async和await的使用总结 ~ 竟然一直用错了c#中的async和await的使用。。

对于c#中的async和await的使用，没想到我一直竟然都有一个错误。。

。。还是总结太少，这里记录下。

这里以做早餐为例

流程如下：

1. 倒一杯咖啡。
2. 加热平底锅，然后煎两个鸡蛋。
3. 煎三片培根。
4. 烤两片面包。
5. 在烤面包上加黄油和果酱。
6. 倒一杯橙汁。

当使用同步方式实现时，代码是这样的：

using System;
using System.Diagnostics;
using System.Threading.Tasks;

namespace AsyncBreakfast
{
    class Program
    {
        static void Main(string[] args)
        {
            var sw = new Stopwatch();
            sw.Start();
            Coffee cup = PourCoffee();
            Console.WriteLine("coffee is ready");

            Egg eggs = FryEggs(2);
            Console.WriteLine("eggs are ready");

            Bacon bacon = FryBacon(3);
            Console.WriteLine("bacon is ready");

            Toast toast = ToastBread(2);
            ApplyButter(toast);
            ApplyJam(toast);
            Console.WriteLine("toast is ready");

            Juice oj = PourOJ();
            Console.WriteLine("oj is ready");
            Console.WriteLine("Breakfast is ready!");

            Console.WriteLine($"totol time:{sw.ElapsedMilliseconds/1000}");
            Console.ReadKey();
        }

        private static Juice PourOJ()
        {
            Console.WriteLine("Pouring orange juice");
            return new Juice();
        }

        private static void ApplyJam(Toast toast) =>
            Console.WriteLine("Putting jam on the toast");

        private static void ApplyButter(Toast toast) =>
            Console.WriteLine("Putting butter on the toast");

        private static Toast ToastBread(int slices)
        {
            for (int slice = 0; slice < slices; slice++)
            {
                Console.WriteLine("Putting a slice of bread in the toaster");
            }
            Console.WriteLine("Start toasting...");
            Task.Delay(3000).Wait();
            Console.WriteLine("Remove toast from toaster");

            return new Toast();
        }

        private static Bacon FryBacon(int slices)
        {
            Console.WriteLine($"putting {slices} slices of bacon in the pan");
            Console.WriteLine("cooking first side of bacon...");
            Task.Delay(3000).Wait();
            for (int slice = 0; slice < slices; slice++)
            {
                Console.WriteLine("flipping a slice of bacon");
            }
            Console.WriteLine("cooking the second side of bacon...");
            Task.Delay(3000).Wait();
            Console.WriteLine("Put bacon on plate");

            return new Bacon();
        }

        private static Egg FryEggs(int howMany)
        {
            Console.WriteLine("Warming the egg pan...");
            Task.Delay(3000).Wait();
            Console.WriteLine($"cracking {howMany} eggs");
            Console.WriteLine("cooking the eggs ...");
            Task.Delay(3000).Wait();
            Console.WriteLine("Put eggs on plate");

            return new Egg();
        }

        private static Coffee PourCoffee()
        {
            Console.WriteLine("Pouring coffee");
            return new Coffee();
        }
    }
    class Coffee { }
    class Egg { }
    class Bacon { }
    class Toast { }
    class Juice { }
}

运行效果如下：

或表示为这样

同步准备的早餐大约花费了 30 分钟，因为总耗时是每个任务耗时的总和。这里的total time只是用来表示记录下程序运行的时间。

而我以前写的异步代码是这样的：

using System;
using System.Diagnostics;
using System.Threading.Tasks;

namespace AsyncBreakfast
{
    class Program
    {
        static async void Main(string[] args)
        {
            var sw = new Stopwatch();
            sw.Start();
            Coffee cup = PourCoffee();
            Console.WriteLine("coffee is ready");

            Egg eggs = await FryEggsAsync(2);
            Console.WriteLine("eggs are ready");

            Bacon bacon = await FryBaconAsync(3);
            Console.WriteLine("bacon is ready");

            Toast toast = await ToastBreadAsync(2);
            ApplyButter(toast);
            ApplyJam(toast);
            Console.WriteLine("toast is ready");

            Juice oj = PourOJ();
            Console.WriteLine("oj is ready");
            Console.WriteLine("Breakfast is ready!");

            Console.WriteLine($"totol time:{sw.ElapsedMilliseconds/1000}");
            Console.ReadKey();
        }

        static async Task<Toast> MakeToastWithButterAndJamAsync(int number)
        {
            var toast = await ToastBreadAsync(number);
            ApplyButter(toast);
            ApplyJam(toast);

            return toast;
        }

        private static Juice PourOJ()
        {
            Console.WriteLine("Pouring orange juice");
            return new Juice();
        }

        private static void ApplyJam(Toast toast) =>
            Console.WriteLine("Putting jam on the toast");

        private static void ApplyButter(Toast toast) =>
            Console.WriteLine("Putting butter on the toast");

        private static async Task<Toast> ToastBreadAsync(int slices)
        {
            for (int slice = 0; slice < slices; slice++)
            {
                Console.WriteLine("Putting a slice of bread in the toaster");
            }
            Console.WriteLine("Start toasting...");
            await Task.Delay(3000);
            Console.WriteLine("Remove toast from toaster");

