« Task » : différence entre les versions

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(114 versions intermédiaires par le même utilisateur non affichées)
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[[Category:CSharp]]
[[Category:CSharp]]
= Description =
= Links =
Permet d'effectuer des tâches en parallèle ou des tâches sans consommer le thread de l'UI.<br />
* [[LINQ#Select_async|Select async]]
L'utilisation de Task permet de gagner en réactivité mais pas en performance.
* [[LINQ#Select_async|SelectMany async]]


= [https://msdn.microsoft.com/en-us/library/system.threading.tasks.task(v=vs.110).aspx Task] =
= Task =
Délégué asynchrone
A Task represents an asynchronous operation.
<kode lang=csharp>
// création et lancement asynchrone de la tâche
Task taskA = Task.Run(() => Thread.Sleep(4000));
Console.WriteLine("taskA Status: {0}", taskA.Status);  // WaitingToRun
Console.WriteLine("taskA Status: {0}", taskA.Status);  // Running
 
// attente de la fin de la tâche, code bloquant
taskA.Wait();
Console.WriteLine("taskA Status: {0}", taskA.Status);  // RanToCompletion
 
// valeur de retour
Task<int> t2 = Task.Run(() => 42 );
// code bloquant, attente de la fin de la tâche
t2.Result;  // 42
</kode>
 
= Enchaînement de tâches =
<kode lang=cs>
<kode lang=cs>
Task<int> t = Task.Run(() => 42)
// create and run a task in a new thread
.ContinueWith((antecedent) => antecedent.Result * 2);
var result = await Task.Run(async () =>  
 
Task<int> t = Task.Run(() => 42);
t.ContinueWith((antecedent) =>
{
{
     Console.WriteLine("Canceled");
     await Task.Delay(4000);
}, TaskContinuationOptions.OnlyOnCanceled); // run only if the antecedent has been canceled
     return 0;
 
});
t.ContinueWith((antecedent) =>
{
    Console.WriteLine("Faulted");
}, TaskContinuationOptions.OnlyOnFaulted); // run only if the antecedent threw an unhandled exception
 
var completedTask = t.ContinueWith((antecedent) =>
{
     Console.WriteLine("Completed");
}, TaskContinuationOptions.OnlyOnRanToCompletion); // run only if the antecedant ran to completion
 
completedTask.Wait();
</kode>
</kode>


= TaskFactory et sous-tâches =
= [https://stackoverflow.com/questions/27464287/what-is-the-difference-between-await-taskt-and-taskt-result await vs Result] =
<kode lang='cs'>
<syntaxhighlight lang='cs' inline>await task;</syntaxhighlight> will "yield" the current thread, it is an asynchronous wait.<br>
Task<int[]> parent = Task.Run(() =>
<syntaxhighlight lang='cs' inline>task.Result</syntaxhighlight> will block the current thread, it is a blocking wait.
{
    var results = new int[3];
    TaskFactory tf = new TaskFactory(TaskCreationOptions.AttachedToParent,
    TaskContinuationOptions.ExecuteSynchronously);
    tf.StartNew(() => results[0] = 0);
    tf.StartNew(() => results[1] = 1);
    tf.StartNew(() => results[2] = 2);
    return results;
});


var finalTask = parent.ContinueWith(parentTask =>
= Cancel a task =
{
    foreach (int i in parentTask.Result)
        Console.WriteLine(i);
});
 
finalTask.Wait();
 
// exécution simultanée des tâches
Task[] tasks;
Task.WaitAll(tasks);
 
// attende de la fin d'une des tâches
int index = Task.WaitAny(tasks);
</kode>
 
= Annuler une tâche =
<kode lang='cs'>
<kode lang='cs'>
var cancellationTokenSource = new CancellationTokenSource();
var cancellationTokenSource = new CancellationTokenSource(5000); // cancel after 5s
CancellationToken token = cancellationTokenSource.Token;
CancellationToken token = cancellationTokenSource.Token;


