Home > Articles

This chapter is from the book

Item 32: Compose Asynchronous Work Using Task Objects

Tasks are the abstraction for the work you’ve offloaded to another resource. The Task type and related classes and structs have rich APIs to manipulate tasks and the work that has been offloaded. Tasks are also objects that can be manipulated using their methods and properties. Tasks can be aggregated to compose larger-grained tasks. They can be ordered, or they can run in parallel. You use await expressions to enforce an ordering: Code that follows the await expression will not execute until the awaited task has completed. You can specify that tasks may start only in response to the completion of another task. Overall, the rich set of APIs used by tasks enables you to write elegant algorithms that work with these objects and the work they represent. The more you learn how to use tasks as objects, the more elegant your asynchronous code will be.

Let’s begin with an asynchronous method that starts a number of tasks and awaits for each to finish. A very naïve implementation would be this:

public static async Task<IEnumerable<StockResult>>
ReadStockTicker(IEnumerable<string> symbols)
{
var results = new List<StockResult>();
foreach (var symbol in symbols)
{
var result = await ReadSymbol(symbol);
results.Add(result);
}
return results;
}

These several tasks are independent, and there’s no reason for you to wait for each task to finish before starting the next. One change you could make is to start all tasks, and then wait for all of them to finish before executing the continuations:

public static async Task<IEnumerable<StockResult>>
ReadStockTicker(IEnumerable<string> symbols)
{
var resultTasks = new List<Task<StockResult>>();
foreach (var symbol in symbols)
{
resultTasks.Add(ReadSymbol(symbol));
}
var results = await Task.WhenAll(resultTasks);
return results.OrderBy(s => s.Price);
}

This would be the correct implementation if the continuation requires the results of all the tasks to continue effectively. Using WhenAll, you create a new task that completes when all the tasks it is watching have completed. The result from Task.WhenAll is an array of all the completed (or faulted) tasks.

At other times, you might start several different tasks, all of which generate the same result. Your goal in such a case is to try different sources, and continue working with the first task that finishes. The Task.WhenAny() method creates a new task that is complete as soon as any one of the tasks it is awaiting is complete.

Suppose you want to read a single stock symbol from multiple online sources, and return the first result that completes. You could use WhenAny to determine which of the started tasks completed first:

public static async Task<StockResult>
ReadStockTicker(string symbol, IEnumerable<string> sources)
{
var resultTasks = new List<Task<StockResult>>();
foreach (var source in sources)
{
resultTasks.Add(ReadSymbol(symbol, source));
}
return await Task.WhenAny(resultTasks);
}

Sometimes you may want to execute the continuation as each task completes. A naïve implementation might look like this:

public static async Task<IEnumerable<StockResult>>
ReadStockTicker(IEnumerable<string> symbols)
{
var resultTasks = new List<Task<StockResult>>();
var results = new List<StockResult>();
foreach (var symbol in symbols)
{
resultTasks.Add(ReadSymbol(symbol));
}
foreach(var task in resultTasks)
{
var result = await task;
results.Add(result);
}
return results;
}

There’s no guarantee that the tasks will finish in the order you’ve started them. This could be a very inefficient algorithm: Any number of completed tasks may be stuck in the queue, awaiting processing behind a task that is simply taking longer.

You might try to improve this using Task.WhenAny(). The implementation would look like this:

public static async Task<IEnumerable<StockResult>>
ReadStockTicker(IEnumerable<string> symbols)
{
var resultTasks = new List<Task<StockResult>>();
var results = new List<StockResult>();
foreach (var symbol in symbols)
{
resultTasks.Add(ReadSymbol(symbol));
}
while (resultTasks.Any())
{
// Each time through the loop, this creates a
// new task. That can be expensive.
Task<StockResult> finishedTask = await
Task.WhenAny(resultTasks);
var result = await finishedTask;
resultTasks.Remove(finishedTask);
results.Add(result);
}
var first = await Task.WhenAny(resultTasks);
return await first;
}

As the comment indicates, this strategy is not a good way to create the desired behavior. You create new tasks each time you call Task.WhenAny(). As the number of tasks you want to manage grows, this algorithm performs more and more allocations, and becomes increasingly more inefficient.

