字母添随机数字编号

private string getAplusRandom()
{
    string strFive = CreateRandom();
    DataSet ds = rule.GetOne(strFive);
    if (ds.Tables[0].Rows.Count == 0)
    {

    }
    else
    {
        getAplusRandom();

    }
    return strFive;
}

private static char[] ConstantNumber = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9' };

protected string CreateRandom()
{
    Random rd = new Random();

    System.Text.StringBuilder strFiveChars = new System.Text.StringBuilder(6);
    strFiveChars.Append("A");

    for (int i = 0; i < 5; i++)
    {
        strFiveChars.Append(ConstantNumber[rd.Next(10)]);
    }
    strFive = strFiveChars.ToString();


    return strFive;
}