HttpClient介绍

使用RestSharp 库消费Restful Service 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Http Client。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ASP.NET Web API 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， MSDN code gallery 有个很详细Get操作的示例，这个示例是向World Bank Data Web API 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 = "http://localhost:2650/api/values";
           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 = "http://localhost:2650/api/values";
           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：