C#中的Task.WhenAll和Task.WhenAny方法介绍

一、简介

Task.WhenAll()、Task.WhenAny()这两个与Task.WaitALL()、Task.WaitAny()是有区别的，When是异步的，Wait是同步的。
Task.WhenAll()：所有提供的任务已完成时，创建将完成的任务。
Task.WhenAny()：任何提供的任务已完成时，创建将完成的任务。

二、代码案例

Task.WhenAll

代码：

    class Program
  {
      public class DownLoadTest
      {
          Stopwatch watch = new Stopwatch();
          public DownLoadTest()
          {
              watch.Start();
          }
          public string DownLoadString(string url)
          {
              Console.WriteLine(string.Format("下載{0}開始運行 :{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
              WebClient wc = new WebClient();
              string str = wc.DownloadString(url);
              Console.WriteLine(string.Format("下載{0}運行結束 :{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
              return str;
          }

          public async Task<string> DoRunStringAsync(string url)
          {
              Console.WriteLine(string.Format("異步程序獲取{0}開始運行:{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
              var task = await DownLoadStringAsync(url);
              Console.WriteLine(string.Format("異步程序獲取{0運行結束:{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
              return task;
          }

          public async Task<string> DownLoadStringAsync(string url)
          {
              string str = await Task.Run(() => DownLoadString(url));
              return str;
          }
      }

      static void Main(string[] args)
      {
          string[] urls = { "https://www.baidu.com/", "https://www.taobao.com/", "https://www.cnblogs.com/" };
          var task = DoRunsAsync(urls);
          //task.Wait(3000);
          Console.WriteLine(string.Format("主線程運行結束."));
          Console.ReadKey();
      }

      private static async Task<string> DoRunsAsync(IEnumerable<string> urls)
      {
          DownLoadTest dwtest = new DownLoadTest();
          List<Task> tasks = new List<Task>();
          foreach (var url in urls)
          {
              var task = dwtest.DoRunStringAsync(url);
              Console.WriteLine("task.Id =" + task.Id + "  url=" + url);
              tasks.Add(task);
          }
          await Task.WhenAll(tasks);
          foreach (var task in tasks)
          {
              Console.WriteLine("task.Id =" + task.Id + "  task.Status=" + task.Status);
          }
          return "";
      }
  }

运行结果：

可以看到await Task.WhenAll(tasks);异步等待了所有的任务完成。

Task.WhenAny

代码：

    class Program
  {
      public class DownLoadTest
      {
          Stopwatch watch = new Stopwatch();
          public DownLoadTest()
          {
              watch.Start();
          }
          public string DownLoadString(string url)
          {
              Console.WriteLine(string.Format("下載{0}開始運行 :{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
              WebClient wc = new WebClient();
              string str = wc.DownloadString(url);
              Console.WriteLine(string.Format("下載{0}運行結束 :{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
              return str;
          }

          public async Task<string> DoRunStringAsync(string url)
          {
              Console.WriteLine(string.Format("異步程序獲取{0}開始運行:{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
              var task = await DownLoadStringAsync(url);
              Console.WriteLine(string.Format("異步程序獲取{0運行結束:{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
              return task;
          }

          public async Task<string> DownLoadStringAsync(string url)
          {
              string str = await Task.Run(() => DownLoadString(url));
              return str;
          }
      }

      static void Main(string[] args)
      {
          string[] urls = { "https://www.baidu.com/", "https://www.taobao.com/", "https://www.cnblogs.com/" };
          var task = DoRunsAsync(urls);
          task.Wait(1000);
          Console.WriteLine(string.Format("主線程運行結束."));
          Console.ReadKey();
      }

      private static async Task<string> DoRunsAsync(IEnumerable<string> urls)
      {
          DownLoadTest dwtest = new DownLoadTest();
          List<Task> tasks = new List<Task>();
          foreach (var url in urls)
          {
              var task = dwtest.DoRunStringAsync(url);
              Console.WriteLine("task.Id =" + task.Id + "  url=" + url);
              tasks.Add(task);
          }
          await Task.WhenAny(tasks);
          foreach (var task in tasks)
          {
              Console.WriteLine("task.Id =" + task.Id + "  task.Status=" + task.Status);
          }
          return "";
      }
  }

运行结果：

Task.WhenAny(tasks)至少等待了一个任务的完成。这里https://www.baidu.com/先下载完，就直接终止等待 。

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