Problem with Response.Write - Changing Dynamic Contents without Corruption

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Introduction

When you need to dynamically control the HTML that is generated from your ASP.Net page or control, a famous technique to do that is to override the render function protected override void Render(HtmlTextWriter writer) It is a function where you can get a great power over your control, the function simply sends HtmlTextWriter which is a stream where you can write anything and it will appear on client screen as we can see in the code below.

        protected override void Render(HtmlTextWriter writer)
        {
            writer.Write("SayAnthing");
        } 

What is the Problem

Now let us change page data instead of replacing it. The attached sample contains two aspx pages, both have the same HTML structure as follows:

<table> <tr> <td>
        Hello ( <%Response.Write(@"Mr. XYZ"); %> $MSG$       
</td> </tr> </table>     

All we need to do is to replace $MSG$ with a message "How R U?" by capturing the rendering event. The required result should be as in the following figure:


Let us write down the code:

protected override void Render(HtmlTextWriter writer) 
{
    // Create your HTMLTestWriter
   System.Text.StringBuilder oPageContents = new System.Text.StringBuilder();
    System.IO.StringWriter oSW = new System.IO.StringWriter(oPageContents);
    System.Web.UI.HtmlTextWriter oHtmlWriter = new System.Web.UI.HtmlTextWriter(oSW);
    
    // Give the page a fake streamer to capture HTML contents.
    base.Render(oHtmlWriter); 
    oPageContents = oPageContents.Replace("$MSG$", "How R U ?");
    
    // Write data back into the original stream
    System.IO.StringReader oSR = new System.IO.StringReader(oPageContents.ToString ());
    writer.Write(oSR.ReadToEnd ()); 
}

Nothing strange about this code and it can be found all over the internet, the idea behind it is that you replace the given HtmlTextWriter that you can only write into it, with another on that streams data into a StringBuilder object oPageContents , once the Html is in oPageContents string builder you can read and modify easily. Next step is to write your contents in the actual HtmlTextWriter that is passed as a parameter in the render function. It is pretty straight forward :) .... wait the result is not what why want to do ??!!

The strange thing is that you dont get the desired result instead you get the following result:


I spend couple of days searching why does thing happen. and finally I found the answer. Response.Write() DOES NOT send its contents through base.Render that is used to render the control. Response.Write() , in fact HtmlTextWriter stream sends the data eventually to Response object.

So what happens here is that while base.Render() is working, when Response.Write() takeplace it sends data to directly out, while the other data are all stored in our local string builder, then we inject them in to the writer but it is too late, as the order of streaming output now has been corrupted.

Solution - HTML Streamer

HTMLStreamer solves this issue by acting as a double streamer, it streams data to StringBuilder object as in our previous sample, in the same time, it streams data to writer parameter of type HtmlTextWriter. The data is submitted synchornized to Response object, while another copy is being saved in the string builder object.

public class HTMLStreamer: System.IO.StringWriter 
{ 
    protected System.Web.UI.HtmlTextWriter mHtmlTextWriter;
    public HTMLStreamer(System.Text.StringBuilder oStringBuilder, System.Web.UI.HtmlTextWriter innerWriter) 
        : base(oStringBuilder)
     { 
        mHtmlTextWriter = innerWriter;
     }
    
    public override void Write(string value) 
    { 
        // Change content dynamically here.
        if (value.IndexOf("$MSG$")!=-1) 
        {
            value = value.Replace("$MSG$","How R U ?"); 
        }
        mHtmlTextWriter.Write(value); base.Write(value); 
    }
    
    public override void Write(char[] buffer, int index, int count)
    { 
        mHtmlTextWriter.Write(buffer, index, count);
        base.Write(buffer, index, count);
    } 

    public override void Write(char value) 
    { 
        mHtmlTextWriter.Write(value);
        base.Write(value);
    } 
} 

Now back to the Render() function it should be handled now as follows:

protected override void Render(HtmlTextWriter writer) 
{ 
    // Create your string builder.
    System.Text.StringBuilder oPageContents = new System.Text.StringBuilder();
   
    // Create HTMLStreamer and put both writer & oPageContents into it.
    HTMLStreamer oSW = new HTMLStreamer(oPageContents, writer); 
   
    // Create HTMLTextWriter that contains the double-stream class "HTMLStreamer"
    System.Web.UI.HtmlTextWriter oHtmlWriter = new System.Web.UI.HtmlTextWriter(oSW);
   
    // Get HTML contents.
    base.Render(oHtmlWriter); 
}

Enhancements

A delegate is called in all Write() functions of HTMLStreamer so that it can be used by the page to change contents without the need to write into HTMLStreamer itself, so it can be used as a separate DLL. For sure if you have any idea how to capture Response.Write() please let me know.

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Simple fix: Use <%= %> instead of <%Response.Write%> Rick L 6:19 9 Oct '08   I've also discovered that replacing <%Response.Write(value)%> with the alternative <%=value%> syntax results in a properly rendered page. However, I've been unable to find how exactly the <%= syntax is handled under the hood. Everything I can find says that this is simply a syntax alternative and is converted directly to a Response.Write, but based on the behavior I'm seeing here, it is obviously a little more complicated than a simple call redirect. Sign In·View Thread·PermaLink Good work !!! dnpro 23:16 21 May '08   Learn t something new !!! dnpro Sign In·View Thread·PermaLink Alternative Matthew Fergusson 1:16 20 May '08   I was trying to work out why I have never come across this problem. In fact why I had never needed to pass a dummy writer to base.Render(). Its because I have always overwritten the Response.Stream for such text-only replacements. What benefits do you see from overriding in the render method over replacement in the actual response? Sign In·View Thread·PermaLink Re: Alternative Mohammad Hefny 3:26 10 Aug '08   Could you please send me a sample. Mohammad S. Hefny Development Manager Sign In·View Thread·PermaLink

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