紧接着前文我们已经得到了HttpControllerHandler实例，它是一个IHttpAsyncHandler类型，我们来看看它的BeginProcessRequest方法是如何实现：

protected virtual IAsyncResult BeginProcessRequest(HttpContextBase httpContextBase, AsyncCallback callback, object state)        {            HttpRequestMessage request = httpContextBase.GetHttpRequestMessage() ?? ConvertRequest(httpContextBase);            request.Properties[HttpPropertyKeys.HttpRouteDataKey] = _routeData;            Task responseBodyTask = _server.Value.SendAsync(request, CancellationToken.None)                .Then(response => ConvertResponse(httpContextBase, response, request));            TaskWrapperAsyncResult result = new TaskWrapperAsyncResult(responseBodyTask, state);            if (callback != null)            {                if (result.IsCompleted)                {                    result.CompletedSynchronously = true;                    callback(result);                }                else                {                    result.CompletedSynchronously = false;                    responseBodyTask.ContinueWith(_ =>                    {                        callback(result);                    });                }            }            return result;        }

 我想这个方法的大致逻辑大家一看也就明白了，这里我们只关心  HttpRequestMessage request = httpContextBase.GetHttpRequestMessage() ?? ConvertRequest(httpContextBase);这句，创建HttpRequestMessage实例，有关 HttpRequestMessage的一些介绍大家可以参考

其中GetHttpRequestMessage方法的实现非常简单：

internal static readonly string HttpRequestMessageKey = "MS_HttpRequestMessage";

public static HttpRequestMessage GetHttpRequestMessage(this HttpContextBase context)

{

     return context.Items[HttpRequestMessageKey] as HttpRequestMessage;

}

public static void SetHttpRequestMessage(this HttpContextBase context, HttpRequestMessage request)

{

    context.Items.Add(HttpRequestMessageKey, request);

}

所以这里的GetHttpRequestMessage并没有真正创建HttpRequestMessage，ConvertRequest方法才是真正创建HttpRequestMessage的地方。

internal static HttpRequestMessage ConvertRequest(HttpContextBase httpContextBase)        {            Contract.Assert(httpContextBase != null);            HttpRequestBase requestBase = httpContextBase.Request;            HttpMethod method = HttpMethodHelper.GetHttpMethod(requestBase.HttpMethod);            Uri uri = requestBase.Url;            HttpRequestMessage request = new HttpRequestMessage(method, uri);            IHostBufferPolicySelector policySelector = _bufferPolicySelector.Value;            bool isInputBuffered = policySelector == null ? true : policySelector.UseBufferedInputStream(httpContextBase);            Stream inputStream = isInputBuffered                                    ? requestBase.InputStream                                    : httpContextBase.ApplicationInstance.Request.GetBufferlessInputStream();            request.Content = new StreamContent(inputStream);            foreach (string headerName in requestBase.Headers)            {                string[] values = requestBase.Headers.GetValues(headerName);                AddHeaderToHttpRequestMessage(request, headerName, values);            }            request.Properties.Add(HttpContextBaseKey, httpContextBase);            request.Properties.Add(HttpPropertyKeys.RetrieveClientCertificateDelegateKey, _retrieveClientCertificate);            request.Properties.Add(HttpPropertyKeys.IsLocalKey, new Lazy
     
      (() => requestBase.IsLocal));            request.Properties.Add(HttpPropertyKeys.IncludeErrorDetailKey, new Lazy
      
       (() => !httpContextBase.IsCustomErrorEnabled));            return request;        }
      
     

 其中

    HttpRequestBase requestBase = httpContextBase.Request;

            HttpMethod method = HttpMethodHelper.GetHttpMethod(requestBase.HttpMethod);

            Uri uri = requestBase.Url;

            HttpRequestMessage request = new HttpRequestMessage(method, uri);

这几句代码很简单也很好明白，我想我就不多说了，而下面的几句代码页很好理解

    IHostBufferPolicySelector policySelector = _bufferPolicySelector.Value;

            bool isInputBuffered = policySelector == null ? true : policySelector.UseBufferedInputStream(httpContextBase);

            Stream inputStream = isInputBuffered

                                    ? requestBase.InputStream

                                    : httpContextBase.ApplicationInstance.Request.GetBufferlessInputStream();

 request.Content = new StreamContent(inputStream);

isInputBuffered是否使用输入缓存，这里默认返回true，这几句主要是 获取请求的输入流isInputBuffered? requestBase.InputStream: httpContextBase.ApplicationInstance.Request.GetBufferlessInputStream();， 然后设置HttpRequestMessage的Content属性，

 foreach (string headerName in requestBase.Headers)

