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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchTo reuse an iframe in ASP.NET Web Forms, put a server-side <iframe runat="server"> in an .ascx user control, expose the attributes you need as public properties, then register and use that control inside a Web Forms page’s server form. An .ascx file is not a standalone page and should not be the iframe’s direct target.
Create the iframe user control
Add an .ascx file, such as IframeWrapper.ascx, and make the iframe a server control by adding runat="server". The title gives the embedded frame an accessible name; loading="lazy" asks the browser to defer loading when appropriate.
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<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="IframeWrapper.ascx.cs" Inherits="WebApp.Controls.IframeWrapper" %>
<iframe id="Frame" runat="server" title="Embedded content" loading="lazy"></iframe>In the code-behind, expose only the settings consumers need. This example provides a source URL and dimensions:
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using System;
using System.Web.UI;
namespace WebApp.Controls
{
public partial class IframeWrapper : UserControl
{
public string Src
{
get => Frame.Attributes["src"] ?? String.Empty;
set
{
if (String.IsNullOrWhiteSpace(value))
throw new ArgumentException("Src is required.", nameof(value));
// Apply your application's URL allow-list or validation policy here.
Frame.Attributes["src"] = ResolveUrl(value);
}
}
public string FrameWidth
{
get => Frame.Attributes["width"] ?? String.Empty;
set => Frame.Attributes["width"] = value;
}
public string FrameHeight
{
get => Frame.Attributes["height"] ?? String.Empty;
set => Frame.Attributes["height"] = value;
}
}
}The designer file should declare Frame using a server-control type compatible with the target framework. If a framework upgrade causes an iframe parser or field-type error, check the generated designer declaration against the framework and regenerate or correct it as needed; a documented .NET 4 versus 4.5 case involved different generated iframe server-control types.
#1 Best Overall
Register and use the control on a page
Register the .ascx file with an @ Register directive, then add the control inside the page’s server form. Microsoft’s inclusion guidance uses TagPrefix, TagName, and Src, and recommends a relative path for flexibility.
<%@ Page Language="C#" %>
<%@ Register TagPrefix="uc" TagName="IframeWrapper" Src="~/Controls/IframeWrapper.ascx" %>
<form id="form1" runat="server">
<uc:IframeWrapper ID="HelpFrame" runat="server"
Src="~/Help/Embedded.aspx" FrameWidth="100%" FrameHeight="600" />
</form>The control’s public properties can also be assigned in page code-behind instead of markup. Keep the server form in the consuming page, not in the reusable user control.
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Set a dynamic iframe URL safely
For a URL chosen at runtime, validate it according to your application’s policy before assigning it to the control. For example, set it during Page_Load on the initial request:
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
HelpFrame.Src = ResolveAllowedEmbedUrl(Request.QueryString["page"]);
}ResolveAllowedEmbedUrl represents application code that validates and returns an allowed destination; it is not a built-in Web Forms method. Treat a configurable URL as untrusted input: allow-list acceptable schemes and hosts, reject dangerous schemes such as javascript:, and apply the site’s content-security policy and framing rules. Microsoft warns that HtmlGenericControl can display user input that might include malicious client script, so assigning an attribute is not a substitute for URL validation.
For simple cases, code-behind can set the server iframe directly with Frame.Attributes["src"] = validatedUrl;. A public property keeps that detail behind the user control’s interface and makes declarative use possible.
Choose how to assign the source and configure the frame
| Choice | Use it when | Trade-off |
|---|---|---|
Declarative Src |
The target is fixed for the page instance, such as an embedded help page. | Clear in markup; the URL is not selected dynamically at runtime. |
Dynamic Src property |
The application selects a destination from validated state or input. | Requires an explicit validation policy before assignment. |
Direct Frame.Attributes["src"] |
Page code needs to change the raw iframe attribute directly. | Couples the caller to the control’s internal iframe rather than its public interface. |
| Same-origin target | The framed page belongs to the same origin and the application has a legitimate need to interact with it. | Origin alignment can permit more parent-frame interaction, subject to browser security rules and application policy. |
| External target | The iframe must display content hosted elsewhere. | Cross-origin restrictions can prevent the parent page from reading the framed document or resizing it through DOM access. |
Expose additional iframe options—such as sandbox, dimensions, or loading behavior—as properties only when callers need to configure them. Keep a meaningful title on the frame. For content whose height may vary, prefer a fixed or responsive container rather than assuming the parent can inspect and measure a cross-origin document.
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Use an .aspx host page when another site needs to frame the content
An .ascx user control is a component, not an independently requestable page. Microsoft states that user controls cannot be called independently; they can only be called from the page or another user control that contains them. To expose the component in an iframe, create an .aspx host page, register the .ascx control inside that page, and set the iframe’s src to the host page URL—not to the .ascx path.
Convert an existing Web Forms page into a user control
When turning a page into a reusable .ascx file, Microsoft’s conversion guidance is to rename the file extension, remove the html, body, and form elements, and replace the @ Page directive with @ Control. The consuming page remains responsible for the server form.
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