c# 2008 ช่วยอธิบายโค้ดแต่ละบรรทัดให้หน่อยนะคับ รบกวนด้วยคับ

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.OleDb;

namespace musical
{
    public partial class brand : Form
    {
        OleDbConnection Conn;   //connet กับ database
        OleDbCommand com;       //ประมวลผลคำสั่ง sql
        StringBuilder sb;
        OleDbTransaction tr;    //การเปลี่ยนแปลงข้อมูล
        OleDbDataReader dr;     //เก็บผลการคิวรี่ข้อมูลจาก command
        public static string strConn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + Application.StartupPath + "\\dbmusical.accdb";

        public int CountRecord;
        public string TempID;
        public string TrimID;
        public int TempNum;

        AutoCompleteStringCollection acBrandID = new AutoCompleteStringCollection();
        AutoCompleteStringCollection acBrandName = new AutoCompleteStringCollection();

        public static string brandd, branddd;
        public brand()
        {
            InitializeComponent();
        }
        public void Connect()
        {
            try
            {
                Conn = new OleDbConnection();
                if (Conn.State == ConnectionState.Open)
                {
                    Conn.Close();
                }
                Conn.ConnectionString = strConn;
                Conn.Open();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
        private void SetAutoCompleteMode()
        {
            txtBrandID.AutoCompleteSource = AutoCompleteSource.CustomSource;
            txtBrandID.AutoCompleteCustomSource = acBrandID;
            txtBrandID.AutoCompleteMode = AutoCompleteMode.SuggestAppend;

            txtBrandName.AutoCompleteSource = AutoCompleteSource.CustomSource;
            txtBrandName.AutoCompleteCustomSource = acBrandName;
            txtBrandName.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
        }
        public void ShowDatabrand()
        {
            Connect();
            string brandSQL = "SELECT * FROM brand";
            com = new OleDbCommand();
            com.CommandType = CommandType.Text;
            com.CommandText = brandSQL;
            com.Connection = Conn;
            dr = com.ExecuteReader();
            if (dr.HasRows)
            {
                DataTable dt = new DataTable();
                dt.Load(dr);
                //โหลด ฟิว ชื่อรหัสยี่ห้อและชื่อยี่ห้อ มาไว้ที่ text
                foreach (DataRow drw in dt.Rows)
                {
                    acBrandID.Add(drw["brandID"].ToString());
                }
                foreach (DataRow drw in dt.Rows)
                {
                    acBrandName.Add(drw["brandName"].ToString());
                }
            }
            else
            {
                MessageBox.Show("ไม่พบข้อมูล");
                return;
            }
            dr.Close();
            this.Cursor = Cursors.Default;
            SetAutoCompleteMode();
        }
        public void ShowCus()
        {
            string brandSQL = "SELECT * FROM brand WHERE " + brandd + "= @brandd";
            com = new OleDbCommand();
            com.CommandType = CommandType.Text;
            com.CommandText = brandSQL;
            com.Connection = Conn;
            com.Parameters.Clear();
            com.Parameters.Add("@brandd", OleDbType.VarChar).Value = branddd.ToString();
            dr = com.ExecuteReader();
            if (dr.HasRows)
            {
                dr.Read();
                txtBrandID.Text = dr["brandID"].ToString();
                txtBrandName.Text = dr["brandName"].ToString();
                ShowDatabrand();
            }
            else
            {
                MessageBox.Show("ไม่พบข้อมูล");
                ClearData();
                return;
            }
            dr.Close();
        }
        private void showdata()
        {
           // ฟังก์ชัน แสดงข้อมูล
            Connect();

            string brandSQL;
            brandSQL = "Select * from brand";

            com = new OleDbCommand();
            com.CommandType = CommandType.Text;
            com.CommandText = brandSQL;
            com.Connection = Conn;
            dr = com.ExecuteReader();
            if (dr.HasRows)
            {
                DataTable dt = new DataTable();
                dt.Load(dr);
                dgvbrand.DataSource = dt;
            }
            else
            {
                MessageBox.Show("ไม่พบข้อมูล");
                return;
            }
            dr.Close();

