.NET HttpWebRequest、WebClient、HttpClient

HttpWebRequest类与HttpRequest类的区别。
HttpRequest类的对象用于服务器端【Request.QueryString、Request.From、Request[],Rquest.Params】，获取客户端传来的请求的信息，包括HTTP报文传送过来的所有信息。
而HttpWebRequest用于客户端[请求服务器的]，拼接请求的HTTP报文并发送等。
HttpWebRequest这个类非常强大，强大的地方在于它封装了几乎HTTP请求报文里需要用到的东西，以致于能够能够发送任意的HTTP请求并获得服务器响应(Response)信息。
HttpWebRequest:
继承 WebRequest
命名空间： System.Net，这是.NET创建者最初开发用于使用HTTP请求的标准类。使用HttpWebRequest可以让开发者控制请求/响应流程的各个方面，如 timeouts, cookies, headers, protocols。另一个好处是HttpWebRequest类不会阻塞UI线程。例如，当您从响应很慢的API服务器下载大文件时，您的应用程序的UI不会停止响应。HttpWebRequest通常和WebResponse一起使用，一个发送请求，一个获取数据。HttpWebRquest更为底层一些，能够对整个访问过程有个直观的认识，但同时也更加复杂一些。
　//POST方法
public static string HttpPost(string Url, string postDataStr)
{
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Url);
request.Method = “POST”;
request.ContentType = “application/x-www-form-urlencoded”;
Encoding encoding = Encoding.UTF8;
byte[] postData = encoding.GetBytes(postDataStr);
request.ContentLength = postData.Length;
Stream myRequestStream = request.GetRequestStream();
myRequestStream.Write(postData, 0, postData.Length);
myRequestStream.Close();
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
Stream myResponseStream = response.GetResponseStream();
StreamReader myStreamReader = new StreamReader(myResponseStream, encoding);
string retString = myStreamReader.ReadToEnd();
myStreamReader.Close();
myResponseStream.Close();

return retString;
}
//GET方法
public static string HttpGet(string Url, string postDataStr)
{
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Url + (postDataStr == “” ? “” : “?”) + postDataStr);
request.Method = “GET”;
request.ContentType = “text/html;charset=UTF-8”;
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
Stream myResponseStream = response.GetResponseStream();
StreamReader myStreamReader = new StreamReader(myResponseStream, Encoding.GetEncoding(“utf-8”));
string retString = myStreamReader.ReadToEnd();
myStreamReader.Close();
myResponseStream.Close();
return retString;
}

WebClient：
public abstract class WebRequest : MarshalByRefObject, ISerializable
public class HttpWebRequest : WebRequest, ISerializable
public class WebClient : Component
命名空间System.Net，WebClient是一种更高级别的抽象，是HttpWebRequest为了简化最常见任务而创建的，使用过程中你会发现他缺少基本的header，timeoust的设置，不过这些可以通过继承httpwebrequest来实现。相对来说，WebClient比WebRequest更加简单，它相当于封装了request和response方法，不过需要说明的是，**Webclient和WebRequest继承的是不同类，两者在继承上没有任何关系。**使用WebClient可能比HttpWebRequest直接使用更慢（大约几毫秒），但却更为简单，减少了很多细节，代码量也比较少。

public class WebClientHelper
{
public static string DownloadString(string url)
{
WebClient wc = new WebClient();
//wc.BaseAddress = url; //设置根目录
wc.Encoding = Encoding.UTF8; //设置按照何种编码访问，如果不加此行，获取到的字符串中文将是乱码
string str = wc.DownloadString(url);
return str;
}
public static string DownloadStreamString(string url)
{
WebClient wc = new WebClient();
wc.Headers.Add(“User-Agent”, “Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/76.0.3809.132 Safari/537.36”);
Stream objStream = wc.OpenRead(url);
StreamReader _read = new StreamReader(objStream, Encoding.UTF8); //新建一个读取流，用指定的编码读取，此处是utf-8
string str = _read.ReadToEnd();
objStream.Close();
_read.Close();
return str;
}

public static void DownloadFile(string url, string filename)
{
  WebClient wc = new WebClient();
  wc.DownloadFile(url, filename);   //下载文件
}

