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-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。。。");
    }
  }