           // ----กำหนดหัวกับขนาดของ datagridview--------------
            dgvbrand.Columns[0].HeaderText = "รหัสยี่ห้อสินค้า";
            dgvbrand.Columns[1].HeaderText = "ชื่อยี่ห้อสินค้า";
            dgvbrand.Columns[0].Width = 100;
            dgvbrand.Columns[1].Width = 200;

        }
        private void ClearData()
        {
            BtnSearch.Enabled = true;       //ค้นหา 
            BtnSearchb.Enabled = true;
            BtnSearchIDb.Enabled = true;
            BtnAdd.Enabled = true;       //เพิ่ม
            BtnDelete.Enabled = true;       //ลบ
            BtnUpdate.Enabled = true;       //แก้ไข
            BtnSave.Enabled = false;       //บันทึก
            BtnCancle.Enabled = false;       //ยกเลิก

            txtBrandID.Clear();
            txtBrandName.Clear();
            txtSearch.Clear();
        }


        public void CreateID()//ฟังก์ชั่นออกรหัสยี่ห้อสินค้า
        {
            Connect();

            string brandSQL = "SELECT * FROM brand ORDER BY BrandID";

            com = new OleDbCommand();
            com.CommandType = CommandType.Text;
            com.CommandText = brandSQL;
            com.Connection = Conn;
            dr = com.ExecuteReader();
            if (dr.HasRows)
            {
                DataTable dt = new DataTable();
                dt.Load(dr);

                CountRecord = dt.Rows.Count; //นับจำนวนเรคคอร์ด
                TempID = dt.Rows[CountRecord - 1]["BrandID"].ToString();
                TrimID = TempID.Substring(1);
                TempNum = Convert.ToInt32(TrimID);

            }
            dr.Close();

            TempNum = Convert.ToInt32(TempNum) + 1;
            if (TempNum <= 9)
                txtBrandID.Text = "b00" + TempNum.ToString();
            else if (TempNum <= 99)
                txtBrandID.Text = "b0" + TempNum.ToString();
            else if (TempNum <= 999)
                txtBrandID.Text = "b" + TempNum.ToString();
        }




        private void ClearText()
        {
            txtBrandID.Clear();
            txtBrandName.Clear();
        }
        public bool check = true;

        private void CheckText()//ตัวแปรเช็ค Text
        {
            check = true;
            if (txtBrandName.Text == "")
            {
                MessageBox.Show("คุณยังไม่ได้กรอกชื่อยี่ห้อสินค้า");
                txtBrandName.Focus();
                check = false;
                return;
            }
        }



        private void txtSearch_KeyDown(object sender, KeyEventArgs e)
        {


        }
        private void txtbrandName_TextChanged(object sender, EventArgs e)
        {

        }
        //datagrid
        private void dgvbrand_CellContentClick_1(object sender, DataGridViewCellEventArgs e)
        {
            txtBrandID.ReadOnly = true;
            if (e.RowIndex == -1)
            {
                return;
            }
            txtBrandID.Text = dgvbrand.Rows[e.RowIndex].Cells["BrandID"].Value.ToString();
            txtBrandName.Text = dgvbrand.Rows[e.RowIndex].Cells["BrandName"].Value.ToString();
            txtBrandName.Focus();
            txtBrandName.SelectAll();
        }

        private void listView1_SelectedIndexChanged_1(object sender, EventArgs e)
        {
            txtBrandID.Text = listView1.FocusedItem.SubItems[1].Text;
            txtBrandName.Text = listView1.FocusedItem.SubItems[2].Text;
        }
        //คลิกปุ่มเพิ่ม 
        private void BtnAdd_Click(object sender, EventArgs e)
        {
            ClearData();
            CreateID();
            BtnSearch.Enabled = false;
            BtnSearchb.Enabled = false;
            BtnSearchIDb.Enabled = false;
            BtnAdd.Enabled = false;
            BtnDelete.Enabled = false;
            BtnUpdate.Enabled = false;
            BtnSave.Enabled = true;
            BtnCancle.Enabled = true;
            txtBrandID.ReadOnly = false;
            txtBrandName.Focus();
        }
        //ลบ
        private void BtnDelete_Click(object sender, EventArgs e)
        {
            txtBrandID.ReadOnly = false;
            if (txtBrandID.Text == "")
            {
                MessageBox.Show("คุณยังไม่ได้เลือกรายการที่ต้องการลบ");
                txtBrandID.Focus();
                return;
            }
            Connect();
            string brandSQL = "SELECT * FROM  product WHERE BrandID=@BrandID";
            com = new OleDbCommand();
            com.CommandType = CommandType.Text;
            com.CommandText = brandSQL;
            com.Connection = Conn;
            com.Parameters.Clear();
            com.Parameters.Add("@BrandID", OleDbType.VarChar).Value = txtBrandID.Text;
            dr = com.ExecuteReader();
            if (dr.HasRows)
            {
                MessageBox.Show("ไม่สามารถลบข้อมูลนี้ได้", "รายงานการลบ");
                ClearData();
                dr.Close();
            }
            else
            {
                tr = Conn.BeginTransaction();
                sb = new StringBuilder();
                sb.Append("DELETE FROM brand WHERE BrandID=@BrandID");
                string sqlDel = sb.ToString();