Task task = Task.Run(() =>
try
{
{
     while (!token.IsCancellationRequested)
     await Task.Run(async () =>
     {
     {
         Console.Write("*");
         var i = 1;
        Thread.Sleep(1000);
        while (!token.IsCancellationRequested) // break the loop if cancellation is requested
    }
        {
    token.ThrowIfCancellationRequested();
            Console.Write($"{i++} ");
}, token);
            await Task.Delay(1000);
        }


Console.WriteLine("Press enter to stop the task");
        token.ThrowIfCancellationRequested(); // or throw an OperationCanceledException
Console.ReadLine();
     }, token);
cancellationTokenSource.Cancel();
try
{
     task.Wait();
}
}
catch (AggregateException e)
catch (OperationCanceledException e)
{
{
     Console.WriteLine(e.InnerExceptions[0].Message);
     Console.WriteLine(e.Message);
}
}
cancellationTokenSource.Token.Register(() => {
    /* what to when the token is cancelled */
});
</kode>
</kode>


= async await =
= [https://www.meziantou.net/fire-and-forget-a-task-in-dotnet.htm Fire and forget] =
Permet d'écrire facilement du code asynchrone, exemple: ne pas bloquer le thread graphique lors de tâches longues à exécuter.<br>
Call an async method without waiting for the response. Exceptions will be lost.
Alternative élégante au [[BackgroundWorker_et_Dispatcher|BackgroundWorker]]<br />
<kode lang='cs'>
Une méthode {{boxx|async}} tourne séquentiellement mais permet de faire des appels {{boxx|await}} sur des blocs de code.<br />
// without await, the method is called and the following lines of code are executed without waiting the end of MyMethodAsync
{{boxx|await}} permet d’exécuter des tâches longues sans bloquer le thread UI.
MyMethodAsync();
* [https://msdn.microsoft.com/en-us/library/mt674882.aspx Programmation asynchrone]
* [http://blog.stephencleary.com/2012/02/async-and-await.html Guidelines]
<kode lang=cs>
button.Clicked += Button_Clicked;


// La méthode async doit retourner Task ou Task<T>, void est possible mais réservé au handler d'événement
// without await, the code is executed in background
async void Button_Clicked(object sender, EventArgs e)
// you may need to create a scope if you want to consume scoped services
Task.Run(async () =>
{
{
     // appel asynchrone d'une méthode asynchrone
     Method1();
     await DoItAsync();
    await Method2Async(); // wait the end of Method2Async before calling Method3
     Method3();
});
 
MyMethodAsync().Forget();


    // ou exécution direct du code avec le mot clé await
async Task MyMethodAsync() {}
    label.Text = "Traitement en cours !!!";
    await Task.Run(() =>
        {
            System.Threading.Thread.Sleep(10000);
        });
    label.Text = "Traitement terminé !!!";
}


// La méthode async doit retourner Task ou Task<T>, void est possible mais réservé au handler d'événement
public static class TaskExtension
// Le nom de la méthode doit se terminer par Async (convention)
async Task DoItAsync()
{
{
     label1.Text = "Traitement en cours !!!";
     public static void Forget(this Task task)
    {
        if (!task.IsCompleted || task.IsFaulted)
        {
            _ = ForgetAwaited(task);
        }


    await Task.Run(() =>
        async static Task ForgetAwaited(Task task)
         {
         {
             System.Threading.Thread.Sleep(5000);
             await task.ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);
         });
         }
     // le code suivant ne sera exécute qu'une fois la tache terminée
     }
    // ce qui simplifie le code en évitant la création d'un event TaskCompleted pour avertir de la fin de la tache
    label1.Text = "Traitement terminé !!!";
}
</kode>


== Callback ==
    // simple version: returns void
<kode lang='cs'>
    public static async void Forget(this Task task)
private async Task DoItAsync(string param, Action callback)
    {
{
        await task.ConfigureAwait(false); // ConfigureAwait to avoid deadlock
    // Do some stuff
     }
    await MyMethodAsync();
 
    // Then, when it's done, call the callback
     callback();
</kode>
 
== Paramètre out ==
Les méthodes async ne supportent pas les paramètres out.<br>
Utiliser un tuple comme paramètre de retour à la place.
<kode lang='cs'>
public async Task<(int status, string result)> MyMethod() {}
</kode>
 
