C# 5.0 异步编程技术详解

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C# 5.0 异步编程 C# 5.0 异步编程技术是微软公司在 C# 语言中引入的一项新的编程特性。这项技术允许开发人员编写异步代码，使得程序更加高效、灵活和可扩展。异步编程技术在 C# 5.0 中的引入，标志着 C# 语言在并发编程方面的重要一步。在 C# 5.0 中，异步编程技术主要通过使用 async 和 await 两个关键字来实现。async 关键字用于标记异步方法，而 await 关键字则用于在异步方法中等待异步操作的完成。通过使用 async 和 await 关键字，开发人员可以轻松地编写异步代码，而不需要手动地管理线程和锁。异步编程技术的引入，解决了传统同步编程中的一些问题。在传统同步编程中，程序需要等待某个操作的完成，然后才能继续执行下一步操作。而异步编程技术则允许程序同时执行多个操作，从而提高了程序的执行效率。在 C# 5.0 中，异步编程技术还提供了一些其他的特性，例如异步 lambda 表达式和异步查询。异步 lambda 表达式允许开发人员编写异步 lambda 表达式，而异步查询则允许开发人员编写异步数据库查询。 C# 5.0 异步编程技术的优点包括： * 提高了程序的执行效率 * 使得程序更加灵活和可扩展 * 降低了编程的复杂度 * 提高了开发人员的生产效率 C# 5.0 异步编程技术的应用场景包括： * 网络编程 * 数据库编程 * 文件操作 * 图形化编程 C# 5.0 异步编程技术是 C# 语言中的一项重要特性，它使得开发人员可以轻松地编写高效、灵活和可扩展的异步代码。异步编程技术的发展历程： * 在早期的 C# 语言中，异步编程技术主要通过使用回调函数和委托来实现。 * 在 C# 2.0 中，引入了 iterator 块和 yield 关键字，允许开发人员编写异步代码。 * 在 C# 4.0 中，引入了 Parallel Framework，允许开发人员编写并发代码。 * 在 C# 5.0 中，引入了 async 和 await 关键字，marking 了一种新的异步编程时代。 Async 和 Await 关键字的使用： * async 关键字用于标记异步方法。 * await 关键字用于在异步方法中等待异步操作的完成。异步编程技术的优缺点： * 优点：提高了程序的执行效率，降低了编程的复杂度，提高了开发人员的生产效率。 * 缺点：增加了编程的难度，需要开发人员具备一定的编程经验和知识。异步编程技术在 C# 语言中的发展前景： * 未来，异步编程技术将继续发展和改进，变得更加强大和灵活。 * 异步编程技术将成为 C# 语言中的一个核心特性。 C# 5.0 异步编程技术是 C# 语言中的一项非常重要的特性，它使得开发人员可以轻松地编写高效、灵活和可扩展的异步代码。

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.

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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

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• 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

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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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