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

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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/

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