using ( var wc = new System.Net.WebClient() ) {var imagebytes = wc.DownloadData( hag.ImagePath );using ( MemoryStream ms = new MemoryStream( imagebytes ) ) {picImage.Image = Image.FromStream( ms );}
}

C#中HttpWebRequest、WebClient、HttpClient的使用

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。

 　　　　//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-8string 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

public 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

public 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

public class Singleton {  private volatile static Singleton singleton;  private Singleton (){}  public static Singleton getSingleton() {  if (singleton == null) {  synchronized (Singleton.class) {  if (singleton == null) {  singleton = new Singleton();  }  }  }  return singleton;  }  
}

HttpClient：

public 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();}}}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;}elsereturn 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/json");//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;}elsereturn 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：

 

_httpClient = new HttpClient() { BaseAddress = new Uri(BASE_ADDRESS) };//帮HttpClient热身_httpClient.SendAsync(new HttpRequestMessage {Method = new HttpMethod("HEAD"), RequestUri = new Uri(BASE_ADDRESS + "/") }).Result.EnsureSuccessStatusCode();

 

三者区别列表：