C#5异步编程简捷法：提升应用性能与响应能力

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《C#5.0中的异步编程（Async in C#5.0）》由Alex Davies撰写，是一本专为理解并掌握C# 5.0中异步编程特性的教程。随着C# 5的引入，作者旨在帮助开发者解决传统异步编程方法所带来的复杂性和挑战，如性能瓶颈和代码维护难度。该书介绍了一种更简洁的异步编程方法，利用.NET Framework 4.5及以上版本和.NET Core的内置支持，使得开发者能够以类似同步代码的方式编写异步程序，提高了代码的可读性和可维护性。书中内容覆盖了以下核心知识点： 1. **异步编程基础**：阐述了为何在高性能和用户体验优化中异步编程的重要性，以及传统的异步模式（如回调函数和代理模式）存在的局限性。 2. **C# 5.0的async/await关键字**：重点介绍了C# 5.0中引入的`async`和`await`关键字，它们使得异步操作更加直观和易于理解。`async`关键字用于标记方法，而`await`用于暂停当前任务直到异步操作完成，同时保持线程的高效利用。 3. **异步方法和任务**：解释了如何创建和管理异步任务，包括使用`Task`类和`Task<T>`泛型类型，以及`Task.Run()`和`Task.Factory.StartNew()`等方法的区别。 4. **错误处理和取消操作**：探讨了如何处理异步操作中的异常和取消请求，确保程序的健壮性。 5. **异步编程与并发控制**：讨论了如何在多线程环境中优雅地结合异步和并行编程，以提高系统性能。 6. **性能分析与最佳实践**：提供了一些关于如何衡量和优化异步代码性能的建议，以及避免常见陷阱的方法。 7. **示例与案例研究**：书中包含大量实例，展示如何将异步编程应用到实际项目中，以便读者能更好地理解和实践。 8. **与时俱进：扩展至.NET Core和Windows运行时**：考虑到.NET框架的最新发展，书中也会介绍如何在.NET Core和跨平台环境中继续使用异步编程。通过阅读《C#5.0中的异步编程》，开发者可以提升其应用程序的响应能力，简化异步编程流程，并确保他们的代码既高效又易于维护。无论是初学者还是经验丰富的开发者，都能从中受益匪浅。

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

Downloa d f r o m W o w ! e B o o k < w w w.woweb o o k . c o m >

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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