Обновить источник данных dropdownlist в ASP.NET

Hi I am new in c# for first assignment. I had successfully load data from dropdownlist into my textfield. Also manage to insert, update and delete the records from database table. Now, I am facing this problem. Each new record insert into db table, update or delete, my dropdownlist which load from Page_Load doesn't not refresh my dropdownlist.

I do some research online, instead of using ajax. I may use another trick code which is DropDownList2.DataBind();

Anyhow,it still doesn't work to refresh my Dropdown list. Any expert can give me some advise. Did I place the line of code wrongly?

Below is my code for delete function.

connection.ConnectionString = 
 ConfigurationManager.ConnectionStrings["constr"].ToString();
 connection.Open();
 NpgsqlCommand cmd = new NpgsqlCommand();
 cmd.Connection = connection;
 cmd.CommandText = "Delete from student_db where id=@id";
 cmd.CommandType = CommandType.Text;
 cmd.Parameters.Add(new NpgsqlParameter("@id", 
 Convert.ToInt32(DropDownList2.SelectedItem.Value)));
 cmd.ExecuteNonQuery();
 cmd.Dispose();
 DropDownList2.DataBind();
 connection.Close();