Async in C# 5.0：深入理解异步编程

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"异步编程在C#5.0中的应用(2012.9版)" 本书《Async in C# 5.0》由Alex Davies撰写，深入探讨了C#编程语言中的异步编程概念，特别是在2012年9月发布的C# 5.0版本中的新特性和最佳实践。C# 5.0引入了`async`和`await`关键字，极大地简化了异步代码的编写，使得开发者能够更加高效地处理I/O密集型和计算密集型任务，而不会阻塞应用程序的主线程。异步编程在现代软件开发中扮演着至关重要的角色，尤其是在构建高性能、响应迅速的用户界面或服务器端应用程序时。C# 5.0的异步功能使得开发者可以创建非阻塞的代码，提高程序的并发性，从而充分利用多核处理器和系统资源。书中的内容可能涵盖了以下几个方面： 1. 异步基础：解释异步编程的重要性，以及为什么在C#中需要异步操作。它可能会讨论线程管理、CPU利用率和用户体验等因素。 2.`async`和`await`关键字：详细阐述`async`和`await`的用法，如何定义异步方法，以及如何在这些方法中使用`await`来等待异步操作的完成。 3. Task类和异步操作：介绍`Task`类在异步编程中的核心作用，包括`Task.Run`, `Task.Factory.StartNew`等方法，以及如何使用`TaskCompletionSource`来创建自定义的异步操作。 4. 错误处理和异常：讲解在异步代码中如何正确处理异常，以及如何利用`try-catch`块在异步方法中捕获和处理错误。 5. 并发和同步：讨论多线程环境下如何控制并发访问，如使用锁、信号量和Monitor等同步机制，以防止数据竞争。 6. 进度和取消：介绍如何使用`CancellationToken`来实现异步操作的取消，以及如何在长时间运行的任务中报告进度。 7. 实践案例：提供实际的代码示例和项目实践，展示如何在Web开发、数据库交互、文件操作等场景中应用异步编程。 8. 性能优化：分析异步编程对性能的影响，如何进行性能测试和优化，以确保异步代码的效率。 9. 最佳实践：总结异步编程的最佳实践，包括代码结构、错误处理策略和设计模式等。通过这本书，读者将不仅了解异步编程的概念，还能掌握实际开发中应用异步技术的技巧，从而提升C#应用程序的性能和响应速度。

The interesting bit is the await keyword. When the compiler sees this, it chops the

method up. Exactly what it does is pretty complicated, so for now I will introduce a

false construct that I find useful as a way to think about simple cases.

1. Everything after await is moved into a separate method.

2. We use a new version of DownloadString called DownloadStringTaskAsync. It does

the same as the original, but is asynchronous.

3. That means we can give it the new second method, which it will call when it fin-

ishes. We do this using some magic that I’ll tell you about later.

4. When the download is done, it will call us back with the downloaded string—

which we can use, in this case, to write to the console.

private void DumpWebPageAsync(string uri)

{

WebClient webClient = new WebClient();

webClient.DownloadStringTaskAsync(uri) <- magic(SecondHalf);

}

private void SecondHalf(string awaitedResult)

{

string page = awaitedResult;

Console.WriteLine(page);

}

What happens to the calling thread when it runs this code? When it reaches the call to

DownloadStringTaskAsync, the download gets started. But not in this thread. In this

thread, we reach the end of the method and return. What the thread does next is up to

our caller. If it is a UI thread, it will go back to processing user actions. Otherwise, its

resources might be released. That means we’ve written asynchronous code!

Async Doesn’t Solve Everything

The async feature has deliberately been designed to look as similar to blocking code as

possible. We can deal with long-running or remote operations almost as if they were

local and fast, but keep the performance benefits of calling them asynchronously.

However, it’s not designed to let you forget that there are background operations and

callbacks happening. You need to be careful with lots of things that behave differently

when you use async, including:

• Exceptions and try..catch...finally blocks

• Return values of methods

• Threads and context

• Performance

Without understanding what’s really happening, your program will fail in surprising

ways, and you won’t understand the error messages or the debugger to be able to fix it.

4 | Chapter 1: Introduction

Asynchronous code, even written manually, means that the UI thread can return to its

primary job of checking the message queue for user events, and responding to them. It

can also perform progress animations, and in recent versions of Windows, mouse hover

animations, which are both important visual cues to users that give a good impression

of the responsiveness of the application.

The reason that all common UI frameworks use only one thread is to

simplify synchronization. If there were many threads, one could try to

read the width of a button, while another is in the process of laying out

the controls. To avoid them conflicting, you’d need to use locking heav-

ily, which would reduce the performance to the same as if there were

only one thread.

An Analogy: The Cafe

I’d like to use an analogy to help with an intuitive grasp of the issues involved. If you

feel you already understand, feel free to skip to the next section.

Imagine there’s a small cafe, which sells customers toast for their breakfast. The only

staff member is the owner. He is very concerned about customer service, but hasn’t

learned about asynchronous techniques.

The UI thread models the owner of the cafe very closely. In the same way that work

inside a computer must be done by a thread, only cafe staff can do work at the cafe. In

this case, there’s only one staff member, just like there’s only one UI thread.