            return new Toast();
        }

        private static async Task<Bacon> FryBaconAsync(int slices)
        {
            Console.WriteLine($"putting {slices} slices of bacon in the pan");
            Console.WriteLine("cooking first side of bacon...");
            await Task.Delay(3000);
            for (int slice = 0; slice < slices; slice++)
            {
                Console.WriteLine("flipping a slice of bacon");
            }
            Console.WriteLine("cooking the second side of bacon...");
            await Task.Delay(3000);
            Console.WriteLine("Put bacon on plate");

            return new Bacon();
        }

        private static async Task<Egg> FryEggsAsync(int howMany)
        {
            Console.WriteLine("Warming the egg pan...");
            await Task.Delay(3000);
            Console.WriteLine($"cracking {howMany} eggs");
            Console.WriteLine("cooking the eggs ...");
            await Task.Delay(3000);
            Console.WriteLine("Put eggs on plate");

            return new Egg();
        }

        private static Coffee PourCoffee()
        {
            Console.WriteLine("Pouring coffee");
            return new Coffee();
        }

    }
    class Coffee { }
    class Egg { }
    class Bacon { }
    class Toast { }
    class Juice { }
}

效果如下：

 可以看出，这样编写的异步和最初同步版本的总共的耗时大致相同。

这是因为这段代码还没有利用异步编程的某些关键功能。

即上面的异步代码的使用在这里是不准确的。

可以看出，这段代码里面的打印输出与同步是一样的。

这是因为：在煎鸡蛋或培根时，此代码虽然不会阻塞，但是此代码也不会启动任何其他任务。

就造成了异步煎鸡蛋的操作完成后，才会开始培根制作。

但是，对于这里而言，我不希望每个任务都按顺序依次执行。

最好是首先启动每个组件任务，然后再等待之前任务的完成。

例如：首先启动鸡蛋和培根。

同时启动任务

在很多方案中，你可能都希望立即启动若干独立的任务。然后，在每个任务完成时，你可以继续

进行已经准备的其他工作。

就像这里同时启动煎鸡蛋，培根和烤面包。

我们这里对早餐代码做些更改。

正确的做法

第一步是存储任务以便在这些任务启动时进行操作，而不是等待：

Coffee cup = PourCoffee();
Console.WriteLine("coffee is ready");

Task<Egg> eggsTask = FryEggsAsync(2);
Egg eggs = await eggsTask;
Console.WriteLine("eggs are ready");

Task<Bacon> baconTask = FryBaconAsync(3);
Bacon bacon = await baconTask;
Console.WriteLine("bacon is ready");

Task<Toast> toastTask = ToastBreadAsync(2);
Toast toast = await toastTask;
ApplyButter(toast);
ApplyJam(toast);
Console.WriteLine("toast is ready");

Juice oj = PourOJ();
Console.WriteLine("oj is ready");
Console.WriteLine("Breakfast is ready!");

接下来，可以在提供早餐之前将用于处理培根和鸡蛋的await语句移动到此方法的末尾：

Coffee cup = PourCoffee();
Console.WriteLine("coffee is ready");

Task<Egg> eggsTask = FryEggsAsync(2);
Task<Bacon> baconTask = FryBaconAsync(3);
Task<Toast> toastTask = ToastBreadAsync(2);

Toast toast = await toastTask;
ApplyButter(toast);
ApplyJam(toast);
Console.WriteLine("toast is ready");
Juice oj = PourOJ();
Console.WriteLine("oj is ready");

Egg eggs = await eggsTask;
Console.WriteLine("eggs are ready");
Bacon bacon = await baconTask;
Console.WriteLine("bacon is ready");

Console.WriteLine("Breakfast is ready!");

运行效果如下：

 或者

 可以看出，这里一次启动了所有的异步任务。而你仅在需要结果时，才会等待每项任务。

这里异步准备的造成大约花费20分钟，这是因为一些任务可以并发进行。

而对于直接  Egg eggs = await FryEggsAsync(2); 的方式，适用于你只需要等待这一个异步操作结果，不需要进行其他操作的时候。

与任务组合

吐司操作由异步操作（烤面包）和同步操作（添加黄油和果酱）组成。

这里涉及到一个重要概念：

异步操作后跟同步操作的这种组合也是一个异步操作。

也就是说，如果操作的任何部分是异步的，整个操作就是异步的。

代码如下：

static async Task<Toast> MakeToastWithButterAndJamAsync(int number)
{
    var toast = await ToastBreadAsync(number);
    ApplyButter(toast);
    ApplyJam(toast);

    return toast;
}

所有，主要代码块现在变为：

static async Task Main(string[] args)
{
    Coffee cup = PourCoffee();
    Console.WriteLine("coffee is ready");
    
    var eggsTask = FryEggsAsync(2);
    var baconTask = FryBaconAsync(3);
    var toastTask = MakeToastWithButterAndJamAsync(2);

    var eggs = await eggsTask;
    Console.WriteLine("eggs are ready");

    var bacon = await baconTask;
    Console.WriteLine("bacon is ready");

    var toast = await toastTask;
    Console.WriteLine("toast is ready");

