DropDownList不会在SelectedIndexChanged上回发

Question

I am attempting to get a DropDownList to AutoPostBack when the selected item is changed. I'm going a little stir-crazy as to why this isn't working.

Does anyone have any quick ideas?Please Help

Here is code:-

 <asp:DropDownList ID="ddl_username" runat="server"
   AutoPostBack="True" 
   onselectedindexchanged="ddl_username_SelectedIndexChanged" 
  DataTextField="Shortname" DataValueField="ID">
  </asp:DropDownList>`



  protected void Page_Load(object sender, EventArgs e)
  {
    if (!IsPostBack)
    {
        Bind_Users();
    }
  }

 private DataSet Bind_Users()
 {
    DataSet ds;
    ds = cs.BindUsers();
    if (ds.Tables[0] != null)
    {
        if (ds.Tables[0].Rows.Count > 0)
        {
            ddl_username.DataSource = ds.Tables[0];
            ddl_username.DataTextField = "Shortname";
            ddl_username.DataValueField = "ID";
            ddl_username.DataBind();
            ListItem li = new ListItem();
            li.Text = "Select";
            li.Value = "-1";
            ddl_username.Items.Insert(0, li);
            ddl_username.SelectedIndex = 0;
        }
        //ddl_username.SelectedIndex = 0;
        //ddl_username.Text = "-----Select-----";
    }
    return ds;
}

Answer 1

Have you checked if the initial loading of the ddl occurs again after the repost? Just put a debug break-point in your selected-index-changed method and one in your oninit/page-load method - whereever you load the data to the ddl. if it loads again before firing your event - you need to put an

if(!IsPostBack)

before the load method - like:

if(!IsPostBack)
  this.MyDDL.DataSource = GetDatasourceForMyDDL();

Answer 2

Page_Load is called each time page do postback , so you have to put Bind_Users() inside a

if(!IsPostBack) block , because Page_Load is called before ddl_username_SelectedIndexChanged