export access database table to csv file using c# windows application

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C#

Windows

application

I'm trying with this below code for export access database table to csv file
but getting error

Quote:

"Object reference not set to an instance of an object." Null reference exception was unhandled.

Please provide solution for this.
this is my code

private void button1_Click(object sender, EventArgs e)
       {
           OleDbConnection cnon = new OleDbConnection();
           cnon.ConnectionString = @"Provider= Microsoft.ACE.OLEDB.12.0; Data Source=D:\WindowsFormsApplication1\barcode.accdb";
           string query = "SELECT * FROM tblbarcode";
           string sep = ",";
           string strFilePath = @"D:\WindowsFormsApplication1\export.txt";


           StreamWriter sw = new StreamWriter(HttpContext.Current.Server.MapPath(strFilePath));


           using (OleDbCommand Cmd = new OleDbCommand(query, cnon))
               {
                   cnon.Open();
                   using (OleDbDataReader dr = Cmd.ExecuteReader())
                   {
                       int fields = dr.FieldCount - 1;
                       while (dr.Read())
                       {
                           StringBuilder sb = new StringBuilder();
                           for (int i = 0; i <= fields; i++)
                           {
                               if (i != fields)
                               {
                                   sep = sep;
                               }
                               else
                               {
                                   sep = "";
                               }
                               sb.Append(dr[i].ToString() + sep);

                           }
                           sw.WriteLine(sb.ToString());
                       }
                   }
               }

       }