Don't Block on Async Code【转】
http://blog.stephencleary.com/2012/07/dont-block-on-async-code.html
This is a problem that is brought up repeatedly on the forums and Stack Overflow. I think it’s the most-asked question by async newcomers once they’ve learned the basics.
UI Example
Consider the example below. A button click will initiate a REST call and display the results in a text box (this sample is for Windows Forms, but the same principles apply to any UI application).
// My "library" method.
public static async Task<JObject> GetJsonAsync(Uri uri)
{
using (var client = new HttpClient())
{
var jsonString = await client.GetStringAsync(uri);
return JObject.Parse(jsonString);
}
}
// My "top-level" method.
public void Button1_Click(...)
{
var jsonTask = GetJsonAsync(...);
textBox1.Text = jsonTask.Result;
}
The “GetJson” helper method takes care of making the actual REST call and parsing it as JSON. The button click handler waits for the helper method to complete and then displays its results.
This code will deadlock.
ASP.NET Example
This example is very similar; we have a library method that performs a REST call, only this time it’s used in an ASP.NET context (Web API in this case, but the same principles apply to any ASP.NET application):
// My "library" method.
public static async Task<JObject> GetJsonAsync(Uri uri)
{
using (var client = new HttpClient())
{
var jsonString = await client.GetStringAsync(uri);
return JObject.Parse(jsonString);
}
}
// My "top-level" method.
public class MyController : ApiController
{
public string Get()
{
var jsonTask = GetJsonAsync(...);
return jsonTask.Result.ToString();
}
}
This code will also deadlock. For the same reason.
What Causes the Deadlock
Here’s the situation: remember from my intro post that after you await a Task, when the method continues it will continue in a context.
In the first case, this context is a UI context (which applies to any UI except Console applications). In the second case, this context is an ASP.NET request context.
One other important point: an ASP.NET request context is not tied to a specific thread (like the UI context is), but it does only allow one thread in at a time. This interesting aspect is not officially documented anywhere AFAIK, but it is mentioned in my MSDN article about SynchronizationContext.
So this is what happens, starting with the top-level method (Button1_Click for UI / MyController.Get for ASP.NET):
- The top-level method calls GetJsonAsync (within the UI/ASP.NET context).
- GetJsonAsync starts the REST request by calling HttpClient.GetStringAsync (still within the context).
- GetStringAsync returns an uncompleted Task, indicating the REST request is not complete.
- GetJsonAsync awaits the Task returned by GetStringAsync. The context is captured and will be used to continue running the GetJsonAsync method later. GetJsonAsync returns an uncompleted Task, indicating that the GetJsonAsync method is not complete.
- The top-level method synchronously blocks on the Task returned by GetJsonAsync. This blocks the context thread.
- … Eventually, the REST request will complete. This completes the Task that was returned by GetStringAsync.
- The continuation for GetJsonAsync is now ready to run, and it waits for the context to be available so it can execute in the context.
- Deadlock. The top-level method is blocking the context thread, waiting for GetJsonAsync to complete, and GetJsonAsync is waiting for the context to be free so it can complete.
For the UI example, the “context” is the UI context; for the ASP.NET example, the “context” is the ASP.NET request context. This type of deadlock can be caused for either “context”.
Preventing the Deadlock
There are two best practices (both covered in my intro post) that avoid this situation:
- In your “library” async methods, use ConfigureAwait(false) wherever possible.
- Don’t block on Tasks; use async all the way down.
Consider the first best practice. The new “library” method looks like this:
public static async Task<JObject> GetJsonAsync(Uri uri)
{
using (var client = new HttpClient())
{
var jsonString = await client.GetStringAsync(uri).ConfigureAwait(false);
return JObject.Parse(jsonString);
}
}
This changes the continuation behavior of GetJsonAsync so that it does not resume on the context. Instead, GetJsonAsync will resume on a thread pool thread. This enables GetJsonAsync to complete the Task it returned without having to re-enter the context.
Consider the second best practice. The new “top-level” methods look like this:
public async void Button1_Click(...)
{
var json = await GetJsonAsync(...);
textBox1.Text = json;
}
public class MyController : ApiController
{
public async Task<string> Get()
{
var json = await GetJsonAsync(...);
return json.ToString();
}
}
This changes the blocking behavior of the top-level methods so that the context is never actually blocked; all “waits” are “asynchronous waits”.
Note: It is best to apply both best practices. Either one will prevent the deadlock, but both must be applied to achieve maximum performance and responsiveness.
Resources
- My introduction to async/await is a good starting point.
- Stephen Toub’s blog post Await, and UI, and deadlocks! Oh, my! covers this exact type of deadlock (in January of 2011, no less!).
- If you prefer videos, Stephen Toub demoed this deadlock live (39:40 - 42:50, but the whole presentation is great!). Lucian Wischik also demoed this deadlock using VB (17:10 - 19:15).