As an alternative, you can use the TaskCompletionSource class. TaskCompletionSource enables you to return a Task object that you manipulate to produce the result at a later point in time. Effectively, you can produce the result for any method asynchronously. The most common use of this strategy is to provide a conduit between a source Task (or Tasks) and a destination Task (or Tasks). You write the code that should execute when the source task completes. Your code then awaits the source task, and updates the destination task by using the TaskCompletionSource.

In the next example, let’s assume you have an array of source tasks. You’ll create an array of destination TaskCompletionSource objects. As each task finishes, you’ll update one of the destination tasks using its TaskCompletionSource. Here’s the code:

public static Task<T>[] OrderByCompletion<T>(
this IEnumerable<Task<T>> tasks)
{
// Copy to List because it gets enumerated multiple times.
var sourceTasks = tasks.ToList();

// Allocate the sources; allocate the output tasks.
// Each output task is the corresponding task from
// each completion source.
var completionSources =
new TaskCompletionSource<T>[sourceTasks.Count];
var outputTasks = new Task<T>[completionSources.Length];
for (int i = 0; i < completionSources.Length; i++)
{
completionSources[i] = new TaskCompletionSource<T>();
outputTasks[i] = completionSources[i].Task;
}

// Magic, part 1:
// Each task has a continuation that puts its
// result in the next open location in the completion
// sources array.
int nextTaskIndex = -1;
Action<Task<T>> continuation = completed =>
{
var bucket = completionSources
[Interlocked.Increment(ref nextTaskIndex)];
if (completed.IsCompleted)
bucket.TrySetResult(completed.Result);
else if (completed.IsFaulted)
bucket.TrySetException(completed.Exception);
};

// Magic, part 2:
// For each input task, configure the
// continuation to set the output task.
// As each task completes, it uses the next location.
foreach (var inputTask in sourceTasks)
{
inputTask.ContinueWith(continuation,
CancellationToken.None,
TaskContinuationOptions.ExecuteSynchronously,
TaskScheduler.Default);
}

return outputTasks;
}

There’s quite a bit going on here, so let’s look at it section by section. First, the method allocates an array of TaskCompletionSource objects. Then, it defines the continuation code that will run as each source task completes. This continuation code sets the next slot in the destination TaskCompletionSource objects to complete. It uses the InterlockedIncrement() method to update the next open slot in a thread-safe manner. Finally, it sets the continuation for each Task object to execute this code. Ultimately, the method returns the sequence of tasks from the array of TaskCompletionSource objects.

The caller can now enumerate the list of tasks, which will be ordered by their completion time. Let’s walk through one typical run that starts 10 tasks. Suppose the tasks finish in this order: 3, 7, 2, 0, 4, 9, 1, 6, 5, 8. When task 3 finishes, its continuation will run, placing its Task result in slot 0 of the destination array. Next, task 7 finishes, placing its result in slot 1. Task 2 places its result in slot 2. This process continues until task 8 finishes, placing its result in slot 9. See Figure 3.1.

03fig01.jpg

Figure 3.1 Ordering tasks based on their completion

Let’s extend the code so that it handles tasks that end up in the faulted state. The only change necessary is in the continuation:

// Magic, part 1:
// Each task has a continuation that puts its
// result in the next open location in the completion
// sources array.
int nextTaskIndex = -1;
Action<Task<T>> continuation = completed =>
{
var bucket = completionSources
[Interlocked.Increment(ref nextTaskIndex)];
if (completed.IsCompleted)
bucket.TrySetResult(completed.Result);
else if (completed.IsFaulted)
bucket.TrySetException(completed.Exception);
};

A number of methods and APIs are available that support programming with tasks and enabling actions when tasks complete or fault. Using these methods makes it easier to construct elegant algorithms that process the results of asynchronous code when they are ready. These extensions, which are found in the Task library, specify actions that take place when tasks finish. The easily readable code manipulates tasks as they finish in a very inefficient fashion.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020