C#中的Task.WaitAll和Task.WaitAny方法介绍

一、简介

Task.WaitAll：等待所有提供的 Task 对象完成执行过程。

Task.WaitAny：等待提供的任一 Task 对象完成执行过程。

二、代码案例

Task.WaitAll

代码：

    class Program
    {
        public class DownLoadTest
        {
            Stopwatch watch = new Stopwatch();
            public  DownLoadTest()
            {
                watch.Start();
            }

            public async Task DoRunTaskAsync(string url, CancellationToken ct)
            {
                if (ct.IsCancellationRequested)
                {
                    Console.WriteLine(string.Format("取消{0}的運行 :{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
                    return;
                }
                Console.WriteLine(string.Format("下載{0}開始運行 :{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
                WebClient wc = new WebClient();
                await Task.Run(() =>
                {
                    var task = wc.DownloadStringTaskAsync(url);
                    while (!task.IsCompleted)
                    {
                        if (ct.IsCancellationRequested)
                        {
                            Console.WriteLine(string.Format("取消{0}的運行 :{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
                            return;
                        }
                    }
                    if (task.IsCompleted)
                        Console.WriteLine(string.Format("下載{0}運行結束 :{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
                });
            }
        }

        static void Main(string[] args)
        {

            DownLoadTest dwtest = new DownLoadTest();
            CancellationTokenSource cts = new CancellationTokenSource();
            CancellationToken token = cts.Token;
            var task1 = dwtest.DoRunTaskAsync("https://www.baidu.com/", token);
            var task2 = dwtest.DoRunTaskAsync("https://www.taobao.com/", token);
            var task3 = dwtest.DoRunTaskAsync("https://www.cnblogs.com/", token);
            Task[] tasks = new Task[] { task1, task2, task3 };
            Console.WriteLine("task.Result等待結果打印");
            Task.WaitAll(tasks, 10000);
            cts.Cancel();//超过10s时间取消运行
            Console.WriteLine(task1.Status);
            Console.WriteLine(task2.Status);
            Console.WriteLine(task3.Status);
            Console.WriteLine(string.Format("主線程運行結束"));
            Console.ReadKey();
        }
    }

运行结果：

通过运行结果可以看出，可以看到Task.WaitAll(tasks)在等待超时的时间内，同步等待了所有的任务完成。

Task.WaitAny

代码：

    class Program
    {
        public class DownLoadTest
        {
            Stopwatch watch = new Stopwatch();
            public  DownLoadTest()
            {
                watch.Start();
            }

            public async Task DoRunTaskAsync(string url, CancellationToken ct)
            {
                if (ct.IsCancellationRequested)
                {
                    Console.WriteLine(string.Format("取消{0}的運行 :{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
                    return;
                }
                Console.WriteLine(string.Format("下載{0}開始運行 :{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
                WebClient wc = new WebClient();
                await Task.Run(() =>
                {
                    var task = wc.DownloadStringTaskAsync(url);
                    while (!task.IsCompleted)
                    {
                        if (ct.IsCancellationRequested)
                        {
                            Console.WriteLine(string.Format("取消{0}的運行 :{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
                            return;
                        }
                    }
                    if (task.IsCompleted)
                        Console.WriteLine(string.Format("下載{0}運行結束 :{1,4:N0}ms", url, watch.Elapsed.TotalMilliseconds));
                });
            }
        }

        static void Main(string[] args)
        {

            DownLoadTest dwtest = new DownLoadTest();
            CancellationTokenSource cts = new CancellationTokenSource();
            CancellationToken token = cts.Token;
            var task1 = dwtest.DoRunTaskAsync("https://www.baidu.com/", token);
            var task2 = dwtest.DoRunTaskAsync("https://www.taobao.com/", token);
            var task3 = dwtest.DoRunTaskAsync("https://www.cnblogs.com/", token);
            Task[] tasks = new Task[] { task1, task2, task3 };
            Console.WriteLine("task.Result等待結果打印");
            Task.WaitAny(tasks, 4000);
            cts.Cancel();//超过4s时间取消运行
            Console.WriteLine(task1.Status);
            Console.WriteLine(task2.Status);
            Console.WriteLine(task3.Status);
            Console.WriteLine(string.Format("主線程運行結束"));
            Console.ReadKey();
        }
    }

运行结果：

通过运行结果截图可以看出，可以看到，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 。

三、总结

Task.WaitAll阻止当前线程，直到所有其他任务完成执行为止。Task.WaitAny阻止当前线程，至少等待一个任务完成执行为止。如果等待时长设置较长，上述案例结果会一样。

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