== Application Console ==
<kode lang='cs'>
// permet d'appeler une méthode async dans la méthode Main d'un projet Console
string result = MyMethodAsync().GetAwaiter().GetResult();
 
static async Task<string> MyMethodAsync()
{
}
}
</kode>
</kode>


== [https://stackoverflow.com/questions/48212998/how-to-call-an-async-method-from-a-property-setter/48217792?noredirect=1#comment83625536_48217792 Propriété WPF] ==
= [https://stackoverflow.com/questions/48212998/how-to-call-an-async-method-from-a-property-setter/48217792?noredirect=1#comment83625536_48217792 Propriété WPF] =
Nuget:
Nuget:
* {{boxx|Nito.Mvvm.Async}} prerelease
* {{boxx|Nito.Mvvm.Async}} prerelease
* {{boxx|FontAwesome.WPF}}
* {{boxx|FontAwesome.WPF}}
<filebox fn='MainWindow.xaml'>
 
<filebox fn='MainWindow.xaml' collapsed>
<Window xmlns:Controls="clr-namespace:System.Windows.Controls;assembly=PresentationFramework"
<Window xmlns:Controls="clr-namespace:System.Windows.Controls;assembly=PresentationFramework"
         xmlns:fa="http://schemas.fontawesome.io/icons/">
         xmlns:fa="http://schemas.fontawesome.io/icons/">
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                                 Converter={StaticResource BooleanToVisibilityConverter}}"/>
                                 Converter={StaticResource BooleanToVisibilityConverter}}"/>
</filebox>
</filebox>
<filebox fn='MainVM.cs'>
 
<filebox fn='MainVM.cs' collapsed>
private string _query;
private string _query;
public string Query
public string Query
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</filebox>
</filebox>


= Exceptions =
= Parallel =
Utile si le code n'est pas séquentiel.
<kode lang='cs'>
// for loop
Parallel.For(1, 20, i =>
{
    Console.WriteLine(i);
    Thread.Sleep(1000);
});
 
// for each loop
Parallel.ForEach(Enumerable.Range(1, 20), i =>
{
    Console.WriteLine(i);
    Thread.Sleep(1000);
});
 
// invoke actions
Parallel.Invoke(
    () => {
        Console.WriteLine(1);
        Thread.Sleep(1000);
    },
    () => {
        Console.WriteLine(2);
        Thread.Sleep(1000);
    }
);
</kode>
 
== Parallel options ==
<kode lang='cs'>
// after 4s throw an OperationCanceledException
// no further operations will start but don't stop currently executing operations
var cancellationTokenSource = new CancellationTokenSource(4000);
 
var parallelOptions = new ParallelOptions
{
    MaxDegreeOfParallelism = 12, // by default use as much computer power as possible
    TaskScheduler = null,
    CancellationToken = cancellationTokenSource.Token
}
 
Parallel.ForEach(
    numbers,
    parallelOptions,
    (int i, ParallelLoopState loopState) =>
{
    if (loopState.ShouldExitCurrentIteration) // check if another iteration has requested to break
    {
        loopState.Break(); // break loop
    }
 
    if (!cancellationTokenSource.Token.IsCancellationRequested) { /* next operation step */ } // useful for long operation to break
});
</kode>
 
== Handling exceptions ==
All the exceptions are catched and when all the tasks have been executed then an AggregateException is thrown if any.
<kode lang='cs'>
<kode lang='cs'>
catch (AggregateException e)
try
{
    Parallel.Invoke(
        () =>
        {
            var waitTime = DateTime.UtcNow.AddSeconds(4);
            while (DateTime.UtcNow < waitTime) { }
        },
        () =>
        {
            throw new Exception("MyException");
        }
    );
}
catch (AggregateException ex)
{
{
     Console.WriteLine("There where {0} exceptions", e.InnerExceptions.Count);
     ex.InnerExceptions; // ReadOnlyCollection<Exception>
}
}
</kode>
</kode>


