Render Control as String

When working with AJAX and Web Services it’s often nice to be able to render ASP.NET controls as strings, so the rendered HTML can be sent back to the client to replace the contents of a <div> or something like that. The standard way of achieving this is to use the RenderControl() method, exposed by all ASP.NET controls. Unfortunately, the RenderControl() method doesn’t simply return a string – that would be too easy. Instead, it takes in an HtmlTextWriter which it will render the control into. No problem, just new one of those up and… not so fast. You can’t actually create an instance of an HtmlTextWriter without first having a TextWriter. And since you really want a string when this is all said and done, a StringBuilder would be nice to have as well. So, a few using() statements later, you end up with something like this as the basic pattern:

You can take this technique and apply it to a real control in a web method like this example, that renders a Panel with two Label controls in it.

I have another blog post on how to render a user control as a string if that is what you really want to do.

The thing to notice about this code for rendering an ASP.NET control as a string is that it’s rather repetitive and verbose. However, it also is the same for any System.Web.UI.Control, and so it is an excellent candidate for an extension method. Add something like this to a namespace that is referenced in your code to create the extension method:

Now the code to render a control becomes simply myControl.RenderControl();. Our GetPanel() web method can be rewritten to use the extension method like so:

I discussed this approach briefly on a recent interview on Craig Shoemaker’s Polymorphic Podcast. You can listen to the show here.

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  • Jarrett

    Note: this only works when you have a HttpContext. If you try rendering a control outside of Asp.Net runtime (such as a non-hosted WCF service), it will not work.

  • Mike Borozdin

    I like the way you implemented that with extension methods, they really come very handy.

  • asp.net developer

    it’s simple to use in simple apps.

  • PhanCongPhuoc

    I use your function, but I get the error message that : Control ‘Mydiv’ of type ‘RadioButton’ must be placed inside a form tag with runat=server.

    But I use master page, and take the form there

    How can I solve it

    Thank you

  • ramzi

    somehow the dynamic VS compiler won’t swallow

    return myPanel.RenderControl(), asking for a stringwriter, hence

    in a simple asp.net page, in the coderbehind, the method can be called as:

    return ControlExtenders.RenderControl(i1);

    it does the trick, misses the point of extenders, but works.

  • Jeremy

    When I try to render a Panel or a Label, this works great. However, when I try to render an AJAX control, more specifically the TabContainer, I get the ever-helpful "…not set to an instance of an object" error.

    Any thoughts?