How to get trace output added to context.response in httphandler?

Question

I've enabled trace pageoutput="true" in my web.config and I like the easy way it provides of seeing all this stuff at the bottom of the page.

I'd like to get the same output from trace at the bottom of the output from my httphandler. Is there a way to dump out the same trace info via code that would follow this code:

public class UploadHandler : IHttpHandler
{

    public void ProcessRequest(HttpContext context)
    {
        context.Response.ContentType = "text/plain";
        context.Response.Write("Hello World");
    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }
}

I particularly want to see the Forms and QueryString collections but all this gives is "Hello World".

-- edit update 7/25/2009:

    public class UploadHandler : IHttpHandler
    {

    public void ProcessRequest(HttpContext context)
    {
        context.Response.ContentType = "text/plain";
        context.Response.Write("Hello World");

        object htw = new System.Web.UI.Html32TextWriter(context.Response.Output);
        {
            typeof(TraceContext)
                .GetMethod("Render", System.Reflection.BindingFlags.NonPublic)
                .Invoke(HttpContext.Current.Trace, new object[] { htw });
        }

    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }
}

I am also open to any other ideas about how to most easily get a formatted dump of the forms and querystring collections like pageOutput trace does.

Answer 1

I don't think that ASP.NET tracing works except for pages.

Does it matter if the output is at the bottom of the page? What if the output were in a separate file, or in the event log? You could probably do thsi with ASP.NET Health Monitoring.

Answer 2

You can get your own trace events from HttpContext.Current.Trace.TraceFinished. Unfortunately, the page tracing (which includes all the goodies of Forms, QueryString, etc.) is locked away in internal methods.

If you're okay with reflection, you can call it like:

using (var htw = new System.Web.UI.Html32TextWriter(response.Output)) {
    typeof(TraceContext)
        .GetMethod("Render",BindingFlags.NonPublic | BindingFlags.Instance)
        .Invoke(HttpContext.Current.Trace, new object[] { htw });
}

Reflectoring over System.Web.TraceContext.EndRequest should give you enough to create your own TracingHttpHandler if you can't use reflection.

Edit: It looks like you forgot BindingFlags.Instance. Also, I'm guessing you changed using (var htw = ...) to using (object htw = ...) which would give you the "type must be implicitly convertible to IDisposable" error. If you can't use var, then you'll have to write it as using (Html32TextWriter htw = ...).

Full sample:

<%@ WebHandler Language="C#" Class="UploadHandler" %>

using System;
using System.Web;
using System.Web.UI;
using System.Reflection;

public class UploadHandler : IHttpHandler {
    public bool IsReusable { 
       get { return true; }
    }

    public void ProcessRequest(HttpContext context) {
       // the output will suck as text/plain - Render outputs HTML.
       context.Response.ContentType = "text/html"; 
       context.Response.Write("Hello World!");

       // depending on web.config settings, you may need to enable tracing manually
       HttpContext.Current.Trace.IsEnabled = true;
       // I had to write a custom trace message, or the Request context wasn't captured - YMMV
       HttpContext.Current.Trace.Write(null);

       using (Html32TextWriter htw = new Html32TextWriter(context.Response.Output)) {
          typeof(TraceContext)
              .GetMethod("Render", BindingFlags.NonPublic | BindingFlags.Instance)
             .Invoke(HttpContext.Current.Trace, new object[] { htw });
       } 
    }
}