A .NET Programmer’s Information to CancellationToken

Generally canceling is an effective factor. In a lot of my .NET initiatives, I’ve had loads of motivation to cancel each inside and exterior processes. Microsoft realized that builders have been approaching this widespread use case in quite a lot of advanced implementations and determined there have to be a greater means. Thus, a standard cancellation communication sample was launched as CancellationToken, which was constructed utilizing lower-level multithreading and interprocess communication constructs. As a part of my preliminary analysis into this sample—and after having dug by way of the precise .NET supply code for Microsoft’s implementation—I discovered that CancellationToken can resolve a wider set of issues: subscriptions on purposes’ run states, timing out operations utilizing completely different triggers, and basic interprocess communications through flags.

The Supposed CancellationToken Use Case

CancellationToken was launched in .NET 4 as a method to boost and standardize the present options for canceling operations. There are 4 basic approaches to dealing with cancellation that common programming languages are likely to implement:

CancellationToken resides within the closing class, the place the cancellation dialog is cooperative.

After Microsoft launched CancellationToken, the event neighborhood rapidly embraced it, notably as a result of many main .NET APIs have been up to date to make use of these tokens natively. For instance, starting with ASP.NET Core 2.0, actions assist an non-obligatory CancellationToken parameter that will sign if an HTTP request has been closed, permitting cancellation of any operation and thus avoiding pointless use of sources.

After a deep dive into the .NET codebase, it grew to become clear that CancellationToken’s utilization will not be restricted to cancellation.

CancellationToken Underneath a Microscope

When wanting extra carefully at CancellationToken’s implementation, we see it’s only a easy flag (i.e., ManualResetEvent) and the supporting infrastructure that gives the power to observe and alter that flag. CancellationToken’s principal utility is in its title, which suggests that is the widespread technique to cancel operations. These days, any .NET library, bundle, or framework with asynchronous or long-running operations permits cancellation by way of these tokens.

CancellationToken could also be triggered both by manually setting its flag to “true” or programming it to vary to “true” after a sure time span has elapsed. No matter how a CancellationToken is triggered, consumer code that’s monitoring this token might decide the token flag’s worth by way of considered one of three strategies:

• Utilizing a WaitHandle
• Polling the CancellationToken’s flag
• Informing the consumer code when the flag’s state is up to date by way of a programmatic subscription

After additional analysis within the .NET codebase, it grew to become evident that the .NET group discovered CancellationTokens helpful in different eventualities not related to cancellation. Let’s discover a few of these superior and off-brand use circumstances, which empower C# builders with multithreaded and interprocess coordination to simplify advanced conditions.

CancellationTokens for Superior Occasions

When writing ASP.NET Core purposes, we typically must know when our utility has began, or we have to inject our code into the host shutdown course of. In these circumstances, we use the IHostApplicationLifetime interface (beforehand IApplicationLifetime). This interface (from .NET Core’s repository) makes use of CancellationToken to speak three main occasions: ApplicationStarted, ApplicationStopping, and ApplicationStopped:

namespace Microsoft.Extensions.Internet hosting
{
    /// <abstract>
    /// Permits customers to be notified of utility lifetime occasions. 
    /// This interface will not be meant to be user-replaceable.
    /// </abstract>
    public interface IHostApplicationLifetime
    {
        /// <abstract>
        /// Triggered when the applying host has totally began.
        /// </abstract>
        CancellationToken ApplicationStarted { get; }

        /// <abstract>
        /// Triggered when the applying host is beginning a sleek shutdown.
        /// Shutdown will block till all callbacks registered on 
        /// this token have accomplished.
        /// </abstract>
        CancellationToken ApplicationStopping { get; }

        /// <abstract>
        /// Triggered when the applying host has accomplished a sleek shutdown.
        /// The applying is not going to exit till all callbacks registered on 
        /// this token have accomplished.
        /// </abstract>
        CancellationToken ApplicationStopped { get; }

        /// <abstract>
        /// Requests termination of the present utility.
        /// </abstract>
        void StopApplication();
    }
}

At first look, it might look like CancellationTokens don’t belong right here, particularly since they’re getting used as occasions. Nonetheless, additional examination reveals these tokens to be an ideal match:

• They’re versatile, permitting for a number of methods for the interface’s consumer to hear to those occasions.
• They’re thread-safe out of the field.
• They are often created from completely different sources by combining CancellationTokens.

Though CancellationTokens aren’t good for each occasion want, they are perfect for occasions that occur solely as soon as, like utility begin or cease.

CancellationToken for Timeout

By default, ASP.NET offers us little or no time during which to close down. In these circumstances the place we would like just a little extra time, utilizing the built-in HostOptions class permits us to vary this timeout worth. Beneath, this timeout worth is wrapped in a CancellationToken and fed into the underlying subprocesses.

