ไม่ให้เลือกสินค้าที่จะขายซ้ำ ผมใส่โค้ดตัวนี้แล้วมัน...Win (Windows App), C#

strSQL = "SELECT COUNT(*) FROM customer WHERE CustomerID = '"+ this.txtCustomerID.Text +"' ";
			objCmd = new SqlCommand(strSQL, objConn);
			intNumRows = Convert.ToInt32(objCmd.ExecuteScalar());

			if(intNumRows > 0)
			{
				this.pnlAdd.Visible = false;
				this.lblStatus.Visible = true;
				this.lblStatus.Text = "CustomerID already exists.";
			}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;
using System.Data.OleDb;

namespace Carservice
{
    public class dbm
    {
        public static string SqlCom { get; set; }
        private static string conString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\\DatabaseCarservice.accdb";

        public static void ExeSql(string sql) {
            using(OleDbConnection con = new OleDbConnection(conString)){
                con.Open();
                using (OleDbCommand cmd = new OleDbCommand(sql, con)) {
                    try
                    {
                        cmd.ExecuteNonQuery();
                    }
                    catch (Exception e)
                    {
                        DevExpress.XtraEditors.XtraMessageBox.Show(e.ToString());
                    }
                }
            }
        }
        public static void ExeSql() {
            using (OleDbConnection con = new OleDbConnection(conString))
            {
                con.Open();
                using (OleDbCommand cmd = new OleDbCommand(SqlCom, con))
                {
                    try
                    {
                        cmd.ExecuteNonQuery();
                    }
                    catch (Exception e)
                    {
                        DevExpress.XtraEditors.XtraMessageBox.Show(e.ToString());
                    }
                }
            }
        }
        public static DataTable GetDataTable(string sql) {
            using (OleDbConnection con = new OleDbConnection(conString))
            {
                con.Open();
                DataTable dt = new DataTable();
                using (OleDbDataAdapter dap = new OleDbDataAdapter(sql, con))
                {
                    dap.Fill(dt);
                    return dt;
                }
            }
        }
        public static DataTable GetDataTable()
        {
            using (OleDbConnection con = new OleDbConnection(conString))
            {
                con.Open();
                DataTable dt = new DataTable();
                using (OleDbDataAdapter dap = new OleDbDataAdapter(SqlCom, con))
                {
                    dap.Fill(dt);
                    return dt;
                }
            }
        }
        public static DataSet GetDataSet(string sql) {
            using (OleDbConnection con = new OleDbConnection(conString))
            {
                con.Open();
                DataSet ds = new DataSet();
                using (OleDbDataAdapter dap = new OleDbDataAdapter(sql, con))
                {
                    dap.Fill(ds);
                    return ds;
                }
            }
        }
        public static DataSet GetDataSet()
        {
            using (OleDbConnection con = new OleDbConnection(conString))
            {
                con.Open();
                DataSet ds = new DataSet();
                using (OleDbDataAdapter dap = new OleDbDataAdapter(SqlCom, con))
                {
                    dap.Fill(ds);
                    return ds;
                }
            }
        }
        public static bool CheckExist(string sql) {
            using (OleDbConnection con = new OleDbConnection(conString))
            {
                con.Open();
                using (OleDbCommand cmd = new OleDbCommand(sql, con))
                {
                    OleDbDataReader dr = cmd.ExecuteReader();
                    dr.Read();
                    if (dr.HasRows)
                    {
                        return true;
                    }
                    else
                    {
                        return false;
                    }
                }
            }
        }
        public static bool CheckExist()
        {
            using (OleDbConnection con = new OleDbConnection(conString))
            {
                con.Open();
                using (OleDbCommand cmd = new OleDbCommand(SqlCom, con))
                {
                    OleDbDataReader dr = cmd.ExecuteReader();
                    dr.Read();
                    if (dr.HasRows)
                    {
                        return true;
                    }
                    else
                    {
                        return false;
                    }
                }
            }
        }
        public static string Get1Value(string sql) {
            using (OleDbConnection con = new OleDbConnection(conString))
            {
                con.Open();
                using (OleDbCommand cmd = new OleDbCommand(sql, con))
                {
                    OleDbDataReader dr = cmd.ExecuteReader();
                    dr.Read();
                    if (dr.HasRows)
                    {
                        return dr[0].ToString();
                    }
                    else
                    {
                        return null;
                    }
                }
            }
        }
        public static string Get1Value()
        {
            using (OleDbConnection con = new OleDbConnection(conString))
            {
                con.Open();
                using (OleDbCommand cmd = new OleDbCommand(SqlCom, con))
                {
                    OleDbDataReader dr = cmd.ExecuteReader();
                    dr.Read();
                    if (dr.HasRows)
                    {
                        return dr[0].ToString();
                    }
                    else
                    {
                        return null;
                    }
                }
            }
        }
    }
}