拦截asp.net输出流做处理, 拦截HTML文本（asp.net webForm版)

对已经生成了HTML的页面做一些输出到客户端之前的处理

方法的原理是：把Response的输出重定向到自定义的容器内，也就是我们的StringBuilder对象里，在HTML所有的向页面输出都变 成了向StringBuilder输出，然后我们对StringBuilder处理完成之后，再把Response的输出重定向到原来的页面上，然后再通 过Response.Write方法把StringBuilder的内容输出到页面上

这里之所以用反射，是因为Response对象的OutPut属性是只读的，通过反编译该类的程序集发现，OutPut实际上是内部私有成员 _writer来实现输出的。因此通过反射来改写该成员的值以实现输出流的重定向。（测试过第三种方法，可行）

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Web;
    using System.Web.UI;
    using System.Web.UI.WebControls;
    using System.Text;
    using System.IO;
    using System.Reflection;  

    public partial class _Default : System.Web.UI.Page
    {
        StringBuilder content = new StringBuilder();
        TextWriter tw_old, tw_new;
        FieldInfo tw_field;  

        protected void Page_Load(object sender, EventArgs e)
        {
            var context = HttpContext.Current;  

            tw_old = context.Response.Output;//Response原来的OutPut
            tw_new = new StringWriter(content);//一个StringWriter,用来获取页面内容
            var type_rp = context.Response.GetType();
            //通过反射获取对象的私有字段
            tw_field = type_rp.GetField("_writer", System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
            tw_field.SetValue(context.Response, tw_new);
        }  

        protected override void Render(HtmlTextWriter writer)
        {
            base.Render(writer);
            //替换回Response的OutPut
            tw_field.SetValue(HttpContext.Current.Response, tw_old);
            //做自己的处理
            content.AppendLine("<!--江湖小子-->");
            HttpContext.Current.Response.Write(content.ToString());
        }
    }  

    方法二，用HttpModul来实现：  

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Web;
    using System.Web.UI;
    using System.IO;
    using System.Text;
    using System.Reflection;  

    /// <summary>
    ///HttpModule 的摘要说明
    /// </summary>
    public class HttpModule : IHttpModule
    {
        private HttpApplication _contextApplication;
        private TextWriter tw_new, tw_old;
        private StringBuilder _content;
        private FieldInfo tw_field;  

        public void Init(HttpApplication context)
        {
            _contextApplication = context;
            _contextApplication.PreRequestHandlerExecute += new EventHandler(_contextApplication_PreRequestHandlerExecute);
        }  

        public void Dispose()
        {
            _contextApplication = null;
            _contextApplication.Dispose();
        }  

        public void _contextApplication_PreRequestHandlerExecute(object sender, EventArgs e)
        {
            HttpContext context = _contextApplication.Context;  

            var _page = context.Handler as System.Web.UI.Page;
            _page.Unload += new EventHandler(_page_Unload);  

            _content = new StringBuilder();
            tw_old = context.Response.Output;//Response原来的OutPut
            tw_new = new StringWriter(_content);//一个StringWriter,用来获取页面内容
            var type_rp = context.Response.GetType();
            tw_field = type_rp.GetField("_writer", System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
            tw_field.SetValue(context.Response, tw_new);
        }  

        void _page_Unload(object sender, EventArgs e)
        {
            //替换回Response的OutPut
            tw_field.SetValue(HttpContext.Current.Response, tw_old);
            //做自己的处理
            _content.AppendLine("<!--江湖小子-->");
            HttpContext.Current.Response.Write(_content.ToString());
        }  

    }  

    方法三：
    public class HttpModule : IHttpModule
    {
        private HttpApplication _contextApplication;
        private TextWriter tw_new, tw_old;
        private StringBuilder _content;
        private FieldInfo tw_field;  

        public void Init(HttpApplication application)
        {
            _contextApplication = application;
            _contextApplication.BeginRequest += new EventHandler(_contextApplication_BeginRequest);
            _contextApplication.EndRequest +=new EventHandler(_contextApplication_EndRequest);
        }  

        void _contextApplication_BeginRequest(object sender, EventArgs e)
        {
            _content = new StringBuilder();
            tw_old = _contextApplication.Response.Output;
            tw_new = new StringWriter(_content);
            var type_rp = _contextApplication.Response.GetType();
            tw_field = type_rp.GetField("_writer", System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
            tw_field.SetValue(_contextApplication.Response, tw_new);
        }  

        void _contextApplication_EndRequest(object sender, EventArgs e)
        {
            tw_field.SetValue(_contextApplication.Response, tw_old);
            //做自己的处理
            _content.AppendLine("<!--jhxz-->");
            _contextApplication.Response.Write(_content.ToString());
        }  

        public void Dispose()
        {
            _contextApplication = null;
            _contextApplication.Dispose();
        }
    }