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：