IHostedService’s StopAsync methodology is a superb instance of this utilization:

namespace Microsoft.Extensions.Internet hosting
{
    /// <abstract>
    /// Defines strategies for objects which might be managed by the host.
    /// </abstract>
    public interface IHostedService
    {
        /// <abstract>
        /// Triggered when the applying host is able to begin the service.
        /// </abstract>
        /// <param title="cancellationToken">Signifies that the beginning
        ///     course of has been aborted.</param>
        Job StartAsync(CancellationToken cancellationToken);

        /// <abstract>
        /// Triggered when the applying host is performing a sleek shutdown.
        /// </abstract>
        /// <param title="cancellationToken">Signifies that the shutdown 
        ///     course of ought to now not be sleek.</param>
        Job StopAsync(CancellationToken cancellationToken);
    }
}

As evident within the IHostedService interface definition, the StopAsync methodology takes one CancellationToken parameter. The remark related to that parameter clearly communicates Microsoft’s preliminary intent for CancellationToken was as a timeout mechanism moderately than a cancellation course of.

For my part, if this interface had existed previous to CancellationToken’s existence, this might have been a TimeSpan parameter—to point how lengthy the cease operation was allowed to course of. In my expertise, timeout eventualities can virtually at all times be transformed to a CancellationToken with nice further utility.

For the second, let’s overlook that we all know how the StopAsync methodology is designed and as a substitute take into consideration how we might design this methodology’s contract. First let’s outline the necessities:

• The StopAsync methodology should attempt to cease the service.
• The StopAsync methodology ought to have a sleek cease state.
• No matter whether or not a sleek cease state is achieved, a hosted service should have a most time during which to cease, as outlined by our timeout parameter.

By having a StopAsync methodology in any type, we fulfill the primary requirement. The remaining necessities are difficult. CancellationToken satisfies these necessities precisely by utilizing a normal .NET flag-based communication device to empower the dialog.

CancellationToken As a Notification Mechanism

The most important secret behind CancellationToken is that it’s only a flag. Let’s illustrate how CancellationToken can be utilized to start out processes as a substitute of stopping them.

Take into account the next:

1. Create a RandomWorker class.
2. RandomWorker ought to have a DoWorkAsync methodology that executes some random work.
3. The DoWorkAsync methodology should enable a caller to specify when the work ought to start.

public class RandomWorker
{
    public RandomWorker(int id)
    {
        Id = id;
    }

    public int Id { get; }

    public async Job DoWorkAsync()
    {
        for (int i = 1; i <= 10; i++)
        {
            Console.WriteLine($"[Worker {Id}] Iteration {i}");
            await Job.Delay(1000);
        }
    }
}

The above class satisfies the primary two necessities, leaving us with the third. There are a number of alternate interfaces we may use to set off our employee, like a time span or a easy flag:

# With a time span
Job DoWorkAsync(TimeSpan startAfter);

# Or a easy flag
bool ShouldStart { get; set; }
Job DoWorkAsync();

These two approaches are high-quality, however nothing is as elegant as utilizing a CancellationToken:

public class RandomWorker
{
    public RandomWorker(int id)
    {
        Id = id;
    }

    public int Id { get; }

    public async Job DoWorkAsync(CancellationToken startToken)
    {
        startToken.WaitHandle.WaitOne();

        for (int i = 1; i <= 10; i++)
        {
            Console.WriteLine($"[Worker {Id}] Iteration {i}");
            await Job.Delay(1000);
        }
    }
}

This pattern consumer code illustrates the ability of this design:

utilizing System;
utilizing System.Linq;
utilizing System.Threading;
utilizing System.Threading.Duties;

namespace CancelToStart
{
    public class Program
    {
        static void Important(string[] args)
        {
            CancellationTokenSource startCts = new CancellationTokenSource();

            startCts.CancelAfter(TimeSpan.FromSeconds(10));

            var duties = Enumerable.Vary(0, 10)
                .Choose(i => new RandomWorker(i))
                .Choose(employee => employee.DoWorkAsync(startCts.Token))
                .ToArray();

            Job.WaitAll(duties, CancellationToken.None);
        }
    }
}

The CancellationTokenSource will create our CancellationToken behind the scenes and coordinate the triggering of all of the related processes. On this case, the related course of is our RandomWorker, which is ready to start out. This strategy permits us to leverage the thread security baked into the default CancellationToken implementation.

These examples show how CancellationToken supplies a toolbox of options which might be helpful exterior of its meant use case. The instruments can come in useful in lots of eventualities that contain interprocess flag-based communication. Whether or not we’re confronted with timeouts, notifications, or one-time occasions, we are able to fall again on this elegant, Microsoft-tested implementation.

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