                com = new OleDbCommand();
                com.CommandType = CommandType.Text;
                com.CommandText = sqlDel;
                com.Connection = Conn;
                com.Transaction = tr;
                com.Parameters.Clear();
                com.Parameters.Add("@BrandID", OleDbType.VarChar).Value = txtBrandID.Text;
                int result = com.ExecuteNonQuery();
                if (result == 0)
                {
                    tr.Rollback();
                    MessageBox.Show("ไม่พบรายการที่ต้องการลบ", "รายงานการลบ");
                    txtBrandID.Focus();
                    return;
                }
                else
                {
                    tr.Commit();
                    MessageBox.Show("ลบข้อมูลเรียบร้อยแล้ว", "รายงานการลบ");
                }
                txtBrandID.ReadOnly = false;
                showdata();
                ClearData();
            }
        }
        //แก้ไข
        private void BtnUpdate_Click(object sender, EventArgs e)
        {
            txtBrandID.ReadOnly = false;
            CheckText();
            if (check != false)
            {
                tr = Conn.BeginTransaction();
                try
                {
                    com = new OleDbCommand();
                    sb = new StringBuilder();
                    sb.Append("UPDATE brand SET BrandID=@BrandID,BrandName=@BrandName WHERE BrandID=@BrandID");
                    string sqladd = "";
                    sqladd = sb.ToString();
                    com.CommandType = CommandType.Text;
                    com.CommandText = sqladd;
                    com.Connection = Conn;
                    com.Transaction = tr;
                    com.Parameters.Clear();
                    com.Parameters.Add("@BrandID", OleDbType.VarChar).Value = txtBrandID.Text;
                    com.Parameters.Add("@BrandName", OleDbType.VarChar).Value = txtBrandName.Text;

                    com.ExecuteNonQuery();
                    int result = com.ExecuteNonQuery();
                    if (result == 0)
                    {
                        tr.Rollback();
                        MessageBox.Show("รหัสที่คุณกรอกมาไม่ถูกต้อง");
                        txtBrandID.Focus();
                        return;
                    }
                    else
                    {
                        if (MessageBox.Show("คุณต้องการแก้ไขข้อมูลใช่หรือไม่", "คำยืนยัน", MessageBoxButtons.YesNo, MessageBoxIcon.Question) == DialogResult.No)
                        {
                            tr.Rollback();
                            return;
                        }
                        tr.Commit();
                        MessageBox.Show("แก้ไขข้อมูลเรียบร้อยแล้ว");
                    }
                }
                catch (Exception ex)
                {
                    tr.Rollback();
                    MessageBox.Show("รายงานผิดพลาด" + ex.Message, "ผิดพลาด", MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
                showdata();
                ClearData();
            }
        }
        //บันทึก
        private void BtnSave_Click_1(object sender, EventArgs e)
        {
            txtBrandID.ReadOnly = false;
            txtBrandID.ReadOnly = false;
            CheckText();
            if (check != false)
            {
                string brandSQL = "SELECT * FROM brand WHERE BrandID = @BrandID";

                com = new OleDbCommand();
                com.CommandType = CommandType.Text;
                com.CommandText = brandSQL;
                com.Connection = Conn;
                com.Parameters.Clear();
                com.Parameters.Add("@BrandID", OleDbType.VarChar).Value = txtBrandID.Text;
                dr = com.ExecuteReader();
                if (dr.HasRows)
                {
                    MessageBox.Show("รหัสนี้มีอยู่แล้ว");
                    txtBrandID.Focus();
                    return;
                }
                dr.Close();