            {

                string[] values = requestBase.Headers.GetValues(headerName);

                AddHeaderToHttpRequestMessage(request, headerName, values);

            }

这句也很好明白，就是把Request的header信息按需添加到HttpRequestMessage的Content.Headers里面， 最后在HttpRequestMessage的Properties属性中添加一些暂存信息。这里的Properties属性没得说它是 IDictionary<string, object>类型。

现在我们好好分析一下这里面的一些细节的东西。

首先我们来看看    IHostBufferPolicySelector policySelector = _bufferPolicySelector.Value;这一句，这里的_bufferPolicySelector是一个延迟加载对象，其定义如下：

  private static readonly Lazy<IHostBufferPolicySelector> _bufferPolicySelector =

            new Lazy<IHostBufferPolicySelector>(() => GlobalConfiguration.Configuration.Services.GetHostBufferPolicySelector());

在GlobalConfiguration类中有这么一句

    config.Services.Replace(typeof(IHostBufferPolicySelector), new WebHostBufferPolicySelector());这里我们可以知道_bufferPolicySelector.Value其实就是一个WebHostBufferPolicySelector实例，该实例的UseBufferedInputStream方法返回true，表示使用输入缓存。

大家应该还记得前面在说HttpConfiguration的构造函数有这么一句吧：

   Services = new DefaultServices(this); 其中Services是ServicesContainer类型。

DefaultServices的代码比较多但是还是比较好理解的，

private readonly ReaderWriterLockSlim _cacheLock = new ReaderWriterLockSlim();    private readonly Dictionary
     
       _cacheMulti = new Dictionary
      
       ();    private readonly Dictionary
       
         _cacheSingle = new Dictionary
        
         ();    private readonly Dictionary
         
           _defaultServicesSingle = new Dictionary
          
           (); private readonly Dictionary
           
            > _defaultServicesMulti = new Dictionary
            
             >(); public DefaultServices(HttpConfiguration configuration) { if (configuration == null) { throw Error.ArgumentNull("configuration"); } _configuration = configuration; // Initialize the dictionary with all known service types, even if the list for that service type is // empty, because we will throw if the developer tries to read or write unsupported types. SetSingle
             
              (new DefaultActionValueBinder()); SetSingle
              
               (new ApiExplorer(configuration)); SetSingle
               
                (new DefaultAssembliesResolver()); SetSingle
                
                 (new DefaultBodyModelValidator()); SetSingle
                 
                  (new DefaultContentNegotiator()); SetSingle
                  
                   (null); // Missing SetMultiple
                   
                    (new ConfigurationFilterProvider(), new ActionDescriptorFilterProvider()); SetSingle
                    
                     (null); SetSingle
                     
                      (new ApiControllerActionInvoker()); SetSingle
                      
                       (new ApiControllerActionSelector()); SetSingle
                       
                        (new DefaultHttpControllerActivator()); SetSingle
                        
                         (new DefaultHttpControllerSelector(configuration)); SetSingle
                         
                          (new DefaultHttpControllerTypeResolver()); SetSingle
                          
                           (new TraceManager()); SetSingle
                           
                            (null); // This is a priority list. So put the most common binders at the top. SetMultiple
                            
                             (new TypeConverterModelBinderProvider(), new TypeMatchModelBinderProvider(), new KeyValuePairModelBinderProvider(), new ComplexModelDtoModelBinderProvider(), new ArrayModelBinderProvider(), new DictionaryModelBinderProvider(), new CollectionModelBinderProvider(), new MutableObjectModelBinderProvider()); SetSingle
                             
                              (new DataAnnotationsModelMetadataProvider()); SetMultiple
                              
                               (new DataAnnotationsModelValidatorProvider(), new DataMemberModelValidatorProvider(), new InvalidModelValidatorProvider()); // This is an ordered list,so put the most common providers at the top. SetMultiple
                               
                                (new QueryStringValueProviderFactory(), new RouteDataValueProviderFactory()); ModelValidatorCache validatorCache = new ModelValidatorCache(new Lazy
                                
                                 
                                  >(() => this.GetModelValidatorProviders())); configuration.RegisterForDispose(validatorCache); SetSingle
                                  
