Question

I’ve developed custom HTTP module that processes .aspx page (it just sets app.Response.Filter for doing some simple string replacing) after it is rendered by ASP.NET. It is working perfectly, but I am running into one small problem – OutputCache HTTP module will not cache changes I’m doing with app.Response.Filter.

Because of performance benefit I would love if it would be somehow possible to inverse String Replacing and Output Caching.

So, is there a way to do this? Would using HttpHandlers be the way to go?

Here is the current source code of replacer:

public class StringReplaceModule : IHttpModule
{
    void IHttpModule.Dispose()
    {
        // Nothing to dispose; 
    }

    void IHttpModule.Init(HttpApplication context)
    {
        context.PreSendRequestHeaders +=
          (sender, e) => HttpContext.Current.Response.Headers.Remove("Server");

        context.BeginRequest += new EventHandler(context_BeginRequest);
    }

    void context_BeginRequest(object sender, EventArgs e)
    {
        HttpApplication app = sender as HttpApplication;
        string url = app.Request.RawUrl.ToLower();
        if (!url.Contains(".aspx/") &&
            (url.Contains(".aspx") || url.Contains(".css") || url.Contains("/shorturl/")))
        {
            app.Response.Filter = new StringReplaceFilter(app.Response.Filter);
        }
    }

    #region Stream filter

    private class StringReplaceFilter : Stream
    {
        public StringReplaceFilter(Stream sink)
        {
            _sink = sink;
        }

        private Stream _sink;
        private static string[] find;
        private static string[] replace;
        static StringReplaceFilter()
        {
            var config = StringReplaceModuleConfig.CurrentConfigSection();

            find = config.Find.ToArray();
            replace = config.Replace.ToArray();
        }

        public override void Write(byte[] buffer, int offset, int count)
        {
            byte[] data = new byte[count];
            Buffer.BlockCopy(buffer, offset, data, 0, count);
            string html = System.Text.Encoding.Default.GetString(buffer);

            for (int i = 0; i < find.Length; i++)
            {
                html = html.Replace(find[i], replace[i]);
            }

            byte[] outdata = System.Text.Encoding.Default.GetBytes(html);
            _sink.Write(outdata, 0, outdata.GetLength(0));
        }


        #region Less Important

        public override bool CanRead
        {
            get { return true; }
        }

        public override bool CanSeek
        {
            get { return true; }
        }

        public override bool CanWrite
        {
            get { return true; }
        }

        public override void Flush()
        {
            _sink.Flush();
        }

        public override long Length
        {
            get { return 0; }
        }

        private long _position;
        public override long Position
        {
            get { return _position; }
            set { _position = value; }
        }

        public override int Read(byte[] buffer, int offset, int count)
        {
            return _sink.Read(buffer, offset, count);
        }

        public override long Seek(long offset, SeekOrigin origin)
        {
            return _sink.Seek(offset, origin);
        }

        public override void SetLength(long value)
        {
            _sink.SetLength(value);
        }

        public override void Close()
        {
            _sink.Close();
        }

        #endregion
    }
    #endregion
}

Answer 1

Can you paste some sample code of what you are doing here?

It sounds like you are trying to find a way of replacing any instance of a certain word/keyword with something else if you find it in the output?

You can try doing this http://forums.asp.net/t/1123505.aspx and adding a explicit expiration rather than using the output cache facilities of asp.net

 using System;
 using System.Web;

 namespace TT.Web.HttpModules
 {
      /// <summary>
      /// HttpModule to prevent caching 
      /// </summary>
      public class NoCacheModule : IHttpModule
      {
         public NoCacheModule()
         {
         }

         #region IHttpModule Members

         public void Init(HttpApplication context)
         {
             context.EndRequest += (new EventHandler(this.Application_EndRequest));
         }

         public void Dispose()
         {
         }

         private void Application_EndRequest(Object source, EventArgs e) 
         {
             HttpApplication application = (HttpApplication)source;
             HttpContext context = application.Context;
             context.Response.Cache.SetLastModified(DateTime.Now);
             context.Response.Cache.SetExpires(DateTime.Now.AddMinutes(GetExpiryTime()));
             context.Response.Cache.SetCacheability(HttpCacheability.Public);
             context.Response.Cache.AppendCacheExtension("post-check=7200");
         //The pre-check is in seconds and the value configured in web.config is in minutes. So need to multiply it with 60
             context.Response.Cache.AppendCacheExtension("pre-check=" + (GetExpiryTime() * 60).ToString());
             context.Response.CacheControl = "public";
         }

         #endregion
     }
 }

• Also – Is this page being loaded only once? Or on a postback?