ASP.NET - FileUpload with PostBack Trigger

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" />
  开发者_开发技巧                              <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?

need to add enctype="multipart/form-data" in the form tag of the page.

faced the same problem and the above work around made it upload the file in the first attempt. hope some one will find it useful.

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> 

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.

According to anbalagan's answer and an answer from another question, you should change the enctype property of the form element to 'multipart/form-data'. As it is said in this question's best answer, 'multipart/form-data' should be used whenever you write client-side code and need to post files to the server, e.g. use FileUpload controls.

I had the same problem as yours, changed the enctype of the form and now everything works fine.