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 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 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 DoRunsAsync(IEnumerable urls)
        {
            DownLoadTest dwtest = new DownLoadTest();
            List tasks = new List();
            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 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 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 DoRunsAsync(IEnumerable urls)
        {
            DownLoadTest dwtest = new DownLoadTest();
            List tasks = new List();
            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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