在网上经常看见有这样的代码
HttpResponse response = HttpContext.Current.Response;
response.Filter = new PageFilter(response.Filter);
来拦截输出流，自己也做个类似的东东，如asp.net中 js 合并 压缩，现在我也来说说这几个东东是什么吧，需要大家对asp.net的生命周期比较熟悉，如不熟悉的朋友建议先看看ASP.NET 请求处理流程 ASP.NET管线与应用程序生命周期
首先我们来看看这3个属性的源代码吧：
public TextWriter Output
{
get
{
return this._writer;
}
set
{
this._writer = value;
}
}
public Stream OutputStream
{
get
{
if (!this.UsingHttpWriter)
{
throw new HttpException(SR.GetString("OutputStream_NotAvail"));
}
return this._httpWriter.OutputStream;
}
}
public Stream Filter
{
get
{
if (this.UsingHttpWriter)
{
return this._httpWriter.GetCurrentFilter();
}
return null;
}
set
{
if (!this.UsingHttpWriter)
{
throw new HttpException(SR.GetString("Filtering_not_allowed"));
}
this._httpWriter.InstallFilter(value);
IIS7WorkerRequest request = this._wr as IIS7WorkerRequest;
if (request != null)
{
request.ResponseFilterInstalled();
}
}
}
我们看到Filter和OutputStream都用到了一个属性UsingHttpWriter，那这个属性是怎么定义的了
private bool UsingHttpWriter
{
get
{
return ((this._httpWriter != null) && (this._writer == this._httpWriter));
}
}
从这个属性我们可以知道_writer 、_httpWriter实际上是同一个东东，它们的类型是HttpWriter ，而HttpWriter 又继承与TextWriter。现在我们可以解释Output就是_httpWriter，而OutputStream是_httpWriter的OutputStream属性。类HttpWriter 主要代码如下
public Stream OutputStream
{
get
{
return this._stream;
}
}
internal HttpWriter(HttpResponse response) : base(null)
{
this._response = response;
this._stream = new HttpResponseStream(this);
this._buffers = new ArrayList();
this._lastBuffer = null;
this._charBuffer = (char[]) s_Allocator.GetBuffer();
this._charBufferLength = this._charBuffer.Length;
this._charBufferFree = this._charBufferLength;
this.UpdateResponseBuffering();
}
internal HttpResponseStream(HttpWriter writer)
{
this._writer = writer;
}
HttpResponse 在Filter属性设置调用了HttpWriter类的InstallFilter方法，而获取调用了该类的GetCurrentFilter
internal void InstallFilter(Stream filter)
{
if (this._filterSink == null)
{
throw new HttpException(SR.GetString("Invalid_response_filter"));
}
this._installedFilter = filter;
}
internal Stream GetCurrentFilter()
{
if (this._installedFilter != null)
{
return this._installedFilter;
}
if (this._filterSink == null)
{
this._filterSink = new HttpResponseStreamFilterSink(this);
}
return this._filterSink;
}
由以上代码我们可以得知HttpResponse的输出流就是Filter属性设置的流,即HttpResponse的Output和OutputStream属性的输出流都是来自Filter中的流。我们来看看_writer 、_httpWriter它们是在什么时候初始化的了？在HttpResonse中有一个方法
internal void InitResponseWriter()
{
if (this._httpWriter == null)
{
this._httpWriter = new HttpWriter(this);
this._writer = this._httpWriter;
}
}
该方法是由HttpRuntime的ProcessRequestInternal来调用
private void ProcessRequestInternal(HttpWorkerRequest wr)
{
HttpContext context;
try
{
context = new HttpContext(wr, false);
}
catch
{
wr.SendStatus(400, "Bad Request");
wr.SendKnownResponseHeader(12, "text/html; charset=utf-8");
byte[] bytes = Encoding.ASCII.GetBytes("<html><body>Bad Request</body></html>");
wr.SendResponseFromMemory(bytes, bytes.Length);
wr.FlushResponse(true);
wr.EndOfRequest();
return;
}
wr.SetEndOfSendNotification(this._asyncEndOfSendCallback, context);
Interlocked.Increment(ref this._activeRequestCount);
HostingEnvironment.IncrementBusyCount();
try
{
try
{
this.EnsureFirstRequestInit(context);
}
catch
{
if (!context.Request.IsDebuggingRequest)
{
throw;
}
}
context.Response.InitResponseWriter();
IHttpHandler applicationInstance = HttpApplicationFactory.GetApplicationInstance(context);
if (applicationInstance == null)
{
throw new HttpException(SR.GetString("Unable_create_app_object"));
}
if (EtwTrace.IsTraceEnabled(5, 1))
{
EtwTrace.Trace(EtwTraceType.ETW_TYPE_START_HANDLER, context.WorkerRequest, applicationInstance.GetType().FullName, "Start");
}
if (applicationInstance is IHttpAsyncHandler)
{
IHttpAsyncHandler handler2 = (IHttpAsyncHandler) applicationInstance;
context.AsyncAppHandler = handler2;
handler2.BeginProcessRequest(context, this._handlerCompletionCallback, context);
}
else
{
applicationInstance.ProcessRequest(context);
this.FinishRequest(context.WorkerRequest, context, null);
}
}
catch (Exception exception)
{
context.Response.InitResponseWriter();
this.FinishRequest(wr, context, exception);
}
}