原创：截取HttpResponse输出流

传统方法如下：

 protected override void Render(HtmlTextWriter writer)
 {
           

string content = string.Empty;
            StringWriter stringWriter = new StringWriter();
            HtmlTextWriter htmlWriter = new HtmlTextWriter(stringWriter);
            try
            {
                // 将当前页面的内容呈现到临时的 HtmlTextWriter 对象中  
                base.Render(htmlWriter);
                htmlWriter.Close();
                // 得到当前页面的全部内容  
                content = stringWriter.ToString();
                // 替换页面中的部分内容  
                string newContent = content.Replace("[mxh]", "孟宪会");
                // 将新页面的内容显示出来  
                writer.Write(newContent);
            }
            catch { }
            finally
            {
                stringWriter.Dispose();
                htmlWriter.Close();
                htmlWriter.Dispose();
            }

}

//原创方法如下：这个方法与在render方法里自己实例化一个HtmlTextWriter ，然后调用base.Render的时候传入自己的实例化的HtmlTextWriter 然后获取输出html的区别是：前者能够截获在aspx页面中通过Response.Write的字符串并且顺序正确，后者在Render方法里的字符串content不能截取页面中通过Response.Write的字符串所以会导致输出以后html内容顺序错乱。

protected override void Render(HtmlTextWriter writer)
        {
           

base.Render(writer);

            //Response.Output其实就是一个HttpWriter,Response.OutputStream其实就是HttpResponseStream,Object.ReferenceEquals (Response.Output,(Response.OutputStream as HttpResponseStream)._writer)为true

            BindingFlags bind = BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.IgnoreCase | BindingFlags.GetField;
            //因为HttpWriter._charBuffer这个字符数组的长度是1024，所以推测，一旦某一段字符串超过1024就会创建一个IHttpResponseElement，然后加入到HttpWriter._buffers,HttpWriter._buffers的每一个元素都实现了IHttpResponseElement接口,具体类型可能是HttpResponseUnmanagedBufferElement，HttpSubstBlockResponseElement等类型
            ArrayList arr = (ArrayList)Response.Output.GetType().GetField("_buffers", bind).GetValue(Response.Output);

            Assembly systemWeb = Assembly.Load("System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a");
            Type type = systemWeb.GetType("System.Web.IHttpResponseElement");
            MethodInfo method = type.GetMethod("GetBytes");
            StringBuilder sb = new StringBuilder(5000);
            //遍历每一个buffer，获取buffer里存储的字符数组，然后转换为字符串
            for (int i = 0; i < arr.Count; i++)
            {
                byte[] buffer = (byte[])method.Invoke(arr[i], null);
                //使用当前编码得出已经存储到HttpWriter._buffers中的字符串
                sb.Append(Response.ContentEncoding.GetString(buffer));
            }
            //获取HttpWriter的字符数组缓冲区
            char[] charBuffer = (char[])Response.Output.GetType().GetField("_charBuffer", bind).GetValue(Response.Output);
            int charBufferLength = (int)Response.Output.GetType().GetField("_charBufferLength", bind).GetValue(Response.Output);
            int charBufferFree = (int)Response.Output.GetType().GetField("_charBufferFree", bind).GetValue(Response.Output);
            //charBufferLength - charBufferFree 等于字符数组缓冲区已经使用的字符数量
            for (int i = 0; i < charBufferLength - charBufferFree; i++)
            {
                sb.Append(charBuffer[i]);
            }

        }