How DropDownList's SelectedIndexChanged() works without PostBack?

Question

DropDownList's SelectedIndexChanged() Event fills the ListBox on the page. Obviously this posts the page back to the server. Is there any way to make it happen without full postback?

protected void ddlTablo_SelectedIndexChanged(object sender, EventArgs e)
{
    List<string> list = new List<string>();
    ListBox1.Items.Clear();
    var columnNames= from t in typeof(Person).GetProperties() select t.Name;
    foreach (var item in columnNames)
    {
         list.Add(item);
    }
    ListBox1.DataSource = list;
    ListBox.DataBind();
}

Solution 1

You could put the DropDownList into an <asp:UpdatePanel> and set the trigger to the SelectedIndexChanged event of the DropDownList.

Something like this (don't forget the script manager)

<asp:ScriptManager ID="ScriptManager1" runat="server" />

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
   <ContentTemplate>
      <asp:DropDownList ID="drop1" runat="server" OnSelectedIndexChanged="ddlTablo_SelectedIndexChanged" />
   </ContentTemplate>
   <Triggers>
      <asp:AsyncPostbackTrigger ControlID="drop1" EventName="SelectedIndexChanged" />
   </Triggers>
</asp:UpdatePanel>

Solution 2

You can send ajax call, using asp.net UpdatePanel or use jQuery ajax. This wont do postback and your whole page wont get refreshed.

The UpdatePanel is quite straight forward and easy to use. ASP.net ajax will generate the asyn calls for you whereas jQuery ajax will probably need you to render html using javascript.

Solution 3

In the code snippet below, add this parameter: AppendDataBoundItems="True"

<asp:DropDownList ID="ddlGroupNameFilter" 
    runat="server" 
    AutoPostBack="true" 
    AppendDataBoundItems="true" 
    OnSelectedIndexChanged="ddlLeadgroupName_SelectedIndexChange">
</asp:DropDownList>

Comments

[Adil]

Any kind of ajax (async call) could be used.

[Green_qaue]

Hi, can you please explain what difference the "AsyncPostbackTrigger" does? To me it looks like I get the same effect without it? Since I'm setting the first dropdown to AutoPostBack="true". (Im updating a second DDL after what is chosen in the first)

[slfan]

You can see an answer to your question here: stackoverflow.com/questions/15330089/…. Basically a sync postback will post and load the whole form whereas an asyncpostback only reloads part of the page.

[Max Voisard]

@slfan You say to set AutoPostBack="true" on the DropDownList but don't do it in the example. Which is right?

[slfan]

@MaxVoisard. You are right, the AutoPostBack is not required, because we have the AsyncPostbackTrigger. I updated my answer