如何使用C#调用雅虎REST服务

.NET Framework提供类处理HTTP请求。这篇文章讲述一下处理GET和POST请求。

概述

简单GET请求

简单POST请求

HTTP授权请求

错误处理

深入阅读

概述

System.Net命名空间包含 HttpWebRequest和 HttpWebResponse类，这两个类可以从web服务器获取数据和使用基于HTTP的服务。通常你需要添加System.Web的引用，这样就可以使用HttpUtility类，这个类可以提供方法对 HTML 和URL的文本进行编码或者解码。

雅虎web服务返回XML数据。有些web服务返回其他格式的数据，比如JSON和序列化的PHP，.NET Framework自从扩展支持处理xml数据之后，处理这种格式的数据就特别简单了。

简单GET请求

接下来的例子获取网页并打印出源码。

C# GET 例1

using System;
using System.IO;
using System.Net;
using System.Text;  // Create the web request
HttpWebRequest request = WebRequest.Create("https://developer.yahoo.com/") as HttpWebRequest;  // Get response
using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)
{  // Get the response stream  StreamReader reader = new StreamReader(response.GetResponseStream());  // Console application output  Console.WriteLine(reader.ReadToEnd());
}

简单POST请求

有些APIs要求你使用POST请求。为了实现这个功能，我们改变请求方法和内容类型，然后将请求的数据写入到数据流（stream）中。

C# POST例1

// We use the HttpUtility class from the System.Web namespace
using System.Web;  Uri address = new Uri("http://api.search.yahoo.com/ContentAnalysisService/V1/termExtraction");  // Create the web request
HttpWebRequest request = WebRequest.Create(address) as HttpWebRequest;  // Set type to POST
request.Method = "POST";
request.ContentType = "application/x-www-form-urlencoded";  // Create the data we want to send
string appId = "YahooDemo";
string context = "Italian sculptors and painters of the renaissance"  + "favored the Virgin Mary for inspiration";
string query = "madonna";  StringBuilder data = new StringBuilder();
data.Append("appid=" + HttpUtility.UrlEncode(appId));
data.Append("&context=" + HttpUtility.UrlEncode(context));
data.Append("&query=" + HttpUtility.UrlEncode(query));  // Create a byte array of the data we want to send
byte[] byteData = UTF8Encoding.UTF8.GetBytes(data.ToString());  // Set the content length in the request headers
request.ContentLength = byteData.Length;  // Write data
using (Stream postStream = request.GetRequestStream())
{  postStream.Write(byteData, 0, byteData.Length);
}  // Get response
using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)
{  // Get the response stream  StreamReader reader = new StreamReader(response.GetResponseStream());  // Console application output  Console.WriteLine(reader.ReadToEnd());
}

HTTP授权请求

del.icio.us API要求你使用授权的请求，使用HTTP授权传递用户名和密码。这个很容易实现的，只要在请求的时候增加NetworkCredentials 就可以了。

C# HTTP AUTHENTICATION

// Create the web request
HttpWebRequest request   = WebRequest.Create("https://api.del.icio.us/v1/posts/recent") as HttpWebRequest;  // Add authentication to request
request.Credentials = new NetworkCredential("username", "password");  // Get response
using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)
{  // Get the response stream  StreamReader reader = new StreamReader(response.GetResponseStream());  // Console application output  Console.WriteLine(reader.ReadToEnd());
}

错误处理

雅虎提供众多基于REST的web服务，不全是使用相同的错误处理方式。有些web服务返回状态码200（表示OK），详细的错误信息在返回来的xml里面，但是有些web服务使用标准的HTTP状态码表示错误信息。请阅读您使用的web服务的文档，了解你所遇到的响应的错误类型。请记住，雅虎基于浏览器授权和HTTP授权是不一样的。调用 HttpRequest.GetResponse() 方法在服务器没有返回状态码200（表示OK），请求超时和网络错误的时候，会引发错误。但是，重定向会自动处理的。

这是一个典型的例子，打印一个网页的内容和基本的HTTP错误代码处理错误。

C# GET 例2

public static void PrintSource(Uri address)
{  HttpWebRequest request;  HttpWebResponse response = null;  StreamReader reader;  StringBuilder sbSource;  if (address == null) { throw new ArgumentNullException("address"); }  try  {  // Create and initialize the web request  request = WebRequest.Create(address) as HttpWebRequest;  request.UserAgent = ".NET Sample";  request.KeepAlive = false;  // Set timeout to 15 seconds  request.Timeout = 15 * 1000;  // Get response  response = request.GetResponse() as HttpWebResponse;  if (request.HaveResponse == true && response != null)  {  // Get the response stream  reader = new StreamReader(response.GetResponseStream());  // Read it into a StringBuilder  sbSource = new StringBuilder(reader.ReadToEnd());  // Console application output  Console.WriteLine(sbSource.ToString());  }  }  catch (WebException wex)  {  // This exception will be raised if the server didn't return 200 - OK  // Try to retrieve more information about the network error  if (wex.Response != null)  {  using (HttpWebResponse errorResponse = (HttpWebResponse)wex.Response)  {  Console.WriteLine(  "The server returned '{0}' with the status code {1} ({2:d}).",  errorResponse.StatusDescription, errorResponse.StatusCode,  errorResponse.StatusCode);  }  }  }  finally  {  if (response != null) { response.Close(); }  }
}

深入阅读

网上相关信息。

HttpWebRequest 类文档
HttpWebResponse 类文档
A Deeper Look at Performing HTTP Requests in an ASP.NET Page
.NET Screen Scraping in depth

翻译这篇文章其实是自己想学习如何使用C#调用REST服务。