ASP.NET - FileUpload with PostBack Trigger

Question

I have an UpdatePanel which has an upload control and a button to upload. The button is set as a trigger, but the event handler for the button fails to execute on the first PostBack.

My ASPX code is:

<asp:UpdatePanel ID="updPages" runat="server" UpdateMode="Conditional">
    <ContentTemplate>
        <div class="tabs">
            <ul>
                <li><asp:LinkButton ID="lnkContentsPages" runat="server" OnClick="updateContents" CommandArgument="pages">Pages</asp:LinkButton></li>
        <%-- these tabs change the ActiveViewIndex to show a different UserControl --%>
                <li><asp:LinkButton ID="lnkContentsImages" runat="server" OnClick="updateContents" CommandArgument="images">Images</asp:LinkButton></li>
            </ul>
            <div class="tabbedContent">
                <asp:MultiView runat="server" ID="mltContentsInner" ActiveViewIndex="0">
                    <asp:View ID="viwContentsImages" runat="server">
                        // ajax usercontrol for a list of images - works fine with ajax
                        <fieldset>
                            <legend>Upload New</legend>
                            <div class="formRow">
                                <asp:Label ID="lblFile" runat="server" Text="Filename" AssociatedControlID="uplFile" />
                                <asp:FileUpload ID="uplFile" runat="server" />
                            </div>
                            <div class="formRow">
                                <asp:Label ID="lblImageDescription" runat="server" Text="Description" AssociatedControlID="txtImageDescription" />
                                <asp:TextBox runat="server" ID="txtImageDescription" />
                            </div>
                            <asp:Button ID="btnUpload" runat="server" Text="Upload" CssClass="c3button btnUpload" CausesValidation="false" OnClick="btnUpload_Click" />
                        </fieldset>
                    </asp:View>
                    <asp:View ID="viwContentsPages" runat="server">
                        // ajax usercontrol - works fine with ajax
                    </asp:View>
                </asp:MultiView>
            </div>
        </div>
    </ContentTemplate>
    <Triggers>
        <asp:PostBackTrigger ControlID="btnUpload" />
    </Triggers>
</asp:UpdatePanel>

The button works without fail on the second and subsequent times, just not the first. Is there any reason for this?

Answer 1

I dont know why this is happening. However you could attempt to add the event yourself by override the OnInit page cycle event in the aspx.cs file. Remember to remove the Click event from your HTML page.

protected override void OnInit(EventArgs e)
{

btnUpload.Click+= new EventHandler(btnUpload_Click);
base.OnInit(e);
}

Alternatively the Trigger might also need to be positioned directly below the UpdatePanel Tag

<asp:UpdatePanel ID="updPages" runat="server" UpdateMode="Conditional">  
 <Triggers> 
        <asp:PostBackTrigger ControlID="btnUpload" /> 
    </Triggers> 

Answer 2

Maybe it's doing an AJAX postback the first time? On your UpdatePanel, try adding the attribute ChildrenAsTriggers="false".

This will stop any controls in the UpdatePanel causing asynchronous postbacks, so for every control that you want to cause a postback, you'll have to add an AsyncPostBackTrigger in the UpdatePanel's Triggers section.