                                   (validatorCache); _serviceTypesSingle = new HashSet
                                   
                                    (_defaultServicesSingle.Keys); _serviceTypesMulti = new HashSet
                                    
                                     (_defaultServicesMulti.Keys); // Reset the caches and the known dependency scope ResetCache(); } private void SetSingle
                                     
                                      (T instance) where T : class { _defaultServicesSingle[typeof(T)] = instance; } private void SetMultiple
                                      
                                       (params T[] instances) where T : class { var x = (IEnumerable
                                       
                                      
                                     
                                    
                                   
                                  
                                 
                                
                               
                              
                             
                            
                           
                          
                         
                        
                       
                      
                     
                    
                   
                  
                 
                
               
              
             
            
           
          
         
        
       
      
     

 在我们的日常开发中会遇到2中情况，一种是一个接口对应着一个具体实现类（Dictionary<Type, object> _defaultServicesSingle），还有一种情况是一个接口对应着多个实现类（Dictionary<Type, List<object>> _defaultServicesMulti），所以这里的DefaultServices实际就是把一个接口和一个或则多个实例关联起来。

现在我们来看看ServicesContainer的GetHostBufferPolicySelector()方法实现，

        public static IHostBufferPolicySelector GetHostBufferPolicySelector(this ServicesContainer services)

        {

            return services.GetService<IHostBufferPolicySelector>();

        }

       private static TService GetService<TService>(this ServicesContainer services)

        {

            if (services == null)

            {

                throw Error.ArgumentNull("services");

            }

            return (TService)services.GetService(typeof(TService));

        }

这里其实还是调用的是DefaultServices的GetService方法，这里的IHostBufferPolicySelector接口 和实例是一一对应的，首先在_cacheSingle中根据type来查找实例，如果找到这直接返回，如果没有找到就根 据 _configuration.DependencyResolver.GetService(serviceType)方法来找实例，如果 _cacheSingle key中不含type，这把该type和实例添加到_cacheSingle中来，

 if (!_cacheSingle.TryGetValue(serviceType, out result))

                {

                    result = dependencyService ?? _defaultServicesSingle[serviceType];

                    _cacheSingle[serviceType] = result;

                }

如果前面的 _configuration.DependencyResolver.GetService(serviceType)方法返回null，这里就用默认的_defaultServicesSingle中type对应的实例。

而HttpConfiguration的DependencyResolver属性如下：

private IDependencyResolver _dependencyResolver = EmptyResolver.Instance;

  public IDependencyResolver DependencyResolver

        {

            get { return _dependencyResolver; }

            set

            {

                if (value == null)

                {

                    throw Error.PropertyNull();

                }

                _dependencyResolver = value;

            }

        }

看见HttpConfiguration的DependencyResolver默认是EmptyResolver.Instance，其GetService实现如下：

  public object GetService(Type serviceType)

        {

            return null;

        }

到这里我们的_bufferPolicySelector的创建就说完了。DependencyResolver这个东东在mvc里面也有，微软做 这个东东，无非就是让我们在需要的时候设置自己的类型实例。可能是自己接触面比较窄，在工作中我还没遇到要设置DependencyResolver这个 属性的需求。

下面我们来看看添加header时候用到一个  AddHeaderToHttpRequestMessage(request, headerName, values);方法，主要的实现代码如下：

  private static void AddHeaderToHttpRequestMessage(HttpRequestMessage httpRequestMessage, string headerName, string[] headerValues)

        {

            if (!httpRequestMessage.Headers.TryAddWithoutValidation(headerName, headerValues))

            {

                httpRequestMessage.Content.Headers.TryAddWithoutValidation(headerName, headerValues);

            }

        }

这里的HttpRequestHeaders、HttpContentHeaders 都继承于HttpHeaders，TryAddWithoutValidation的实现在HttpHeaders类中

public bool TryAddWithoutValidation(string name, IEnumerable<string> values)

{

    if (values == null)

    {

        throw new ArgumentNullException("values");

    }

    if (!this.TryCheckHeaderName(name))

    {

        return false;

    }

    HeaderStoreItemInfo orCreateHeaderInfo = this.GetOrCreateHeaderInfo(name, false);

    foreach (string str in values)

    {

        AddValue(orCreateHeaderInfo, str ?? string.Empty, StoreLocation.Raw);

    }

    return true;

}

这里的具体是怎么添加的我们也就忽略它吧。

到这里我想我们大家对HttpRequestMessage的创建应该比较清楚了吧，直接实例 化   HttpRequestMessage request = new HttpRequestMessage(method, uri);，然后主要设置它的Content属性和添加Content.Headers成员。