Pausing for a Period of Time
Problem:
You need to (asynchronously) wait for a period of time. This can be useful when unit
testing or implementing retry delays. This solution can also be useful for simple time‐
outs.
Solution:
The Task type has a static method Delay that returns a task that completes after the
specified time。

This example defines a task that completes asynchronously, for use with unit testing.
When faking an asynchronous operation, it’s important to test at least synchronous
success and asynchronous success as well as asynchronous failure. This example returns
a task used for the asynchronous success case:

static async Task<T> DelayResult<T>(T result, TimeSpan delay)
{
await Task. Delay(delay);
return result;
}

This next example is a simple implementation of an exponential backoff, that is, a retry
strategy where you increase the delays between retries. Exponential backoff is a best
practice when working with web services to ensure the server does not get flooded with
retries.
For production code, I would recommend a more thorough solu‐
tion, such as the Transient Error Handling Block in Microsoft’s En‐
terprise Library; the following code is just a simple example of
Task.Delay usage.

static async Task<string> DownloadStringWithRetries(string uri)
{
using (var client = new HttpClient())
{
// Retry after 1 second, then after 2 seconds, then 4.
var nextDelay = TimeSpan. FromSeconds();
for (int i = ; i != ; ++i)
{
try
{
return await client. GetStringAsync(uri);
}
catch
{
}
await Task. Delay(nextDelay);
nextDelay = nextDelay + nextDelay;
}
// Try one last time, allowing the error to propogate.
return await client. GetStringAsync(uri);
}
}

This final example uses Task.Delay as a simple timeout; in this case, the desired se‐
mantics are to return null if the service does not respond within three seconds:

static async Task<string> DownloadStringWithTimeout(string uri)
{
using (var client = new HttpClient())
{
var downloadTask = client. GetStringAsync(uri);
var timeoutTask = Task. Delay();
var completedTask = await Task. WhenAny(downloadTask, timeoutTask);
if (completedTask == timeoutTask)
return null;
return await downloadTask;
}
}

Discussion
Task.Delay is a fine option for unit testing asynchronous code or for implementing
retry logic. However, if you need to implement a timeout, a CancellationToken is usu‐
ally a better choice.
See Also
Recipe 2.5 covers how Task.WhenAny is used to determine which task completes first.
Recipe 9.3 covers using CancellationToken as a timeout

2.2. Returning Completed Tasks
Problem
You need to implement a synchronous method with an asynchronous signature. This
situation can arise if you are inheriting from an asynchronous interface or base class
but wish to implement it synchronously. This technique is particularly useful when unit
testing asynchronous code, when you need a simple stub or mock for an asynchronous
interface.
Solution
You can use Task.FromResult to create and return a new Task<T> that is already com‐
pleted with the specified value

interface IMyAsyncInterface
{
Task<int> GetValueAsync();
}
class MySynchronousImplementation : IMyAsyncInterface
{
public Task<int> GetValueAsync()
{
return Task. FromResult();
}
}

2.3. Reporting Progress
Problem
You need to respond to progress while an asynchronous operation is executing.
Solution
Use the provided IProgress<T> and Progress<T> types. Your async method should
take an IProgress<T> argument; the T is whatever type of progress you need to report

static async Task MyMethodAsync(IProgress<double> progress = null)
{
double percentComplete = ;
while (! done)
{
...
if (progress != null)
progress. Report(percentComplete);
}
}

Calling code can use it as such:

static async Task CallMyMethodAsync()
{
var progress = new Progress<double>();
progress. ProgressChanged += (sender, args) =>
{
...
};
await MyMethodAsync(progress);
}

By convention, the IProgress<T> parameter may be null if the caller does not need
progress reports, so be sure to check for this in your async method.
Bear in mind that the IProgress<T>.Report method may be asynchronous. This means
that MyMethodAsync may continue executing before the progress is actually reported.
For this reason, it’s best to define T as an immutable type or at least a value type. If T is
a mutable reference type, then you’ll have to create a separate copy yourself each time
you call IProgress<T>.Report.
Progress<T> will capture the current context when it is constructed and will invoke its
callback within that context. This means that if you construct the Progress<T> on the
UI thread, then you can update the UI from its callback, even if the asynchronous
method is invoking Report from a background thread.
When a method supports progress reporting, it should also make a best effort to support
cancellation

