紧接着上文Asp.net web Api源码分析-Action的执行 我们的Action已经执行完毕，现在需要把Action的返回结果转化为HttpResponseMessage 实例，我们也知道转化工作主要在HttpRequestMessage的CreateResponse附加方法中，

  public static HttpResponseMessage CreateResponse<T>(this HttpRequestMessage request, HttpStatusCode statusCode, T value, HttpConfiguration configuration)
        {
            if (request == null)
            {
                throw Error.ArgumentNull("request");
            }

            configuration = configuration ?? request.GetConfiguration();
            if (configuration == null)
            {
                throw Error.InvalidOperation(SRResources.HttpRequestMessageExtensions_NoConfiguration);
            }

            IContentNegotiator contentNegotiator = configuration.Services.GetContentNegotiator();
            if (contentNegotiator == null)
            {
                throw Error.InvalidOperation(SRResources.HttpRequestMessageExtensions_NoContentNegotiator, typeof(IContentNegotiator).FullName);
            }

            IEnumerable<MediaTypeFormatter> formatters = configuration.Formatters;

            // Run content negotiation
            ContentNegotiationResult result = contentNegotiator.Negotiate(typeof(T), request, formatters);

            if (result == null)
            {
                // no result from content negotiation indicates that 406 should be sent.
                return new HttpResponseMessage
                {
                    StatusCode = HttpStatusCode.NotAcceptable,
                    RequestMessage = request,
                };
            }
            else
            {
                MediaTypeHeaderValue mediaType = result.MediaType;
                return new HttpResponseMessage
                {
                    // At this point mediaType should be a cloned value (the content negotiator is responsible for returning a new copy)
                    Content = new ObjectContent<T>(value, result.Formatter, mediaType),
                    StatusCode = statusCode,
                    RequestMessage = request
                };
            }
        }

首先这里需要一个IContentNegotiator实例，这里有这么一句代码：

  IContentNegotiator contentNegotiator = configuration.Services.GetContentNegotiator();在DefaultServices中有   SetSingle<IContentNegotiator>(new DefaultContentNegotiator());这句，我们知道默认的contentNegotiator是DefaultContentNegotiator实例。DefaultContentNegotiator构造函数也比较普通，这里我们还需要一个数据的格式化Formatters，这里有这么一句 IEnumerable<MediaTypeFormatter> formatters = configuration.Formatters;，在Asp.net web Api源码分析-HttpServer的创建
曾提到Formatters主要有JsonMediaTypeFormatter,XmlMediaTypeFormatter, FormUrlEncodedMediaTypeFormatter,JQueryMvcFormUrlEncodedFormatter这4个。接着我们知道要干什么了，需要把我们的value转换为需要的格式，这里创建一个  ContentNegotiationResult result = contentNegotiator.Negotiate(typeof(T), request, formatters);实例，其中Negotiate的主要实现如下：

public virtual ContentNegotiationResult Negotiate(Type type, HttpRequestMessage request, IEnumerable<MediaTypeFormatter> formatters)
        {
             ....。。。。。。
            // Go through each formatter to compute how well it matches.
            Collection<MediaTypeFormatterMatch> matches = ComputeFormatterMatches(type, request, formatters);

            // Select best formatter match among the matches
            MediaTypeFormatterMatch bestFormatterMatch = SelectResponseMediaTypeFormatter(matches);

            // We found a best formatter
            if (bestFormatterMatch != null)
            {
                // Find the best character encoding for the selected formatter
                Encoding bestEncodingMatch = SelectResponseCharacterEncoding(request, bestFormatterMatch.Formatter);
                if (bestEncodingMatch != null)
                {
                    bestFormatterMatch.MediaType.CharSet = bestEncodingMatch.WebName;
                }

                MediaTypeHeaderValue bestMediaType = bestFormatterMatch.MediaType;
                MediaTypeFormatter bestFormatter = bestFormatterMatch.Formatter.GetPerRequestFormatterInstance(type, request, bestMediaType);
                return new ContentNegotiationResult(bestFormatter, bestMediaType);
            }

            return null;
        }
这里的Negotiate方法实现比较复杂，首先找到当前请求支持的一个MediaTypeFormatterMatch集合，然后再从这个集合中找到找最合适一个bestFormatterMatch （在实际开发中这个实例往往都是JsonMediaTypeFormatter），如果找到那么我们继续找一个bestEncodingMatch （最合适的一个编码很多时候这里是utf-8），这里最后调用GetPerRequestFormatterInstance方法得到一个MediaTypeFormatter实例，然后用这么实例创建一个ContentNegotiationResult实例。

现在我们回到CreateResponse方法中来，最后直接返回一个 HttpResponseMessage实例。

MediaTypeHeaderValue mediaType = result.MediaType;

                return new HttpResponseMessage
                {
                    // At this point mediaType should be a cloned value (the content negotiator is responsible for returning a new copy)
                    Content = new ObjectContent<T>(value, result.Formatter, mediaType),
                    StatusCode = statusCode,
                    RequestMessage = request
                };