                brandSQL = "SELECT * FROM brand WHERE BrandName = @BrandName";
                com = new OleDbCommand();
                com.CommandType = CommandType.Text;
                com.CommandText = brandSQL;
                com.Connection = Conn;
                com.Parameters.Clear();
                com.Parameters.Add("@BrandName", OleDbType.VarChar).Value = txtBrandName.Text;
                dr = com.ExecuteReader();
                if (dr.HasRows)
                {
                    MessageBox.Show("ชื่อประเภทสินค้านี้มีแล้ว");
                    txtBrandName.Focus();
                    return;
                }
                dr.Close();

                tr = Conn.BeginTransaction();
                try
                {
                    com = new OleDbCommand();
                    sb = new StringBuilder();
                    sb.Append("INSERT INTO brand(BrandID,BrandName) VALUES (@BrandID,@BrandName)");
                    string sqladd = "";
                    sqladd = sb.ToString();
                    com.CommandType = CommandType.Text;
                    com.CommandText = sqladd;
                    com.Connection = Conn;
                    com.Transaction = tr;
                    com.Parameters.Clear();
                    com.Parameters.Add("@BrandID", OleDbType.VarChar).Value = txtBrandID.Text;
                    com.Parameters.Add("@BrandName", OleDbType.VarChar).Value = txtBrandName.Text;
                    com.ExecuteNonQuery();
                    tr.Commit();
                    MessageBox.Show("บันทึกข้อมูลเรียบร้อยแล้ว", "รายงานการบันทึก", MessageBoxButtons.OK);
                }
                catch (Exception ex)
                {
                    tr.Rollback();
                    MessageBox.Show("รายงานการผิดพลาด" + ex.Message, "ผิดพลาด", MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
                showdata();
                ClearData();
            }
        }
        //ยกเลิก
        private void BtnCancle_Click(object sender, EventArgs e)
        {
            txtBrandID.ReadOnly = false;
            listView1.Items.Clear();
            ClearData();
            ClearText();
        }

        private void brand_Load_1(object sender, EventArgs e)
        {
            Connect();
            showdata();
            ClearData();
            ShowDatabrand();

            listView1.Clear();
            listView1.Columns.Add("ลำดับ");
            listView1.Columns.Add("รหัสยี่ห้อสินค้า", 100, HorizontalAlignment.Center);
            listView1.Columns.Add("ชื่อยี่ห้อสินค้า", 90, HorizontalAlignment.Left);

            listView1.View = View.Details;
            listView1.GridLines = true;
            listView1.FullRowSelect = true;
        }
        //ค้นหา
        private void BtnSearch_Click(object sender, EventArgs e)
        {
            txtBrandID.ReadOnly = true;
            BtnAdd.Enabled = false;
            BtnDelete.Enabled = true;
            BtnUpdate.Enabled = true;
            BtnSave.Enabled = false;
            BtnCancle.Enabled = true;
            if (txtSearch.Text == "")
            {
                MessageBox.Show("กรุณากรอกข้อมูลที่ต้องการค้นหา");
                txtSearch.Focus();
                showdata();
            }
            if (CmbSearch.Text == "รหัสยี่ห้อสินค้า")
            {
                Connect();
                string brandSQL = "SELECT * FROM brand WHERE BrandID=@BrandID";
                com = new OleDbCommand();
                com.CommandType = CommandType.Text;
                com.CommandText = brandSQL;
                com.Connection = Conn;
                com.Parameters.Clear();
                com.Parameters.Add("@BrandID", OleDbType.VarChar).Value = txtSearch.Text;
                dr = com.ExecuteReader();
                if (dr.HasRows)
                {
                    dr.Read();
                    txtBrandID.Text = dr["BrandID"].ToString();
                    txtBrandName.Text = dr["BrandName"].ToString();
                }
                else
                {
                    MessageBox.Show("ไม่พบข้อมูลที่ต้องการค้นหา");
                    showdata();
                    return;
                }
                dr.Close();
            }
            if (CmbSearch.Text == "ชื่อยี่ห้อสินค้า")
            {
                Connect();
                string brandSQL = "SELECT * FROM brand WHERE BrandName=@BrandName";
                com = new OleDbCommand();
                com.CommandType = CommandType.Text;
                com.CommandText = brandSQL;
                com.Connection = Conn;
                com.Parameters.Clear();
                com.Parameters.Add("@BrandName", OleDbType.VarChar).Value = txtSearch.Text;
                dr = com.ExecuteReader();
                if (dr.HasRows)
                {
                    dr.Read();
                    txtBrandID.Text = dr["BrandID"].ToString();
                    txtBrandName.Text = dr["BrandName"].ToString();
                }
                else
                {
                    MessageBox.Show("ไม่พบข้อมูลที่ต้องการค้นหา");
                    showdata();
                    return;
                }
                dr.Close();
            }
        }

