HttpClient介绍

http://www.cnblogs.com/shanyou/archive/2012/03/21/2410739.html

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：