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中介绍了一个开源的Http Client库RestSharp。在ASP.NET MVC 4中也带来.NET框架中的。它提供了一个灵活的、可扩展的API来访问一切通过HTTP公开的RESTful服务。HttpClient是 的一部分，直接包含在.NET 4.5中，也可以单独安装ASP.NET MVC4,可以通过Nuget包获取，包里面包含以下3部分：

• System.Net.Http: The main NuGet package providing the basic HttpClient and related classes
• System.Net.Http.Formatting: Adds support for serialization, deserialization as well as for many additional features building on top of System.Net.Http
• System.Json: Adds support for JsonVaue which is a mechanism for reading and manipulating JSON documents

HttpClient是接收HttpResponseMessages和发送HttpRequestMessages的主要类，如果你习惯了使用WebClient或者是HttpWebRequest， 需要注意HttpClient和他们不同的地方：

1、在HttpClient实例上配置扩展，设置默认的头部，取消未完成的请求和更多的设置。

2、你通过一个单一的HttpClient实例，它有自己的连接池。

3、HttpClients不与特定的HTTP服务器绑定,你可以使用相同的HttpClient实例提交任何HTTP请求。

4、你可以用HttpClient为特定的站点创建特殊的Client

5、HttpClient采用新的型模式处理异步请求使它更容易管理和协调更多的请求。

下面我们看下具体的代码， 有个很详细Get操作的示例，这个示例是向 发送一个Get请求，获取到Json格式的数据

namespace WorldBankSample 

{      /// <summary>      /// Sample download list of countries from the World Bank Data sources at http://data.worldbank.org/      /// </summary>      class Program      {          static string _address = "http://api.worldbank.org/countries?format=json";            static void Main(string[] args)          {              // Create an HttpClient instance              HttpClient client = new HttpClient();                // Send a request asynchronously continue when complete              client.GetAsync(_address).ContinueWith(                  (requestTask) =>                  {                      // Get HTTP response from completed task.                      HttpResponseMessage response = requestTask.Result;                        // Check that response was successful or throw exception                      response.EnsureSuccessStatusCode();                        // Read response asynchronously as JsonValue and write out top facts for each country                      response.Content.ReadAsAsync<JsonArray>().ContinueWith(                          (readTask) =>                          {                              Console.WriteLine("First 50 countries listed by The World Bank...");                              foreach (var country in readTask.Result[1])                              {                                  Console.WriteLine("   {0}, Country Code: {1}, Capital: {2}, Latitude: {3}, Longitude: {4}",                                      country.Value["name"],                                      country.Value["iso2Code"],                                      country.Value["capitalCity"],                                      country.Value["latitude"],                                      country.Value["longitude"]);                              }                          });                  });                Console.WriteLine("Hit ENTER to exit...");              Console.ReadLine();          }      }  }  下面介绍一个发送Post 请求的示例，示例代码使用默认创建的ASP.NET Web API模板项目：

public class ValuesController : ApiController

   {        // GET /api/values        public IEnumerable<string> Get()        {            return new string[] { "value1", "value2" };        }

       // GET /api/values/5

       public string Get(int id)        {            return "value";        }

       // POST /api/values

       public string Post(string value)        {            return value;        }

       // PUT /api/values/5

       public void Put(int id, string value)        {        }

       // DELETE /api/values/5

       public void Delete(int id)        {        }    }

使用HttpClient示例的代码如下：

          string serviceAddress = "

           HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(serviceAddress);            request.Method = "POST";            request.ContentType = "application/json";            string strContent = @"{ ""value"": ""test""}" ;            using (StreamWriter dataStream = new StreamWriter(request.GetRequestStream()))            {                dataStream.Write(strContent);                dataStream.Close();            }            HttpWebResponse response = (HttpWebResponse)request.GetResponse();            using (StreamReader dataStream = new StreamReader(response.GetResponseStream()))            {                var result = dataStream.ReadToEnd();                Console.WriteLine(result);            }

            Uri serviceReq = new Uri(serviceAddress);

            HttpClient client = new HttpClient();             HttpContent content = new StringContent(@"{ ""value"": ""test""}");             content.Headers.ContentType = new MediaTypeHeaderValue("application/json");

            // Send a request asynchronously continue when complete

            client.PostAsync(serviceReq, content).ContinueWith(                 (requestTask) =>                 {                     // Get HTTP response from completed task.                     HttpResponseMessage response = requestTask.Result;

                    // Check that response was successful or throw exception

                    response.EnsureSuccessStatusCode();

                    // Read response asynchronously as JsonValue and write out top facts for each country

                    response.Content.ReadAsAsync<string>().ContinueWith(                         (readTask) =>                         {                             Console.WriteLine(readTask.Result);

                        });

                });

上面代码首先是创建一个HttpClient实例，设置要Post的数据的格式，然后调用HttpClient的异步请求，获取到的是一个HttpResponseMessage实例，可以在这个实例中检查请求的状态，调用的是一个扩展方法EnsureSuccessStatusCode，如果不是HttpStatusCode.OK，就会抛出一个异常。通过HttpResponseMessage的Content属性获取HttpContent，HttpContent有一些方法辅助处理接收的内容。

对应于使用HttpWebRequest的示例如下：

          string serviceAddress = "

           HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(serviceAddress);            request.Method = "POST";            request.ContentType = "application/json";            string strContent = @"{ ""value"": ""test""}" ;            using (StreamWriter dataStream = new StreamWriter(request.GetRequestStream()))            {                dataStream.Write(strContent);                dataStream.Close();            }            HttpWebResponse response = (HttpWebResponse)request.GetResponse();            using (StreamReader dataStream = new StreamReader(response.GetResponseStream()))            {                var result = dataStream.ReadToEnd();                Console.WriteLine(result);            }

上面是通过代码发送的Http请求，也可以用Fiddlers来验证请求，例如：