To initialize the billId value as Billid='BID0' in c#.net

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.NET2.0

C#2.0

C#

.NET

i used to increment the BillId from my table Hbill.I called the increment() to increase my billid like BID1,BID2..... The below code is working but when entering the first record,its showing error since the billid is null.How can i initialize the billId value as Billid='BID0' or any other way to set the default value while starting.

public void increment()
    {
        con.Open();           

        SqlCommand cmd = new SqlCommand("select Max(billid) from hbill ", con);
        SqlDataReader dr = cmd.ExecuteReader();

            while (dr.Read())
            {
                id = dr[0] as string;

                //department = reader[1] as string;

            }

            cur_id = IncrementID(id, 3);                       


    con.Close();
}


static string IncrementID(string startValue, int numNonDigits)
        {
            string nonDigits = startValue.Substring(0, numNonDigits);
            int len = startValue.Length - numNonDigits;
            int number = int.Parse(startValue.Substring(numNonDigits));
            number++;
            if (number >= Math.Pow(10, len)) number = 1; // start again at 1
            return String.Format("{0}{1:D" + len.ToString() + "}", nonDigits, number);
        }