的位数
/// 生成的字母字符串
private string GenerateCheckCode(int codeCount)
{
string str = string.Empty;
long num2 = DateTime.Now.Ticks + this.rep;
this.rep++;
Random random = new Random(((int)(((ulong)num2) & 0xffffffffL)) | ((int)(num2 >> this.rep)));
for (int i = 0; i < codeCount; i++)
{
char ch;
int num = random.Next();
if ((num % 2) == 0)
{
ch = (char)(0x30 + ((ushort)(num % 10)));
}
else
{
ch = (char)(0x41 + ((ushort)(num % 0x1a)));
}
str = str + ch.ToString();
}
return str;
}