The first customer asks the owner for a slice of toast. The owner gets the bread and

starts the toaster. Then he watches the toaster while it cooks the toast. The customer

asks where she can find some butter, but the owner ignores her, as he’s blocked,

watching the toaster. Five minutes later, the toast is done, and he brings it to the cus-

tomer. By this time, a queue has built up, and the customer is annoyed about being

ignored. Not ideal.

Now, lets see if we can teach the cafe owner how to be asynchronous.

First, he needs to make sure his toaster can operate asynchronously. When writing

asynchronous code, we need to ensure that the long-running operation we are calling

is able to call us back when it’s done. In the same way, the toaster must have a timer,

and must pop up the toast loudly when it’s cooked, so he notices it.

The next thing is for him to ignore the toaster once he’s started it. He should go back

to serving the customers. In the same way, our asynchronous code must return once

the long-running operation is started, so the UI thread can respond to user actions.

There are two reasons for this:

6 | Chapter 2: Why Programs Need to Be Asynchronous

• It feels more responsive to the user—the customer can ask for butter and isn’t

ignored

• The user can start another operation simultaneously—the next customer can also

ask for their order to be started

The cafe owner can now process multiple customers at the same time, limited only by

the number of toasters he has, and the time it takes him to fetch and carry the toast.

But this comes with its own problems: he now finds it hard to remember which slices

of toast are intended for which customers. In fact, the UI thread has no memory at all

of which operations it’s waiting for once it returns to processing user events.

So we need to attach a callback to the jobs as we start them, to remind us what to do

when they are finished. For the cafe owner, this is as simple as writing the name of the

customer on a label clipped to the toast. We may need something more complicated,

and in general we’d like to be able to provide full instructions for what we need to do

once the job is done.

With all of those things in place, the cafe owner is now fully asynchronous, and business

booms. The customer experience is much better. There’s less waiting, and the service

feels much more responsive. I hope the analogy has helped with your intuition of why

asynchrony is so important in UI applications.

Web Application Server Code

ASP.NET web servers don’t have the same hard limit of one thread as UI code does.

That said, there are still benefits to using asynchronous code. Long-running operations,

especially remote database queries, are very common in web application code.

Depending on your version of IIS, there will be a limit on either the total number of

threads used to process web requests, or the total number of concurrent requests being

handled. If your requests spend most of their time waiting for a database query, it may

seem a good idea to increase the number of simultaneous requests to increase the

throughput your server can handle.

When a thread is blocked, waiting for something, it doesn’t use any CPU time. How-

ever, don’t assume that means it isn’t using any of your server’s resources. In fact,

threads cause two significant overheads, even when they’re blocked:

Memory

Each managed thread reserves around a megabyte of virtual memory on Windows.

This is no problem at all if you have a few tens of threads, but can easily get out of

hand if you start using hundreds of threads. If the memory gets swapped out to

disk, resuming the threads becomes slow.

Web Application Server Code | 7

Scheduler overhead

The operating system’s scheduler is responsible for choosing which thread should

be executed on which CPU, and when. Even when threads are blocked, the sched-

uler must consider them, to find whether they’re become unblocked. This slows

down context switches, and can slow the entire system.

Between them, these overheads can add to the load on your server, increasing latency

and decreasing throughput.

Remember: the main characteristic of asynchronous code is that the thread that started

a long-running operation is released to do other things. In the case of ASP.NET code,

this thread is from the thread pool, so it is returned to the thread pool during the long-

running operation. It can then process other requests, so fewer threads are needed to

process the same number of requests.

Another Analogy: The Restaurant Kitchen

A web server is a close model of a restaurant. Many customers order food, and the

kitchen tries to satisfy them as soon as it can.

Our kitchen has many chefs, with each chef representing a thread. They cook the dishes

that the customers order, but at points during the preparation, each dish just needs to

be in the oven for a while, and the chef has nothing to do. This mirrors the way that

web requests usually need to make a database query that the web server has no part in.

In a blocking implementation of the kitchen, the chef will sit in front of the oven, waiting

for the dish to be cooked. To model a thread exactly, these chefs have an odd contract

where they aren’t paid while they are waiting for food to cook, because a thread doesn’t

use CPU time when it is blocked. Maybe they read a newspaper.

But even if we don’t have to pay them, and we can hire a new chef for every dish we

need to cook, waiting chefs still take up space in the kitchen. We can’t fit more than a

few tens of chefs in the kitchen before it becomes hard to move around, and everyone’s

work slows down.

Of course, the asynchronous system works much better. Each time food is put in the

oven, the chef notes down what dish it is, and what stage of preparation it’s at, then

finds a new task to do. When the time in the oven is done, any chef can pick the dish

up and continue preparing it.

It’s this efficient system that’s so powerful in web servers. Only a few threads can man-

age a number of simultaneous requests that would have required hundreds before, or

would have just been unfeasible because of the overheads. In fact, some web frame-

works, notably node.js, reject the idea of multiple threads altogether, opting to use a

single thread to process all the requests asynchronously. They can often handle more

requests with one thread than a multithreaded, but blocking, system can handle in

total. In the same way, one well-organized chef in an empty kitchen can cook more

8 | Chapter 2: Why Programs Need to Be Asynchronous

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