when i added data in my program,datas dont saved

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SQL-Server-2005

C#

SQL-Server

Hi,

I am running my project in VS2008 and give datas to insert to table.

In current program, datas are added and when the program selects from that table, I see changes.
When rerunning the program after changing a little unrelated codes or close VS2008 and Open a new and run program again, that data does not appear to be added (but when I add data in server explorer of VS2008, without running the program, the data is added).
in class #1

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

namespace Mallmanagement
{
    public class mydataaccess
    {
        public string ServerName;
        public string DBName = "goldDB.mdf";
        public string username;
        public string password;
        SqlConnection con;
        SqlCommand cmd;
        SqlDataAdapter da;
        public mydataaccess()
        {
            con = new SqlConnection();
            cmd = new SqlCommand();
            da = new SqlDataAdapter();
            cmd.Connection = con;
            da.SelectCommand = cmd;
        }
        public void connect()
        {
            string cs = "Data source=.\\SQLEXPRESS;Attachdbfilename=|DataDirectory|\\{0};Integrated security=true;user Instance=true";
            cs = string.Format(cs, this.DBName);
            con.ConnectionString = cs;
            con.Open();
        }
        public void Disconnect()
        {
            con.Close();
        }
        public DataTable select(string sql)
        {
            DataTable dt = new DataTable();
            cmd.CommandText = sql;
            da.Fill(dt);
            return dt;
        }
        public void docommand(string sql)
        {
            cmd.CommandText = sql;
            cmd.ExecuteNonQuery();
        }
    }
}

and class #2

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

namespace Mallmanagement
{
    class sanadlogic
    {
        mydataaccess md = new mydataaccess();
        public int codehesab;
        public int codesanad;
        public int day;
        public int month;
        public int year;
        public DataTable select()
        {
            md.connect();
            string ss = "Select * from sanadha";
            DataTable dt = new DataTable();
            dt = md.select(ss);
            md.Disconnect();
            return dt;
        }
        public void Insert()
        {
            md.connect();
            string AA = "Insert into sanadha(codesanad,codehesab,day,month,year)values({0},{1},{2},{3},{4}";
            AA = string.Format(AA, this.codesanad, this.codehesab, this.day, this.month, this.year);
            md.docommand(AA);
            md.Disconnect();
        }
    }
}

What is the problem?
Can I to change, add, delete data in running program?
And can I change, add, delete data in tables?