C# pocetak u programiranju(ADO)

Code:

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

namespace Ucenje_north
{
    public partial class Form1 : Form
    {
        private string cn_string = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=c:\\Baze\\Northwind.mdb";

        private OleDbConnection cn;
        private OleDbCommand select_cmd;
        private OleDbCommand update_cmd;
        private OleDbCommand delete_cmd;
        private OleDbCommand insert_cmd;
        private OleDbDataAdapter da;
        private DataSet ds;
        private DataTable dt;

        public Form1()
        {
            InitializeComponent();

            cn = new OleDbConnection(cn_string);
            
            string select_string = "SELECT * FROM Customers";
            string update_string = "UPDATE Customers SET CompanyName=?, ContactName=?, ContactTitle=?, Address=?, City=?, Region=?, PostalCode=?, Country=?, Phone=?, Fax=? WHERE CustomerID=?";
            string delete_string = "DELETE Customer WHERE CustomerID=?";
            string insert_string = "INSERT INTO Customers (CustomerID, CompanyName, ContactName, ContactTitle, Address, City, Region, PostalCode, Country, Phone, Fax) VALUES(?,?,?,?,?,?,?,?,?,?,?)";

            select_cmd = new OleDbCommand(select_string, cn);
            update_cmd = new OleDbCommand(update_string, cn);
            delete_cmd = new OleDbCommand(delete_string, cn);
            insert_cmd = new OleDbCommand(insert_string, cn);
        }

        private void ucitajParametre(OleDbCommand cmd)
        {
            cmd.Parameters.Add("@CustomerID", OleDbType.VarChar);
            cmd.Parameters.Add("@CompanyName", OleDbType.VarChar);
            cmd.Parameters.Add("@ContactName", OleDbType.VarChar);
            cmd.Parameters.Add("@ContactTitle", OleDbType.VarChar);
            cmd.Parameters.Add("@Address", OleDbType.VarChar);
            cmd.Parameters.Add("@City", OleDbType.VarChar);
            cmd.Parameters.Add("@Region", OleDbType.VarChar);
            cmd.Parameters.Add("@PostalCode", OleDbType.VarChar);
            cmd.Parameters.Add("@Country", OleDbType.VarChar);
            cmd.Parameters.Add("@Phone", OleDbType.VarChar);
            cmd.Parameters.Add("@Fax", OleDbType.VarChar);
        }

        private void podesiParametre(OleDbCommand cmd)
        {
            cmd.Parameters["@CustomerID"].Value = customerIDTextBox.Text;
            cmd.Parameters["@CompanyName"].Value = companyNameTextBox.Text;
            cmd.Parameters["@ContactName"].Value = contactNameTextBox.Text;
            cmd.Parameters["@ContactTitle"].Value = contactTitleTextBox.Text;
            cmd.Parameters["@Address"].Value = addressTextBox.Text;
            cmd.Parameters["@City"].Value = cityTextBox.Text;
            cmd.Parameters["@Region"].Value = regionTextBox.Text;
            cmd.Parameters["@PostalCode"].Value = postalCodeTextBox.Text;
            cmd.Parameters["@Country"].Value = countryTextBox.Text;
            cmd.Parameters["@Phone"].Value = phoneTextBox.Text;
            cmd.Parameters["@Fax"].Value = faxTextBox.Text;
        }

        private void customersBindingNavigatorSaveItem_Click(object sender, EventArgs e)
        {
            this.Validate();
            this.customersBindingSource.EndEdit();
            this.customersTableAdapter.Update(this.dsCustomers.Customers);

        }

        private void Form1_Load(object sender, EventArgs e)
        {
            // TODO: This line of code loads data into the 'dsCustomers.Customers' table. You can move, or remove it, as needed.
            this.customersTableAdapter.Fill(this.dsCustomers.Customers);

        }

        private void button3_Click(object sender, EventArgs e) //za novi unos
        {
            customerIDTextBox.Text = "";
            companyNameTextBox.Text = "";
            contactNameTextBox.Text = "";
            contactTitleTextBox.Text = "";
            addressTextBox.Text = "";
            cityTextBox.Text = "";
            regionTextBox.Text = "";
            postalCodeTextBox.Text = "";
            countryTextBox.Text = "";
            phoneTextBox.Text = "";
            faxTextBox.Text = "";
        }

        private void button2_Click(object sender, EventArgs e)
        {
            customersBindingSource.MoveNext();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            customersBindingSource.MovePrevious();
        }

        private void button6_Click(object sender, EventArgs e) //za update, naravno.
        {
            cn.Open();
            ucitajParametre(update_cmd);
            podesiParametre(update_cmd);
            try
            {

                int r = update_cmd.ExecuteNonQuery();
                MessageBox.Show(r.ToString());
            }

            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }

            finally
            {
               
                cn.Close();
            }
        }

        private void button5_Click(object sender, EventArgs e) //za delete
        {
            cn.Open();
            ucitajParametre(delete_cmd);
            podesiParametre(delete_cmd);

            try
            {
                int r = delete_cmd.ExecuteNonQuery();
                MessageBox.Show(r.ToString());
            }

            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }

            finally
            {
                cn.Close();
            }
        }

    }
}

Code:

string naredba = "select a,b,c from tabela where a=@1";
SqlCommand  komanda = new Sqlcommand(naredba);
komanda.Parameters.Add(new SqlParameter("@1",SqldbType.Int)).Value = aId;