        private void BtnSearchIDb_Click(object sender, EventArgs e)
        {
            txtBrandID.ReadOnly = false;
            listView1.Visible = true;
            if (txtBrandID.Text == "")
            {
                MessageBox.Show("กรุณากรอกข้อมูลที่ต้องการค้นหา");
                txtBrandID.Focus();
                showdata();
                return;
            }
            string BrandID, BrandName;
            string customer1SQL = "SELECT * FROM brand WHERE BrandID= @BrandID";
            com = new OleDbCommand();
            com.CommandType = CommandType.Text;
            com.CommandText = customer1SQL;
            com.Connection = Conn;
            com.Parameters.Clear();
            com.Parameters.Add("@BrandID", OleDbType.VarChar).Value = txtBrandID.Text;
            dr = com.ExecuteReader();
            if (dr.HasRows)
            {
                DataTable dt = new DataTable();
                dt.Load(dr);

                int CountID = dt.Rows.Count;
                int Num = 1;
                for (int i = 0; i <= CountID - 1; i++)
                {
                    BrandID = dt.Rows[i]["BrandID"].ToString();
                    BrandName = dt.Rows[i]["BrandName"].ToString();


                    ListViewItem lvi;
                    string[] anyData;
                    anyData = new string[] { Num.ToString(), BrandID, BrandName };

                    lvi = new ListViewItem(anyData);
                    listView1.Items.Add(lvi);

                    Num++;
                }

            }
            else
            {
                MessageBox.Show("ไม่พบข้อมูลที่ต้องการค้นหา");
                txtBrandID.Focus();
                return;
            }
            dr.Close();
        }

        private void BtnSearchb_Click(object sender, EventArgs e)
        {
            txtBrandID.ReadOnly = false;
            listView1.Visible = true;
            if (txtBrandName.Text == "")
            {
                MessageBox.Show("กรุณากรอกข้อมูลที่ต้องการค้นหา");
                txtBrandName.Focus();
                showdata();
                return;
            }
            string BrandID, BrandName;
            string customer1SQL = "SELECT * FROM brand WHERE BrandName= @BrandName";
            com = new OleDbCommand();
            com.CommandType = CommandType.Text;
            com.CommandText = customer1SQL;
            com.Connection = Conn;
            com.Parameters.Clear();
            com.Parameters.Add("@BrandName", OleDbType.VarChar).Value = txtBrandName.Text;
            dr = com.ExecuteReader();
            if (dr.HasRows)
            {
                DataTable dt = new DataTable();
                dt.Load(dr);

                int CountID = dt.Rows.Count;
                int Num = 1;
                for (int i = 0; i <= CountID - 1; i++)
                {
                    BrandID = dt.Rows[i]["BrandID"].ToString();
                    BrandName = dt.Rows[i]["BrandName"].ToString();


                    ListViewItem lvi;
                    string[] anyData;
                    anyData = new string[] { Num.ToString(), BrandID, BrandName };

                    lvi = new ListViewItem(anyData);
                    listView1.Items.Add(lvi);

                    Num++;
                }

            }
            else
            {
                MessageBox.Show("ไม่พบข้อมูลที่ต้องการค้นหา");
                txtBrandName.Focus();
                return;
            }
            dr.Close();
        }

        private void CmbSearch_SelectedIndexChanged(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            type page2 = new type();
            page2.Show();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            supplier sup = new supplier();
            sup.Show();
        }

        private void button3_Click(object sender, EventArgs e)
        {
            customer cus = new customer();
            cus.Show();
        }
    }
}