这里的ObjectContent的创建也比较一般，我们也就忽略它吧。

现在我们回到HttpControllerHandler的BeginProcessRequest方法中来，

 Task responseBodyTask = _server.Value.SendAsync(request, CancellationToken.None)
                .Then(response => ConvertResponse(httpContextBase, response, request));

SendAsync已经执行完毕了，我们再来看看ConvertResponse吧，它主要就是把HttpResponseMessage的信息写到HttpResponseBase信息做。

   internal static Task ConvertResponse(HttpContextBase httpContextBase, HttpResponseMessage response, HttpRequestMessage request)
        {
            Contract.Assert(httpContextBase != null);
            Contract.Assert(request != null);

            // A null response creates a 500 with no content
            if (response == null)
            {
                CreateEmptyErrorResponse(httpContextBase.Response);
                return TaskHelpers.Completed();
            }

            CopyResponseStatusAndHeaders(httpContextBase, response);

            CacheControlHeaderValue cacheControl = response.Headers.CacheControl;

            // TODO 335085: Consider this when coming up with our caching story
            if (cacheControl == null)
            {
                // DevDiv2 #332323. ASP.NET by default always emits a cache-control: private header.
                // However, we don't want requests to be cached by default.
                // If nobody set an explicit CacheControl then explicitly set to no-cache to override the
                // default behavior. This will cause the following response headers to be emitted:
                //     Cache-Control: no-cache
                //     Pragma: no-cache
                //     Expires: -1
                httpContextBase.Response.Cache.SetCacheability(HttpCacheability.NoCache);
            }

            // Asynchronously write the response body.  If there is no body, we use
            // a completed task to share the Finally() below.
            // The response-writing task will not fault -- it handles errors internally.
            Task writeResponseContentTask = (response.Content == null)
                                        ? TaskHelpers.Completed()
                                        : WriteResponseContentAsync(httpContextBase, response, request);

            return writeResponseContentTask.Finally(() =>
            {
                request.DisposeRequestResources();
                request.Dispose();
                response.Dispose();
            });
        }

这里的CopyResponseStatusAndHeaders方法比较简单我也不多说了，就是把HttpResponseMessage中的StatusAndHeaders信息写到 httpContextBase.Response中去，

我们还是关注一下WriteResponseContentAsync的实现吧：

 internal static Task WriteResponseContentAsync(HttpContextBase httpContextBase, HttpResponseMessage response, HttpRequestMessage request)
        {
            HttpResponseBase httpResponseBase = httpContextBase.Response;
            HttpContent responseContent = response.Content;

            var unused = response.Content.Headers.ContentLength;
            CopyHeaders(response.Content.Headers, httpContextBase);

            // Select output buffering based on the user-controlled buffering policy
            bool isBuffered = _bufferPolicySelector.Value != null ? _bufferPolicySelector.Value.UseBufferedOutputStream(response) : true;
            httpResponseBase.BufferOutput = isBuffered;

            return isBuffered
                    ? WriteBufferedResponseContentAsync(httpContextBase, responseContent, request)
                    : WriteStreamedResponseContentAsync(httpResponseBase, responseContent);
        }

前面CopyResponseStatusAndHeaders方法把HttpResponseMessage中的Headers信息写到 httpContextBase.Response中去了，这里继续把response.Content.Headers信息写到 httpContextBase.Response中去。

private static readonly Lazy<IHostBufferPolicySelector> _bufferPolicySelector =
            new Lazy<IHostBufferPolicySelector>(() => GlobalConfiguration.Configuration.Services.GetHostBufferPolicySelector());

在GlobalConfiguration中有这么一句 config.Services.Replace(typeof(IHostBufferPolicySelector), new WebHostBufferPolicySelector());所以我们知道_bufferPolicySelector这里其实是一个WebHostBufferPolicySelector实例，这里调用WebHostBufferPolicySelector的UseBufferedOutputStream方法来获取当前输出信息是否采用输出缓存，一般情况下这个个方法返回true，主要实现如下：

 public virtual bool UseBufferedOutputStream(HttpResponseMessage response)
        {
            HttpContent content = response.Content;
            if (content != null)
            {
                long? contentLength = content.Headers.ContentLength;
                if (contentLength.HasValue && contentLength.Value >= 0)
                {
                    return false;
                }
                return !(content is StreamContent || content is PushStreamContent);
            }

            return false;
        }
    }

一般情况下response.Content.Headers.ContentLength为null。所以后面调用的是WriteBufferedResponseContentAsync方法，而该方法的主要实现就一句代码

 Task  writeResponseContentTask = HttpResponseMessage.Content.CopyToAsync(HttpContextBase.Response.OutputStream);把当前HttpResponseMessage.Content中的数据全部写到HttpContextBase.Response.OutputStream中去。到这里我们的BeginProcessRequest就差不多执行完了，后面就是调用回调函数的事情了。

到本章为止，整个web api的主要流程就说完了，这个系列中有关参数的具体绑定和返回值的格式化我是忽略了的，他们的实现都相对比较复杂，后面再抽时间来看看他们是如何实现 吧，个人对web api也不是很熟悉，发现它和mvc 中的很多代码相似，相比之下mvc的使用比web api要广泛得多，所以这里建议大家多读读mvc的源码，读了之后再来读web api源码相对要轻松很多了。