50

If I create a UserControl and add some objects to it, how can I grab the HTML it would render?

ex.

UserControl myControl = new UserControl();
myControl.Controls.Add(new TextBox());

// ...something happens

return strHTMLofControl;

I'd like to just convert a newly built UserControl to a string of HTML.

2
  • 2
    Please, make azamsharp's answer "accepted", if his solution works for you. Be a good SO member :) Nov 13, 2008 at 22:07
  • You will also need to be sure System.Web is added to your references so you can import System.Web.UI (which HtmlTextWriter needs) Aug 24, 2012 at 17:18

7 Answers 7

56

You can render the control using Control.RenderControl(HtmlTextWriter).

Feed StringWriter to the HtmlTextWriter.

Feed StringBuilder to the StringWriter.

Your generated string will be inside the StringBuilder object.

Here's a code example for this solution:

string html = String.Empty;
using (TextWriter myTextWriter = new StringWriter(new StringBuilder()))
{
    using (HtmlTextWriter myWriter = new HtmlTextWriter(myTextWriter))
    {
        myControl.RenderControl(myWriter);
        html = myTextWriter.ToString();
    }
}
4
  • 1
    you could also add the control in a "live" controls collection to avoid random exceptions. Nov 13, 2008 at 22:34
  • 23
    This answer could be vastly improved by simply having the code sample here with the actual code.
    – theJerm
    Jan 30, 2013 at 4:00
  • @theJerm: here? where? Can't see any link Aug 6, 2015 at 5:37
  • StringWriter and HtmlTextWriter instances are not being disposed properly. Wrap them in using block.
    – Kon
    May 14, 2020 at 21:03
32
//render control to string
StringBuilder b = new StringBuilder();
HtmlTextWriter h = new HtmlTextWriter(new StringWriter(b));
this.LoadControl("~/path_to_control.ascx").RenderControl(h);
string controlAsString = b.ToString();
1
14
UserControl uc = new UserControl();
MyCustomUserControl mu = (MyCustomUserControl)uc.LoadControl("~/Controls/MyCustomUserControl.ascx");

TextWriter tw = new StringWriter();
HtmlTextWriter hw = new HtmlTextWriter(tw);

mu.RenderControl(hw);

return tw.ToString();
1
  • 4
    LoadControl solved my problem. (creating a new instance doesn't work)
    – BrunoLM
    Sep 28, 2011 at 12:13
7

Seven years late, but this deserves to be shared.

The generally accepted solution - StringBuilder into StringWriter into HtmlWriter into RenderControl - is good. But there are some gotchas, which I unfortunately ran across while trying to do the same thing. Some controls will throw errors if they're not inside of a Page, and some will throw errors if they're not inside of a <form> with runat="server". The ScriptManager control exhibits both of these behaviours.

I eventually found a workaround here. The gist of it is basically just instantiating a new Page and Form before doing the writer work:

Page page = new Page();
page.EnableEventValidation = false;

HtmlForm form = new HtmlForm();
form.Name = "form1";
page.Controls.Add(form1);

MyControl mc = new MyControl();
form.Controls.Add(mc);

StringBuilder sb = new StringBuilder();
StringWriter sw = new StringWriter(sb);
HtmlTextWriter writer = new HtmlTextWriter(sw);

page.RenderControl(writer);

return sb.ToString();

Unfortunately, this gives you more markup than you actually need (since it includes the dummy form). And the ScriptManager will still fail for some arcane reason I haven't puzzled out yet. Honestly, it's a whole lot of trouble and not worth doing; the whole point of generating controls in the code-behind is so that you don't have to fiddle around with the markup, after all.

1
  • Exactly - thank you Reikim. The other part of these approaches that is bothering me is the quantity of instantiation that it can require. The amount of overhead is nagging at me - all to sling a string back to a browser.
    – Michael
    Apr 13, 2017 at 19:36
4

override the RenderControl method

protected override void Render(HtmlTextWriter output)
{       
   output.Write("<br>Message from Control : " + Message);       
   output.Write("Showing Custom controls created in reverse" +
                                                    "order");         
   // Render Controls.
   RenderChildren(output);
}

This will give you access to the writer which the HTML will be written to.

You may also want to look into the adaptive control architecture of asp.net adaptive control architecture of asp.net where you can 'shape' the default html output from controls.

1

You could utilize the HttpServerUtility.Execute Method, available through HttpContext.Current.Server.Execute:

var page = new Page();
var myControl = (MyControl)page.LoadControl("mycontrol.ascx");
myControl.SetSomeProperty = true;
page.Controls.Add(myControl);
var sw = new StringWriter();
HttpContext.Current.Server.Execute(page, sw, preserveForm: false);

The benefit would be that you also trigger the Page_Load event of your user control.

MSDN documentation can be found here: https://msdn.microsoft.com/en-us/library/fb04e8f7(v=vs.110).aspx.

1

Call its .RenderControl() method.

0

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