Tuesday, February 16, 2010

ASP.NET LoadControl overloads

Yesterday, one my friends asked me a ASP.NET question about LoadControl. According to MSDN, http://msdn.microsoft.com/en-us/library/system.web.ui.templatecontrol.loadcontrol.aspx, the LoadControl method are overloaded.
He had created a user control, with a constructor he would like to initialize, he also had a empty constructor. I've created my own code which describes the situation. The user control looked something like this:
<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="UserControl.ascx.cs" Inherits="LoadControl.UserControl1" %>
<asp:TextBox runat="server" ID="TextBox1" />
public partial class UserControl1 : System.Web.UI.UserControl
   private string text = "TextBox";
   public UserControl1()
   public UserControl1(string text) {
      this.text = text;
The page look something like this:
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="LoadControl._Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
   <head runat="server">
      <form id="form1" runat="server">
            <asp:Panel ID="Panel1" runat="server" />
public partial class _Default : System.Web.UI.Page {
   protected void Page_Load(object sender, EventArgs e)  {
      Panel1.Controls.Add(LoadControl(typeof(UserControl1), new object[] { "Test TextBox"}));
When the page was accessed it surprisingly look like this:
The second instance of the user control, was never added to the page. The reason for this is the way ASP.NET handles user controls. When you use LoadControl(string), .NET processes the acsx file. When you use LoadControl(Type, Object[]), .NET does not process the ascx file, instead it uses the controls Render method. If this method doesn't exists nothing are rendered. To get code above to work like it was intended a Render method have to be added, the method could look something like this:
protected override void Render(HtmlTextWriter writer) {
   writer.Write("<p>" + text + "</p>");
   writer.Write("<input type='text' />");
The important lesson here is that user controls only uses the acsx markup file, if the markup file are specified to load. If only the type are used, the user control knows nothing about the ascx file.

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