Question

I have a non-databound Dropdownlist inside of an UpdatePanel:

<asp:UpdatePanel runat="server" ID="FiltersUpdPnl">
    <ContentTemplate>
        <div class="filters">
            <asp:Button runat="server" ID="ExportBtn" Text="Export Map to Image" />
            <br />
            Show: 
            <asp:DropDownList runat="server" ID="CapNumProjectsDDL" AutoPostBack="true" OnSelectedIndexChanged="ApplyFilters" OnPreRender="CapNumProjectsDDL_PreRender">
                <%--<asp:ListItem Value="0" Text="" Selected="True"></asp:ListItem>--%>
                <asp:ListItem Value="1" Text="Capacity"></asp:ListItem>
                <asp:ListItem Value="2" Text="Number of Projects"></asp:ListItem>
            </asp:DropDownList>
        </div>
    </ContentTemplate>
</asp:UpdatePanel>

The event handler isn’t terribly important, it just does stuff:

protected void ApplyFilters(object sender, EventArgs e)
{
    //Do Stuff relating to the selected Value
}

When the page loads, “Capacity” is selected by default, as it’s the first ListItem. When I switch off to “Number of Projects”, the event handler fires as expected, executing the code. But when I switch back to “Capacity”, the handler does NOT fire. The Postback is occurring, but I want it to specifically hit the event handler on both listitems.

You can see that I have a commented-out “0-value” ListItem in there. When I uncomment that, both “Capacity” and “Number of Projects” will hit the event handler as expected.

The issue is that when the page loads, it’s loading the data relevant to the “Capacity” dropdown, so I want the “Capacity” ListItem to be showing, but be able to fire the event handler when selected.

Am I missing something obvious here?

E: I tried adding a handler for the DDL_Prerender event, setting the SelectedIndex to like 200 or something clearly not in the list, hoping it would de-select “Capacity”, but that didn’t work either.

This is the Pre-render code:

protected void CapNumProjectsDDL_PreRender(object sender, EventArgs e)
{
    CapNumProjectsDDL.SelectedIndex = 200;
}

This didn’t change the way it worked.

Answer 1

It seems that you want that drop down element raise change even if you select in drop down box already selected item. I believe that this will not work, and the cause of the problem is not in server side asp.net dropdownlist control, but in how HTML select element is working to which DropDownList control is rendered.

The problem is that HTML select element doesn’t fire change event if user select the same item as was before drop box was shown (because from the point of view of control state – it was not changed).

So, I believe that the behavior that you want can be implement, but you should not use HTML select control and instead of it implement custom solution.