C#中HttpWebRequest、WebClient、HttpClient的使⽤详解HttpWebRequest:
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。
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3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35　//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：
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res
ponse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。
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5 6 7 8 9 10 11 12 13 14 15 16public 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");
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8 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67      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。。。");
}
}
HttpClient：
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
单例模式：
单例模式（Singleton Pattern）这种类型的设计模式属于创建型模式，它提供了⼀种创建对象的最佳⽅式。
这种模式涉及到⼀个单⼀的类，该类负责创建⾃⼰的对象，同时确保只有单个对象被创建。这个类提供了⼀种访问其唯⼀的对象的⽅式，可以直接访问，不需要实例化该类的对象。
单例创建步骤：1、定义静态私有对象；2、定义私有构造函数；3、提供公共获取对象⽅法；
单例模式⼀般分为两种实现模式：懒汉模式、饿汉模式（以下为Java代码实现）
懒汉模式:  默认不会实例化，什么时候⽤什么时候new
1 2 3 4 5 6 7 8 9 10 11public class Singleton {
private static Singleton instance = null;  private Singleton (){}
public static Singleton getInstance() {  if(instance == null) {
instance = new Singleton();
}
return instance;
}
}
这种⽅式是最基本的实现⽅式，这种实现最⼤的问题就是不⽀持多线程。因为没有加锁 synchronized，所以严格意义上它并不算单例模式。这种⽅式 lazy loading 很明显，不要求线程安全，在多线程不能正常⼯作。
饿汉模式：  类初始化时，会⽴即加载该对象，线程天⽣安全，调⽤效率⾼
1 2 3 4 5 6 7public class Singleton {
private static Singleton instance = new Singleton();  private Singleton (){}
public static Singleton getInstance() {
return instance;
}
}
双检锁/双重校验锁（DCL，即 double-checked locking）：这种⽅式采⽤双锁机制，安全且在多线程情况下能保持⾼性能
1 2 3 4 5 6 7 8 9 10 11 12 13 14public class Singleton {
private volatile static Singleton singleton;  private Singleton (){}
public static Singleton getSingleton() {  if(singleton == null) {
synchronized (Singleton.class) {
java单例模式懒汉和饿汉if(singleton == null) {
singleton = new Singleton();
}
}
}
return singleton;
}
}
HttpClient：1
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5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22public class HttpClientHelper
{
private static readonly object LockObj = new object();    private static HttpClient client = null;
public HttpClientHelper() {
GetInstance();
}
public static HttpClient GetInstance()
{
if(client == null)
{
lock(LockObj)
{
if(client == null)
{
client = new HttpClient();
}
}
23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81      }
return client;
}
public async Task<string> PostAsync(string url, string strJson)//post异步请求⽅法
{
try
{
HttpContent content = new StringContent(strJson);
content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/json");        //由HttpClient发出异步Post请求
HttpResponseMessage res = await client.PostAsync(url, content);
if(res.StatusCode == System.Net.HttpStatusCode.OK)
{
string str = res.Content.ReadAsStringAsync().Result;
return str;
}
else
return null;
}
catch(Exception ex)
{
return null;
}
}
public string Post(string url, string strJson)//post同步请求⽅法
{
try
{
HttpContent content = new StringContent(strJson);
content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/js
on");        //client.DefaultRequestHeaders.Connection.Add("keep-alive");
//由HttpClient发出Post请求
Task<HttpResponseMessage> res = client.PostAsync(url, content);
if(res.Result.StatusCode == System.Net.HttpStatusCode.OK)
{
string str = res.Result.Content.ReadAsStringAsync().Result;
return str;
}
else
return null;
}
catch(Exception ex)
{
return null;
}
}
public string Get(string url)
{
try
{
var responseString = client.GetStringAsync(url);
return responseString.Result;
}
catch(Exception ex)
{
return null;
}
}
}
HttpClient有预热机制，第⼀次请求⽐较慢；可以通过初始化前发送⼀次head请求解决：
1 2 3 4 5_httpClient = new HttpClient() { BaseAddress = new Uri(BASE_ADDRESS) };    //帮HttpClient热⾝
_httpClient.SendAsync(new HttpRequestMessage {
Method = new HttpMethod("HEAD"),
6 7        RequestUri = new Uri(BASE_ADDRESS + "/") })      .Result.EnsureSuccessStatusCode();
三者区别列表：

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