Generate alphanumerical code

dear Programmer,

string was generate code and give error for second time as input string was not in correct format-

code behind-

private string GenerateNewCID(string connection, string query)
        {
            string newCID = string.Empty;
            using (SqlConnection cnn = new SqlConnection(connection))
            {
                cnn.Open();
                SqlCommand cmd = new SqlCommand(query, cnn);
                SqlDataReader dr = cmd.ExecuteReader();
                while (dr.Read())
                {
                    string i = dr[0].ToString();
                    if (string.IsNullOrEmpty(i))
                    {
                        newCID = DateTime.Now.ToString("yyMM") + ddlstu.SelectedValue + "0001";
                    }
                    else
                    {
                        i = i.Substring(4);
                        int j = Convert.ToInt32(i);
                        j = j + 1;
                        newCID = DateTime.Now.ToString("yyMM") + ddlstu.SelectedValue + j.ToString().PadLeft(4, '0');
                    }
                }
                cnn.Close();
            }
            return string.Concat(newCID);
        }

input code behind-

protected void btnsave_Click(object sender, EventArgs e)
        {
            string connstr = ConfigurationManager.ConnectionStrings["xxxx"].ConnectionString;
            SqlConnection cnn = new SqlConnection();
            cnn.ConnectionString = System.Configuration.ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
            cnn.Open();
            string previousCIDquery = "SELECT MAX(RegdNo) RegdNo FROM StudentRegd";
            string newCID = GenerateNewCID(connstr, previousCIDquery);
            SqlCommand cmd = new SqlCommand("insert into stutablevalues('" + newCID + "','" + null + "','" + ddlstunamecat.SelectedItem + "','" + stuFN.Text + "','" + stuMN.Text + "','" + stuLN.Text + ",'" + stulandmark.Text + "','" + stucity.Text + "','"+DateTime.Now+"')", cnn);
            cmd.ExecuteNonQuery();
            cnn.Close();