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:

StringBuilder sb = new StringBuilder();
using (StringWriter sw = new StringWriter(sb))
{
    using (HtmlTextWriter textWriter = new HtmlTextWriter(sw))
    {
        myControl.RenderControl(textWriter);
    }
}
return sb.ToString();

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.

[WebMethod]
public string GetPanel()
{
    Panel myPanel = new Panel();
    Label myLabel1 = new Label();
    myLabel1.Text = "First Label";
    Label myLabel2 = new Label();
    myLabel2.Text = "Second Label";
    myPanel.Controls.Add(myLabel1);
    myPanel.Controls.Add(myLabel2);
 
    StringBuilder sb = new StringBuilder();
    using (StringWriter sw = new StringWriter(sb))
    {
        using (HtmlTextWriter textWriter = new HtmlTextWriter(sw))
        {
            myPanel.RenderControl(textWriter);
        }
    }
    return sb.ToString();
}

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:

public static class ControlExtenders
{
    public static string RenderControl(this Control control)
    {
        StringBuilder sb = new StringBuilder();
        using (StringWriter sw = new StringWriter(sb))
        {
            using (HtmlTextWriter textWriter = new HtmlTextWriter(sw))
            {
                control.RenderControl(textWriter);
            }
        }
        return sb.ToString();
    }
}

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:

[WebMethod]
public string GetPanel()
{
    Panel myPanel = new Panel();
    Label myLabel1 = new Label();
    myLabel1.Text = "First Label";
    Label myLabel2 = new Label();
    myLabel2.Text = "Second Label";
    myPanel.Controls.Add(myLabel1);
    myPanel.Controls.Add(myLabel2);
 
    return myPanel.RenderControl(); // extension method
}

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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