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ASP.NET custom control: when is LoadPostData() called?

Answer 1

A:

Sounds really odd to be working only when an item is selected. A quick way to check if LoadPostData is being invoked would to enable tracing and put the following in IPostBackDataHandler.LoadPostData(...).

Page.Trace.Write("My control", "LoadPostData");

If that is the case, you should make sure that you've got the following:

Page.RegisterRequiresPostBack(this) in OnInit

Here is a full sample control.

using System;
using System.Collections.Generic;
using System.Text;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.ComponentModel.Design;
using System.ComponentModel;
using System.Web.UI.Design;

namespace Controls
{
    public sealed class ExtendedListBoxDesigner : ControlDesigner
    {

        public override string GetDesignTimeHtml()
        {
            StringBuilder sb = new StringBuilder();
            sb.Append("<div>My designer</div>");
            return sb.ToString();
        }

    }

    [DesignerAttribute(typeof(ExtendedListBoxDesigner), typeof(IDesigner))]
    public class ExtendedListBox : ListBox, INamingContainer, IPostBackDataHandler 
    {
        bool IPostBackDataHandler.LoadPostData(string postDataKey, System.Collections.Specialized.NameValueCollection postCollection)
        {
            Page.Trace.Write("ExtendedListBox", "LoadPostData");
            return true;
        }


        protected override void OnInit(EventArgs e)
        {
            Page.RegisterRequiresPostBack(this);
            base.OnInit(e);
        }

        protected override void RenderContents(HtmlTextWriter writer)
        {
            base.RenderContents(writer);
            writer.Write(string.Format("<input type=\"hidden\" name=\"{0}_dummy\" value=\"alwaysPostBack\">", this.ID));
        }

    }
}

and the page looks like this.

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="ControlSamlpe._Default" Trace="true" %>
<%@ Register Assembly="Controls" Namespace="Controls" TagPrefix="sample" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"&gt;
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>Untitled Page</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
    <sample:ExtendedListBox runat="server" ID="extListBox"></sample:ExtendedListBox>
    <asp:Button runat="server" ID="go" />
    </div>
    </form>
</body>
</html>

When you click on go, you should see "LoadPostData" in the trace.

Martin Clarke 2010-07-09 09:45:16

It's not at all odd that it works only when an item is selected. The LoadPostData() method is only called if the postdata contains an item with the same name as the control's UniqueID - which it won't if the user hasn't selected an item.

David 2010-07-09 09:50:40

Well my first reaction was that it felt odd when I started writing it! :) Did you try adding the dummy field in RenderContents? That way LoadPostData will always get called.

Martin Clarke 2010-07-09 09:59:22

Thanks for your suggestion about creating an 'fake' element with the same name attribute as the UniqueID of the control. I sort of see what you mean. To my mind it's not quite as clean as the solution I've offered though. What do you think?

David 2010-07-09 10:17:40

Although it is a piece of twisted genius. If I have any problems with my way, I'll certainly try it! :)

David 2010-07-09 10:18:30

Hold on - what would happen if you added the fake element and the user made a selection? Wouldn't the post data contain two items with the same name? I'm not sure what would happen then.

David 2010-07-09 10:19:26

You can have multiple hidden fields as long as they have different names. So in LoadPostData: postCollection[string.Format("{0}_value", this.ID)] and postCollection[string.Format("{0}_hidden", this.ID)] would refer to two different items! The presence of either when you post would fire LoadPostData for your control.

Martin Clarke 2010-07-09 10:28:02

But surely it's the name attribute of the posted element which fires LoadPostData - and it must match the UniqueID of the control perfectly. If the control's UniqueID is 'foo', why do you think elements which names of 'foo_hidden' or 'foo_value' would fire LoadPostData? Am I misunderstanding you?

David 2010-07-09 10:46:00

You're right, you still need Page.RegisterRequiresBostBack(this). I've amended my answer with a full sample.

Martin Clarke 2010-07-09 12:19:39

Thanks for bringing my attention to Page.RegisterRequiresPostback. It is the way to ensure that LoadPostData() is always called, irrespective of whether there is an item in the post data with the same name as the control's UniqueID. However, I have found that using this method sometimes the selected value of the list box is not preserved. My solution (using a method called during OnLoad()) is the only one that consistently seems to work so I've marked that one as correct.

David 2010-07-12 09:11:49

Answer 2

A:

I've established that the LoadPostData() method is not called unless the post data contains an item with the same name as the control's UniqueID. [Edit: calling Page.RegisterRequiresPostback during Init() overcomes this.] I can see why, but it is quite limiting.

I have overcome the problem by not handling it during the LoadPostData() method at all. Instead, I have handled it in a method which I call in OnLoad() instead.

Two things need to be borne in mind when using this approach:

1) You no longer have access to the postCollection NameValueCollection object which is passed in to the LoadPostData() method as an argument. This means you have to extract the post data from the Request.Form collection, which is slightly harder work. 2) Since OnLoad() occurs after the ViewState processing code, you will need to manually set the SelectedValue after you create the ListItems. If you don't, if the listbox is populated via AJAX and the user makes a selection, the selection will be lost.

I hope this helps someone in the future.

David 2010-07-09 09:55:28

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ASP.NET custom control: when is LoadPostData() called?

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