Writing A Page To A String

ASP.NET Pages are designed to stream their output directly to a response stream. This can be a huge performance benefit for large pages as it doesn’t require buffering and allocating very large strings before rendering. Allocating large strings can put them on the Large Object Heap which means they’ll be sticking around for a while.

However, there are many cases in which you really want to render a page to a string so you can perform some post processing. I wrote about one means using a Response filter eons ago.

However, recently, I learned about a method of the Page class I never noticed which allows me to use a much lighter weight approach to this problem.

The method in question is CreateHtmlTextWriter which is protected, but also virtual.

So here’s an example of the code-behind for a page that can leverage this method to filter the output before its sent to the browser.

public partial class FilterDemo : System.Web.UI.Page
{
  HtmlTextWriter _oldWriter = null;
  StringWriter _stringWriter = new StringWriter();

  protected override HtmlTextWriter CreateHtmlTextWriter(TextWriter tw)
  {
    _oldWriter = base.CreateHtmlTextWriter(tw);
    return base.CreateHtmlTextWriter(_stringWriter);
  }

  protected override void Render(HtmlTextWriter writer)
  {
    base.Render(writer);
    string html = _stringWriter.ToString();
    html = html.Replace("REPLACE ME!", "IT WAS REPLACED!");
    _oldWriter.Write(html);
  }
}

In the CreateHtmlTextWriter method, we simply use the original logic to create the HtmlTextWriter and store it away in an instance variable.

Then we use the same logic to create a new HtmlTextWriter, but this one has our own StringWriter as the underlying TextWriter. The HtmlTextWriter passed into the Render method is the one we created. We call Render on that and grab the output from the StringWriter and now can do all the replacements we want. We finally write the final output to the original HtmlTextWriter which is hooked up to the response.

A lot of caveats apply in using this technique. First, as I mentioned before, for large pages, you could be killing scalability and performance by doing this. Also, I haven’t tested this with output caching, async pages, etc… etc…, so your mileage may vary.

Note, if you want to call one page from another, and get the output as a string within the first page, you can pass your own TextWriter to Server.Execute, so this technique is not necessary in that case.