C# 5.0异步编程实战：探索Async的威力

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"Async in C# 5.0: Unleash the Power of Async" C# 5.0 引入了一项重要的特性，即异步编程（Async/Await），这使得开发者能够更有效地处理长时间运行的操作，如网络请求或数据库查询，而不会阻塞应用程序的主线程。该资源，"Async in C# 5.0"，由 Alex Davies 撰写，深入探讨了这一强大的功能，旨在帮助开发者释放 C# 5.0 异步编程的潜力。在 C# 5.0 中，异步编程的核心是 `async` 和 `await` 关键字。`async` 关键字用于标记一个方法，表示该方法是异步的，而 `await` 关键字用于等待一个异步操作完成。这种方式允许代码在等待操作完成时，将控制权返回给调用者，使程序能够继续执行其他任务，从而提高程序的响应性和效率。异步方法通常以 `async` 关键字修饰，并返回一个 `Task` 或 `Task<T>` 对象。当方法中的 `await` 关键字遇到一个 `Task` 对象时，它会暂停当前方法的执行，直到所等待的任务完成。一旦任务完成，`await` 后面的代码将继续执行，而不会阻塞调用线程。 `async` 和 `await` 的一个关键优势是它们简化了异步代码的编写，使得代码更加清晰和易于理解。在传统的回调函数或事件处理方式下，异步代码往往变得复杂且难以维护，而 `async/await` 提供了一种更直观的编程模型。此外，C# 5.0 异步支持还包含对 `async` 构造函数、`async void` 方法（主要用于事件处理程序）以及 `ConfigureAwait(false)` 方法的处理。`ConfigureAwait(false)` 可以防止异步方法在完成后重新获取原始上下文，这对于性能优化和避免死锁至关重要。本书涵盖了异步编程的基础，包括如何创建和使用异步方法，以及如何正确处理异常和取消异步操作。它可能还讨论了与UI交互的异步编程，因为这是异步编程在许多应用程序中的常见应用场景，例如桌面应用和Web应用。通过阅读 "Async in C# 5.0"，开发者可以学习如何在自己的项目中有效利用异步编程，提高应用性能，减少阻塞，同时保持代码的可读性和可维护性。对于任何想要深入理解和掌握 C# 5.0 异步特性的开发者来说，这本书都是一个宝贵的资源。

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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C# 5.0异步编程实战：探索Async的威力

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auto fut = std::async(std::launch::async, [&]

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