- The Async/Await FAQ goes into detail on exactly when contexts are captured and used for continuations.
This kind of deadlock is always the result of mixing synchronous with asynchronous code. Usually this is because people are just trying out async with one small piece of code and use synchronous code everywhere else. Unfortunately, partially-asynchronous code is much more complex and tricky than just making everything asynchronous.
If you do need to maintain a partially-asynchronous code base, then be sure to check out two more of Stephen Toub’s blog posts: Asynchronous Wrappers for Synchronous Methods and Synchronous Wrappers for Asynchronous Methods, as well as my AsyncEx library.
Answered Questions
There are scores of answered questions out there that are all caused by the same deadlock problem. It has shown up on WinRT, WPF, Windows Forms, Windows Phone, MonoDroid, Monogame, and ASP.NET.
Don't Block on Async Code【转】的更多相关文章
- Async方法死锁的问题 Don't Block on Async Code(转)
今天调试requet.GetRequestStreamAsync异步方法出现不返回的问题,可能是死锁了.看到老外一篇文章解释了异步方法死锁的问题,懒的翻译,直接搬过来了. http://blog.st ...
- Don't Block on Async Code
http://blog.stephencleary.com/2012/07/dont-block-on-async-code.html This is a problem that is brough ...
- Practical Node.js (2018版) 14章, async code in Node
Asynchronous Code in Node 历史上,Node开发者只能用回调和事件emitters. 现在可以使用一些异步的语法: async module Promises Async/aw ...
- Async All the Way
Asynchronous code reminds me of the story of a fellow who mentioned that the world was suspended in ...
- ASP.NET sync over async(异步中同步,什么鬼?)
async/await 是我们在 ASP.NET 应用程序中,写异步代码最常用的两个关键字,使用它俩,我们不需要考虑太多背后的东西,比如异步的原理等等,如果你的 ASP.NET 应用程序是异步到底的, ...
- Async/Await - Best Practices in Asynchronous Programming
https://msdn.microsoft.com/en-us/magazine/jj991977.aspx Figure 1 Summary of Asynchronous Programming ...
- 小心C# 5.0 中的await and async模式造成的死锁
平时在使用C# 5.0中的await and async关键字的时候总是没注意,直到今天在调试一个ASP.NET项目时,发现在调用一个声明为async的方法后,程序老是莫名其妙的被卡住,就算声明为as ...
- Async/Await - Best Practices in Asynchronous Programming z
These days there’s a wealth of information about the new async and await support in the Microsoft .N ...
- async/await 的一些知识
博文 Don't Block on Async Code What is the purpose of "return await" in C#? Any difference b ...
随机推荐
- 分支界定法 branch-and-bound 分析与实现)(转载)
1. 介绍分支界定法之前需要了解一下广度优先搜索breadth-First-search(BFS) 1.从图中某个顶点V0出发,并访问此顶点:以层为顺序,一层一层往下遍历 2.从V0出发,访问V0的各 ...
- xpath爬取网页评论,网址的的调用方法,数据库特殊字符的替换
# -*- coding:utf-8-*-from lxml import etreeimport urllibimport jsonimport requestsimport MySQLdbid=0 ...
- appium远程调用appium server
如何通过本地的代码,调用远程的server呢? 例如:我有两台电脑A(192.168.112.10)和B(192.168.112.11),那我怎么能在A执行本地脚本,但是使用B上的server呢? ...
- cookie小记
清除cookie可用下面2种的方法.注意如果要清除的cookie的域是指定的,则这里也应该指定,否则无效 1: Response.Cooki ...
- (转载)HTML5 LocalStorage 本地存储
原文地址:http://www.cnblogs.com/xiaowei0705/archive/2011/04/19/2021372.html HTML5 LocalStorage 本地存储 说到本地 ...
- spring整合struts2
1. Spring 如何在 WEB 应用中使用 ? 1). 需要额外加入的 jar 包: spring-web-4.0.0.RELEASE.jarspring-webmvc-4.0.0.RELEASE ...
- 使用新浪云 Java 环境搭建一个简单的微信处理后台
前一段时间,写了一篇在新浪云上搭建自己的网站的教程,通过简单构建了一个 maven 的项目,展示部署的整个流程,具体的操作可以参看这里. 新浪云服务器除了可以搭建自己的网站以外,也非常的适合作为微信公 ...
- transitionend的运用案例
transitionend事件代表着过渡动画结束后 原生的绑定方法 obj.addEventListener('transitionEnd', function(){ //do soming }) 我 ...
- eclipse- Web-app verson=2.5 调整将Dynamic Web Module3.0降为2.5
如果提示cannot change version of project facet Dynamic Web Module to 2.5 1.把Dynamic Web Module复选框,勾选去掉,点 ...
- jQuery 的属性
一.显示和隐藏的属性 hide(隐藏),show(显示) 下面是例子 <script type="text/javascript"> $(document).read ...