public class RemoveWhitespacesAttribute : ActionFilterAttribute
    {

        public override void OnActionExecuted(ActionExecutedContext filterContext)
        {

            var response = filterContext.HttpContext.Response;
      
            if (filterContext.HttpContext.Request.RawUrl != "/sitemap.xml")
            {

                if (response.ContentType == "text/html" && response.Filter != null)
                {
                    response.Filter = new HelperClass(response.Filter);
                }
            }
        }

        private class HelperClass : Stream
        {

            private System.IO.Stream Base;

            public HelperClass(System.IO.Stream ResponseStream)
            {

                if (ResponseStream == null)
                    throw new ArgumentNullException("ResponseStream");
                this.Base = ResponseStream;
            }

            StringBuilder s = new StringBuilder();

            public override void Write(byte[] buffer, int offset, int count)
            {

                string HTML = Encoding.UTF8.GetString(buffer, offset, count);

                Regex reg = new Regex(@"(?<=\s)\s+(?![^<>]*</pre>)");
                HTML = reg.Replace(HTML, string.Empty);

                buffer = System.Text.Encoding.UTF8.GetBytes(HTML);
                this.Base.Write(buffer, 0, buffer.Length);
            }

            #region Other Members

            public override int Read(byte[] buffer, int offset, int count)
            {

                throw new NotSupportedException();
            }

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

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

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

            public override long Length { get { throw new NotSupportedException(); } }

            public override long Position
            {

                get { throw new NotSupportedException(); }
                set { throw new NotSupportedException(); }
            }

            public override void Flush()
            {

                Base.Flush();
            }

            public override long Seek(long offset, SeekOrigin origin)
            {

                throw new NotSupportedException();
            }

            public override void SetLength(long value)
            {

                throw new NotSupportedException();
            }

            #endregion
        }

    }

 protected void Application_Start()
        {
            try
            {
                GlobalFilters.Filters.Add(new App_Start.RemoveWhitespacesAttribute());
            }
            catch
            {
                HttpRuntime.UnloadAppDomain(); // سبب ری استارت برنامه و آغاز مجدد آن با درخواست بعدی می‌شود
                throw;
            }

        }