插入更新删除后，如何从页面加载刷新下拉菜单？

Question

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();

Answer 1

The database will not fire an event to your code base when data has changed. You need to use a wrapper class like an Observable around an in-memory collection.

https://docs.microsoft.com/en-us/dotnet/api/system.collections.objectmodel.observablecollection-1?redirectedfrom=MSDN&view=netframework-4.8

If you need to be updated when other client applications update data then you'll either need to use a Message broker like RabbitMQ or present an external API for receiving notification like MVC WebAPI.

Last option would be a re-query after an action is performed, but that could heavily impact your network traffic.