ListFieldIterator control doesn't work when dynamically setting values with SharePoint 2010

Question

I am trying to load ListFieldIterator control dynamically and it is not working as expected.

Step1: Created Application Page

<%@ Assembly Name="$SharePoint.Project.AssemblyFullName$" %>
<%@ Import Namespace="Microsoft.SharePoint.ApplicationPages" %>
<%@ Register Tagprefix="SharePoint" Namespace="Microsoft.SharePoint.WebControls" Assembly="Microsoft.SharePoint, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Register Tagprefix="Utilities" Namespace="Microsoft.SharePoint.Utilities" Assembly="Microsoft.SharePoint, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Register Tagprefix="asp" Namespace="System.Web.UI" Assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" %>
<%@ Import Namespace="Microsoft.SharePoint" %>
<%@ Assembly Name="Microsoft.Web.CommandUI, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="ApplicationPage1.aspx.cs" Inherits="TaskList.Layouts.Utilities.ApplicationPage1" 
DynamicMasterPageFile="~masterurl/default.master" %>

<asp:Content ID="PageHead" ContentPlaceHolderID="PlaceHolderAdditionalPageHead" runat="server">

</asp:Content>

<asp:Content ID="Main" ContentPlaceHolderID="PlaceHolderMain" runat="server">
   <SharePoint:ListFieldIterator ID="listFieldIterator" ControlMode="New" runat="server" ></SharePoint:ListFieldIterator>
</asp:Content>

<asp:Content ID="PageTitle" ContentPlaceHolderID="PlaceHolderPageTitle" runat="server">
Application Page
</asp:Content>

<asp:Content ID="PageTitleInTitleArea" ContentPlaceHolderID="PlaceHolderPageTitleInTitleArea" runat="server" >
My Application Page
</asp:Content>

Code behind

using System;
using Microsoft.SharePoint;
using Microsoft.SharePoint.WebControls;

namespace TaskList.Layouts.Utilities
{
    public partial class ApplicationPage1 : LayoutsPageBase
    {
        protected void Page_Load(object sender, EventArgs e)
        {
        }

        protected void Page_Init(object sender, EventArgs e)
        {
            if (listFieldIterator.ListId == Guid.Empty)
            {
                string listName = Convert.ToString(this.Page.Request.QueryString["ListName"]);
                if (!string.IsNullOrEmpty(listName))
                {
                    listFieldIterator.ListId = SPContext.Current.Web.Lists[Convert.ToString(this.Page.Request.QueryString["ListName"])].ID;
                }


                switch (Convert.ToString(this.Page.Request.QueryString["ControlMode"]))
                {
                    case "New":
                        listFieldIterator.ControlMode = SPControlMode.New;
                        break;
                    case "Display":
                        listFieldIterator.ControlMode = SPControlMode.Display;
                        break;
                    case "Edit":
                        listFieldIterator.ControlMode = SPControlMode.Edit;
                        break;
                    default:
                        listFieldIterator.ControlMode = SPControlMode.New;
                        break;
                }
            }
        }
    }
}

I am passing two values from query string 1. ListName 2. ControlMode

Whatever value I pass it gives me below error message

[NullReferenceException: Object reference not set to an instance of an object.]
   Microsoft.SharePoint.WebControls.ListFieldIterator.CreateChildControls() +167
   System.Web.UI.Control.EnsureChildControls() +145
   Microsoft.SharePoint.WebControls.TemplateBasedControl.OnLoad(EventArgs e) +61
   System.Web.UI.Control.LoadRecursive() +65
   System.Web.UI.Control.LoadRecursive() +190
   System.Web.UI.Control.LoadRecursive() +190
   System.Web.UI.Control.LoadRecursive() +190
   System.Web.UI.Control.LoadRecursive() +190
   System.Web.UI.Control.LoadRecursive() +190
   System.Web.UI.Control.LoadRecursive() +190
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2427

If I hard code ListID on HTML page it works fine but when I set it dinamically its giving me above mentioned error. While doing debugging I came to know that ListFieldIterator control's field collection is not being populated and because of that control is not being rendered.

Answer 1

Removing below line solved the issue.

if (listFieldIterator.ListId == Guid.Empty)

When we access property of ListFieldIterator control its Create Child control method will be called and in my code i have not set value LISTID property at that point and due to that control is throwing exception.

How silly I am... :)

Answer 2

ListFieldIterator is designed to show one item from the list, designated by the ItemId property. Set that property in addition to the ListId property.