public static void DownloadData(string url, string filename)
{
  WebClient wc = new WebClient();
  byte [] bytes = wc.DownloadData(url);  //下载到字节数组
  FileStream fs = new FileStream(filename, FileMode.Create);
  fs.Write(bytes, 0, bytes.Length); 
  fs.Flush();
  fs.Close();
}

public static void DownloadFileAsync(string url, string filename)
{
  WebClient wc = new WebClient();
  wc.DownloadFileCompleted += DownCompletedEventHandler;
  wc.DownloadFileAsync(new Uri(url), filename);
  Console.WriteLine("下载中。。。");
}
private static void DownCompletedEventHandler(object sender, AsyncCompletedEventArgs e)
{
  Console.WriteLine(sender.ToString());  //触发事件的对象
  Console.WriteLine(e.UserState);
  Console.WriteLine(e.Cancelled);
  Console.WriteLine("异步下载完成！");
}

public static void DownloadFileAsync2(string url, string filename)
{
  WebClient wc = new WebClient();
  wc.DownloadFileCompleted += (sender, e) =>
  {
    Console.WriteLine("下载完成!");
    Console.WriteLine(sender.ToString());
    Console.WriteLine(e.UserState);
    Console.WriteLine(e.Cancelled);
  };
  wc.DownloadFileAsync(new Uri(url), filename);
  Console.WriteLine("下载中。。。");
}
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}
HttpClient：
public class HttpClient : HttpMessageInvoker
HttpClient是.NET4.5引入的一个HTTP客户端库，其命名空间为 System.Net.Http ，.NET 4.5之前我们可能使用WebClient和HttpWebRequest来达到相同目的。HttpClient利用了最新的面向任务模式，使得处理异步请求非常容易。它适合用于多次请求操作，一般设置好默认头部后，可以进行重复多次的请求，基本上用一个实例可以提交任何的HTTP请求。HttpClient有预热机制，第一次进行访问时比较慢，所以不应该用到HttpClient就new一个出来，应该使用单例或其他方式获取HttpClient的实例
static void Main(string[] args)
{
const string GetUrl = “http://xxxxxxx/api/UserInfo/GetUserInfos”;//查询用户列表的接口,Get方式访问
const string PostUrl = “http://xxxxxxx/api/UserInfo/AddUserInfo”;//添加用户的接口，Post方式访问

        //使用Get请求
        GetFunc(GetUrl);

        UserInfo user = new UserInfo { Name = "jack", Age = 23 };
        string userStr = JsonHelper.SerializeObject(user);//序列化
        //使用Post请求
        PostFunc(PostUrl, userStr);
        Console.ReadLine();
    }

    /// 

    /// Get请求
    /// 
    /// 
    static async void GetFunc(string path)
    {
        //消息处理程序
        HttpClientHandler handler = new HttpClientHandler() { AutomaticDecompression = DecompressionMethods.GZip };
        HttpClient httpClient = new HttpClient();
        //异步get请求
        HttpResponseMessage response = await httpClient.GetAsync(path);
        //确保响应正常，如果响应不正常EnsureSuccessStatusCode()方法会抛出异常
        response.EnsureSuccessStatusCode();
        //异步读取数据，格式为String
        string resultStr = await response.Content.ReadAsStringAsync();
        Console.WriteLine(resultStr);
    }

    /// 

    /// Post请求
    /// 
    /// 
    /// 
    static async void PostFunc(string path, string data)
    {
        HttpClientHandler handler = new HttpClientHandler() { AutomaticDecompression = DecompressionMethods.GZip };
        HttpClient httpClient = new HttpClient(handler);
        //HttpContent是HTTP实体正文和内容标头的基类。
        HttpContent httpContent = new StringContent(data, Encoding.UTF8, "text/json");
        //httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("BasicAuth", Ticket);//验证请求头赋值
        //httpContent.Headers.Add(string name,string value) //添加自定义请求头

        //发送异步Post请求
        HttpResponseMessage response = await httpClient.PostAsync(path, httpContent);
        response.EnsureSuccessStatusCode();
        string resultStr = await response.Content.ReadAsStringAsync();
        Console.WriteLine(resultStr);
    }
}
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注意：因为HttpClient有预热机制，第一次进行访问时比较慢，所以我们最好不要用到HttpClient就new一个出来，应该使用单例或其他方式获取HttpClient的实例