    Juice oj = PourOJ();
    Console.WriteLine("oj is ready");
    Console.WriteLine("Breakfast is ready!");
}

高效的等待任务

可以通过使用Task类的方法改进上述代码末尾一系列await语句。

WhenAll 是其中的一个api ， 它将返回一个其参数列表中的所有任务都已完成时猜完成的Task，

代码如下

await Task.WhenAll(eggsTask, baconTask, toastTask);
Console.WriteLine("eggs are ready");
Console.WriteLine("bacon is ready");
Console.WriteLine("toast is ready");
Console.WriteLine("Breakfast is ready!");

另一种选择是 WhenAny, 它将返回一个，当其参数完成时猜完成的 Task<Task>。

var breakfastTasks = new List<Task> { eggsTask, baconTask, toastTask };
while (breakfastTasks.Count > 0)
{
    Task finishedTask = await Task.WhenAny(breakfastTasks);
    if (finishedTask == eggsTask)
    {
        Console.WriteLine("eggs are ready");
    }
    else if (finishedTask == baconTask)
    {
        Console.WriteLine("bacon is ready");
    }
    else if (finishedTask == toastTask)
    {
        Console.WriteLine("toast is ready");
    }
    breakfastTasks.Remove(finishedTask);
}

处理已完成任务的结果之后，可以从传递给 WhenAny 的任务列表中删除此已完成的任务。

 进行这些更改后，代码的最终版本将如下所示：

using System;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace AsyncBreakfast
{
    class Program
    {
        static async Task Main(string[] args)
        {
            Coffee cup = PourCoffee();
            Console.WriteLine("coffee is ready");

            var eggsTask = FryEggsAsync(2);
            var baconTask = FryBaconAsync(3);
            var toastTask = MakeToastWithButterAndJamAsync(2);

            var breakfastTasks = new List<Task> { eggsTask, baconTask, toastTask };
            while (breakfastTasks.Count > 0)
            {
                Task finishedTask = await Task.WhenAny(breakfastTasks);
                if (finishedTask == eggsTask)
                {
                    Console.WriteLine("eggs are ready");
                }
                else if (finishedTask == baconTask)
                {
                    Console.WriteLine("bacon is ready");
                }
                else if (finishedTask == toastTask)
                {
                    Console.WriteLine("toast is ready");
                }
                breakfastTasks.Remove(finishedTask);
            }

            Juice oj = PourOJ();
            Console.WriteLine("oj is ready");
            Console.WriteLine("Breakfast is ready!");
        }

        static async Task<Toast> MakeToastWithButterAndJamAsync(int number)
        {
            var toast = await ToastBreadAsync(number);
            ApplyButter(toast);
            ApplyJam(toast);

            return toast;
        }

        private static Juice PourOJ()
        {
            Console.WriteLine("Pouring orange juice");
            return new Juice();
        }

        private static void ApplyJam(Toast toast) =>
            Console.WriteLine("Putting jam on the toast");

        private static void ApplyButter(Toast toast) =>
            Console.WriteLine("Putting butter on the toast");

        private static async Task<Toast> ToastBreadAsync(int slices)
        {
            for (int slice = 0; slice < slices; slice++)
            {
                Console.WriteLine("Putting a slice of bread in the toaster");
            }
            Console.WriteLine("Start toasting...");
            await Task.Delay(3000);
            Console.WriteLine("Remove toast from toaster");

            return new Toast();
        }

        private static async Task<Bacon> FryBaconAsync(int slices)
        {
            Console.WriteLine($"putting {slices} slices of bacon in the pan");
            Console.WriteLine("cooking first side of bacon...");
            await Task.Delay(3000);
            for (int slice = 0; slice < slices; slice++)
            {
                Console.WriteLine("flipping a slice of bacon");
            }
            Console.WriteLine("cooking the second side of bacon...");
            await Task.Delay(3000);
            Console.WriteLine("Put bacon on plate");

            return new Bacon();
        }

        private static async Task<Egg> FryEggsAsync(int howMany)
        {
            Console.WriteLine("Warming the egg pan...");
            await Task.Delay(3000);
            Console.WriteLine($"cracking {howMany} eggs");
            Console.WriteLine("cooking the eggs ...");
            await Task.Delay(3000);
            Console.WriteLine("Put eggs on plate");
            
            return new Egg();
        }

        private static Coffee PourCoffee()
        {
            Console.WriteLine("Pouring coffee");
            return new Coffee();
        }
    }
}

效果如下：

或者

 这种异步的代码实现最终大约花费15分钟，因为一些任务能同时运行，

并且该代码能够同时监视多个任务，只在需要时才执行操作。

总结：

async 和 await的功能最好能做到：

尽可能启动任务，不要在等待任务完成时造成阻塞。

即可以先把任务存储到task，然后在后面需要用的时候，调用await task（）方法。

参考网址：https://docs.microsoft.com/zh-cn/dotnet/csharp/programming-guide/concepts/async/