If all the tasks have the same result type and they all complete successfully, then the
Task.WhenAll task will return an array containing all the task results:

Task task1 = Task. FromResult();
Task task2 = Task. FromResult();
Task task3 = Task. FromResult();
int[] results = await Task. WhenAll(task1, task2, task3);
// "results" contains { 3, 5, 7 }

There is an overload of Task.WhenAll that takes an IEnumerable of tasks; however, I
do not recommend that you use it. Whenever I mix asynchronous code with LINQ, I
find the code is clearer when I explicitly “reify” the sequence (i.e., evaluate the sequence,
creating a collection):

static async Task<string> DownloadAllAsync(IEnumerable<string> urls)
{
var httpClient = new HttpClient();
// Define what we're going to do for each URL.
var downloads = urls. Select(url => httpClient. GetStringAsync(url));
// Note that no tasks have actually started yet
// because the sequence is not evaluated.
// Start all URLs downloading simultaneously.
Task<string>[] downloadTasks = downloads. ToArray();
// Now the tasks have all started.
// Asynchronously wait for all downloads to complete.
string[] htmlPages = await Task. WhenAll(downloadTasks);
return string. Concat(htmlPages);
}
// Returns the length of data at the first URL to respond.
private static async Task<int> FirstRespondingUrlAsync(string urlA, string urlB)
{
var httpClient = new HttpClient();
// Start both downloads concurrently.
Task<byte[]> downloadTaskA = httpClient. GetByteArrayAsync(urlA);
Task<byte[]> downloadTaskB = httpClient. GetByteArrayAsync(urlB);
// Wait for either of the tasks to complete.
Task<byte[]> completedTask =
await Task. WhenAny(downloadTaskA, downloadTaskB);
// Return the length of the data retrieved from that URL.
byte[] data = await completedTask;
return data. Length;
}
static async Task<int> DelayAndReturnAsync(int val)
{
await Task. Delay(TimeSpan. FromSeconds(val));
return val;
}
static async Task AwaitAndProcessAsync(Task<int> task)
{
var result = await task;
Trace. WriteLine(result);
}
// This method now prints "1", "2", and "3".
static async Task ProcessTasksAsync()
{
// Create a sequence of tasks.
Task<int> taskA = DelayAndReturnAsync();
Task<int> taskB = DelayAndReturnAsync();
Task<int> taskC = DelayAndReturnAsync();
var tasks = new[] { taskA, taskB, taskC };
var processingTasks = (from t in tasks
select AwaitAndProcessAsync(t)). ToArray();
// Await all processing to complete
await Task. WhenAll(processingTasks);
}

Alternatively, this can be written as:

static async Task<int> DelayAndReturnAsync(int val)
{
await Task. Delay(TimeSpan. FromSeconds(val));
return val;
}
// This method now prints "1", "2", and "3".
static async Task ProcessTasksAsync()
{
// Create a sequence of tasks.
Task<int> taskA = DelayAndReturnAsync();
Task<int> taskB = DelayAndReturnAsync();
Task<int> taskC = DelayAndReturnAsync();
var tasks = new[] { taskA, taskB, taskC };
var processingTasks = tasks. Select(async t =>
{
var result = await t;
Trace. WriteLine(result);
}). ToArray();
// Await all processing to complete
await Task. WhenAll(processingTasks);
}

Concurrency in C# Cookbook 笔记的更多相关文章

  1. [C#] 《Concurrency in C# Cookbook》读书笔记(一)- 并发编程概述

    并发编程概述 前言 我们经常在耳边听说一些关于高性能.并发和并行等名词,并且很多人都对并发编程有一些误解. 误解 1:并发就是多线程? 答:多线程只不过是并发编程的其中一种形式而已.并发编程的种类很多 ...

  2. Java Concurrency in Practice 读书笔记 第十章

    粗略看完<Java Concurrency in Practice>这部书,确实是多线程/并发编程的一本好书.里面对各种并发的技术解释得比较透彻,虽然是面向Java的,但很多概念在其他语言 ...

