Asp.net web Api源码分析-HttpRequestMessage的创建

紧接着前文Asp.net web Api源码分析-如何获取IHttpHandler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MVC4 WebAPI(二)——Web API工作方式
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<bool>(() => requestBase.IsLocal));            request.Properties.Add(HttpPropertyKeys.IncludeErrorDetailKey, new Lazy<bool>(() => !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<Type, object[]> _cacheMulti = new Dictionary<Type, object[]>();    private readonly Dictionary<Type, object> _cacheSingle = new Dictionary<Type, object>();    private readonly Dictionary<Type, object> _defaultServicesSingle = new Dictionary<Type, object>();    private readonly Dictionary<Type, List<object>> _defaultServicesMulti = new Dictionary<Type, List<object>>();    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<IActionValueBinder>(new DefaultActionValueBinder());            SetSingle<IApiExplorer>(new ApiExplorer(configuration));            SetSingle<IAssembliesResolver>(new DefaultAssembliesResolver());            SetSingle<IBodyModelValidator>(new DefaultBodyModelValidator());            SetSingle<IContentNegotiator>(new DefaultContentNegotiator());            SetSingle<IDocumentationProvider>(null); // Missing            SetMultiple<IFilterProvider>(new ConfigurationFilterProvider(),                                      new ActionDescriptorFilterProvider());            SetSingle<IHostBufferPolicySelector>(null);            SetSingle<IHttpActionInvoker>(new ApiControllerActionInvoker());            SetSingle<IHttpActionSelector>(new ApiControllerActionSelector());            SetSingle<IHttpControllerActivator>(new DefaultHttpControllerActivator());            SetSingle<IHttpControllerSelector>(new DefaultHttpControllerSelector(configuration));            SetSingle<IHttpControllerTypeResolver>(new DefaultHttpControllerTypeResolver());            SetSingle<ITraceManager>(new TraceManager());            SetSingle<ITraceWriter>(null);            // This is a priority list. So put the most common binders at the top.             SetMultiple<ModelBinderProvider>(new TypeConverterModelBinderProvider(),                                        new TypeMatchModelBinderProvider(),                                        new KeyValuePairModelBinderProvider(),                                        new ComplexModelDtoModelBinderProvider(),                                        new ArrayModelBinderProvider(),                                        new DictionaryModelBinderProvider(),                                        new CollectionModelBinderProvider(),                                        new MutableObjectModelBinderProvider());            SetSingle<ModelMetadataProvider>(new DataAnnotationsModelMetadataProvider());            SetMultiple<ModelValidatorProvider>(new DataAnnotationsModelValidatorProvider(),                                        new DataMemberModelValidatorProvider(),                                        new InvalidModelValidatorProvider());            // This is an ordered list,so put the most common providers at the top.             SetMultiple<ValueProviderFactory>(new QueryStringValueProviderFactory(),                                           new RouteDataValueProviderFactory());            ModelValidatorCache validatorCache = new ModelValidatorCache(new Lazy<IEnumerable<ModelValidatorProvider>>(() => this.GetModelValidatorProviders()));            configuration.RegisterForDispose(validatorCache);            SetSingle<IModelValidatorCache>(validatorCache);            _serviceTypesSingle = new HashSet<Type>(_defaultServicesSingle.Keys);            _serviceTypesMulti = new HashSet<Type>(_defaultServicesMulti.Keys);            // Reset the caches and the known dependency scope            ResetCache();        }       private void SetSingle<T>(T instance) where T : class        {            _defaultServicesSingle[typeof(T)] = instance;        }        private void SetMultiple<T>(params T[] instances) where T : class        {            var x = (IEnumerable<object>)instances;            _defaultServicesMulti[typeof(T)] = new List<object>(x);        }       private void ResetCache()        {            _cacheLock.EnterWriteLock();            try            {                _cacheSingle.Clear();                _cacheMulti.Clear();                _lastKnownDependencyResolver = _configuration.DependencyResolver;            }            finally            {                _cacheLock.ExitWriteLock();            }        }      public override object GetService(Type serviceType)        {            if (serviceType == null)            {                throw Error.ArgumentNull("serviceType");            }            if (!_serviceTypesSingle.Contains(serviceType))            {                throw Error.Argument("serviceType", SRResources.DefaultServices_InvalidServiceType, serviceType.Name);            }            // Invalidate the cache if the dependency scope has switched            if (_lastKnownDependencyResolver != _configuration.DependencyResolver)            {                ResetCache();            }            object result;            _cacheLock.EnterReadLock();            try            {                if (_cacheSingle.TryGetValue(serviceType, out result))                {                    return result;                }            }            finally            {                _cacheLock.ExitReadLock();            }            // Get the service from DI, outside of the lock. If we're coming up hot, this might            // mean we end up creating the service more than once.            object dependencyService = _configuration.DependencyResolver.GetService(serviceType);            _cacheLock.EnterWriteLock();            try            {                if (!_cacheSingle.TryGetValue(serviceType, out result))                {                    result = dependencyService ?? _defaultServicesSingle[serviceType];                    _cacheSingle[serviceType] = result;                }                return result;            }            finally            {                _cacheLock.ExitWriteLock();            }        } public override IEnumerable<object> GetServices(Type serviceType)        {            if (serviceType == null)            {                throw Error.ArgumentNull("serviceType");            }            if (!_serviceTypesMulti.Contains(serviceType))            {                throw Error.Argument("serviceType", SRResources.DefaultServices_InvalidServiceType, serviceType.Name);            }            // Invalidate the cache if the dependency scope has switched            if (_lastKnownDependencyResolver != _configuration.DependencyResolver)            {                ResetCache();            }            object[] result;            _cacheLock.EnterReadLock();            try            {                if (_cacheMulti.TryGetValue(serviceType, out result))                {                    return result;                }            }            finally            {                _cacheLock.ExitReadLock();            }            // Get the service from DI, outside of the lock. If we're coming up hot, this might            // mean we end up creating the service more than once.            IEnumerable<object> dependencyServices = _configuration.DependencyResolver.GetServices(serviceType);            _cacheLock.EnterWriteLock();            try            {                if (!_cacheMulti.TryGetValue(serviceType, out result))                {                    result = dependencyServices.Where(s => s != null)                                               .Concat(_defaultServicesMulti[serviceType])                                               .ToArray();                    _cacheMulti[serviceType] = result;                }                return result;            }            finally            {                _cacheLock.ExitWriteLock();            }        }

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。