Prevent duplication datas...

Question

Hi!

 

I have this code to prevent duplicate records but its give me an error message that says:

Object reference not set to an instance of an object. 

 

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 WindowsFormsApplication1

{

public partial class Form1 : Form

{

OleDbConnection conn = new OleDbConnection(@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\Dont_save_when_record_duplicated\WindowsFormsApplication1\App_data\test.mdb;Persist Security Info=False");

OleDbCommand comm;

string connstr = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\Dont_save_when_record_duplicated\WindowsFormsApplication1\App_data\test.mdb;Persist Security Info=False";

public Form1()

{

InitializeComponent();

}

 

 

 

private void button1_Click(object sender, EventArgs e)

{

//conn = new OleDbConnection(connstr);

//comm = new OleDbCommand();

//comm.Connection = conn;

conn.Open();

comm.Connection = conn;

connstr = "SELECT COUNT(*) FROM Not_duplicated_datas WHERE [username] = @username";

comm.CommandText = connstr;

comm.Parameters.Clear();

comm.Parameters.AddWithValue("@username", textBox1.Text);

int numRecords = (int)comm.ExecuteScalar();

if (numRecords == 0)

{

connstr = "INSERT INTO Not_duplicated_datas([username],[fillingcode],[branch],[department],[agency])VALUES(@username,@fillingcode,@branch,@department,@agency)";

comm.CommandText = connstr;

comm.Parameters.Clear();

comm.Parameters.AddWithValue("@username", textBox1.Text);

comm.Parameters.AddWithValue("@fillingcode", textBox2.Text);

comm.Parameters.AddWithValue("@branch", textBox3.Text);

comm.Parameters.AddWithValue("@department", textBox4.Text);

comm.Parameters.AddWithValue("@agency", textBox5.Text);

comm.ExecuteNonQuery();

MessageBox.Show("Created Successfully ..");

}

else

{

MessageBox.Show("A record with a user name of {0} already exists", textBox1.Text);

}

conn.Close();

}