  3. Python Cookbook 笔记--12章并发编程

    <Python Cookbook(第3版)中文版> 1.队列queue的有些方法是线程不安全的,在多线程中最好别用 2.需要限制一段代码的并发访问量时,用信号量.不要把信号量当做普通的锁来 ...

  4. Java Concurrency in Practice 读书笔记 第二章

    第二章的思维导图(代码迟点补上):

  5. java concurrency in practice读书笔记---ThreadLocal原理

    ThreadLocal这个类很强大,用处十分广泛,可以解决多线程之间共享变量问题,那么ThreadLocal的原理是什么样呢?源代码最能说明问题! public class ThreadLocal&l ...

  6. Python Cookbook笔记

    字符串:s.strip()  删除字符串开始和结尾的空白字符. s.lstrip() 删除左边的,s.rstrip()  删除右边的. 随机数:random.random()  生成0-1之间的数. ...

  7. Java Concurrency in Practice——读书笔记

    Thread Safety线程安全 线程安全编码的核心,就是管理对状态(state)的访问,尤其是对(共享shared.可变mutable)状态的访问. shared:指可以被多个线程访问的变量 mu ...

  8. python cookbook 笔记三

    分组: rows = [ {'address': '5412 N CLARK', 'date': '07/01/2012'}, {'address': '5148 N CLARK', 'date': ...

  9. python cookbook 笔记二

    去重和排序: #coding=utf-8 def dedupe(items): seen = set() for item in items: if item not in seen: yield i ...

随机推荐

  1. BFC的形成和排版规则

    何为bfc? BFC(Block Formatting Context)直译为“块级格式化范围”.是 W3C CSS 2.1 规范中的一个概念,它决定了元素如何对其内容进行定位,以及与其他元素的关系和 ...

  2. Centos安装Samba共享服务器

    安装Samba 查看Samba是否已安装 1.# rpm -qa | grep samba

  3. Codeforces 1017F The Neutral Zone (看题解)

    这题一看就筛质数就好啦, 可是这怎么筛啊, 一看题解, 怎么会有这么骚的操作. #include<bits/stdc++.h> #define LL long long #define f ...

  4. 2018牛客网暑假ACM多校训练赛(第十场)F Rikka with Line Graph 最短路 Floyd

    原文链接https://www.cnblogs.com/zhouzhendong/p/NowCoder-2018-Summer-Round10-F.html 题目传送门 - https://www.n ...

  5. 025 SSM简单搭建

    参考了同事的文档,自己也写一篇文档. 同时,补充了一下,程序是如何运行的. 一:SSM框架 1.说明 SSM(Spring+SpringMVC+MyBatis)框架集由Spring.SpringMVC ...

  6. ASP.NET 页面执行顺序

    1.对象初始化(Onlnit方法) 页面中的控件(包括页面本身)都是在它们最初的form中被首次初始化的,通过在aspx页面的后台代码文件的构造器中声明你的对象,页面将知道对象的类型,并知道需要创建多 ...

  7. NSL:SOFM神经网络实现预测哪个样本与哪个样本处在同一层,从而科学规避我国煤矿突水灾难—Jason niu

    load water_data.mat attributes = mapminmax(attributes); P_train = attributes(:,1:35); T_train = clas ...

  8. notepad++ 去空行

    Press Ctrl+H (Replace) Select Extended from SearchMode Put \r\n\r\n in Find What Put \r\n in Replace ...

  9. shell 运算符

    shell的逻辑运算符 涉及有以下几种类型,因此只要适当选择,可以解决我们很多复杂的判断,达到事半功倍效果. 一.逻辑运算符 逻辑卷标 表示意思 1. 关于档案与目录的侦测逻辑卷标! -f 常用!侦测 ...

  10. Spring:获取容器中的Bean

    某些情况下我们要获取 IOC 容器中指定注解.类型.名字的 Bean 要获取 IOC 容器中指定条件的 Bean 可以通过 ApplicationContext 相应的方法 @Autowired pr ...