= Accès aux ressources =
== Shared variable ==
=== [https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/statements/lock Lock] ===
<kode lang='cs'>
private readonly object sumLock = new();
 
var sum = 0m; // shared variable, updated by threads
Parallel.For(0, 100, i =>
{
    lock(sumLock) // only 1 thread at a time can access
    {
        sum += 0.5m; // code inside the lock should take as little time as possible
    }
});
</kode>
{{warn | To avoid deadlocks:
* use 1 lock object for each shared resource
* avoid nested locks
* use a {{boxx|new object}}}}
 
=== Interlocked ===
Create thread-safe atomic operations.
{{warn | Faster than lock, but {{boxx|Interlocked}} only works with integers.}}
<kode lang='cs'>
int sum = 0; // shared variable, updated by threads
Parallel.For(0, 100, i =>
{
    Interlocked.Increment(ref sum); // add 1 to sum and return sum + 1
    Interlocked.Add(ref sum, 2);    // add 2 to sum and return sum + 2
});
</kode>
 
== AsyncLocal ==
Allow to have a different variable for each async task.
<kode lang='cs'>
private static AsyncLocal<decimal?> asyncLocal = new();
Parallel.For(0, 100, async (i) =>
{
    asyncLocal.Value = 10; // the asyncLocal is not shared among async tasks
});
</kode>
 
== Concurrent collections ==
== Concurrent collections ==
{| class="wikitable wtp wtmono1"  
{| class="wikitable wtp wtmono1"  
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</kode>
</kode>


== lock ==
== Tasks ==
Permet de synchroniser l'accès aux ressources.
{{warn | Deadlock si 2 threads s'attendent l'un l'autre.}}
<kode lang='cs'>
<kode lang='cs'>
// pour lock, utiliser un élément private de type ref sauf string
// AsParallel PLINQ
object _lock = new object();
var tasks = Enumerable.Range(1, 30).AsParallel().Select(x => MyTaskAsync(x));
await Task.WhenAll(tasks);


int n = 0;
// ForEachAsync
var up = Task.Run(() =>
await Parallel.ForEachAsync(Enumerable.Range(1, 30), async (x, token) => await MyTaskAsync(x));
 
// Task.Run run task in a new thread
// even without Task.Run, Task.WhenAll will run the tasks in parallel and then for all of them to be done
var tasks = Enumerable.Range(1, 30).Select(x => Task.Run(() => MyTaskAsync(x)));
await Task.WhenAll(tasks);
 
private async Task MyTaskAsync(int i)
{
{
     for (int i = 0; i < 1_000_000; i++)
     Console.WriteLine(i);
     {
    await Task.Delay(4000);
        lock (_lock)
}
            n++;
</kode>
     }
 
});
== Handle exceptions with Task.WhenAll ==
=== The problem ===
If at least one exception occurred among the tasks, it is not possible to get the result of the working tasks.
<kode lang='cs' collapsed>
// create 10 jobs in parallel, the jobs 5 and 7 will raise an Exception
IEnumerable<Task<Job>> createJobTasks = Enumerable.Range(1, 10).Select(x => CreateJobAsync($"Job {x}"));
Task<Job[]> mainTask = Task.WhenAll(createJobTasks);
try
{
     var jobs = await mainTask;
}
catch (Exception e)
{
    e.Message; // Error Job 5, only the first exception is catched
     // AggregateException
    mainTask.Exception.Message; // One or more errors occurred. (Error Job 5) (Error Job 7)
    // ReadOnlyCollection<Exception>
    mainTask.InnerExceptions; // [0] Error Job 5, [1] Error Job 7
}


for (int i = 0; i < 1_000_000; i++)
async Task<Job> CreateJobAsync(string name)
{
{
     lock (_lock)
     await Task.Delay(1);
         n--;
 
    if (name.EndsWith("5") || name.EndsWith("7") )
         throw new Exception($"Error {name}");
 
    return new Job { Name = name };
}
}


up.Wait();
class Job
Console.WriteLine(n);
{
// sans lock, le résultat est toujours différent
    public string Name { get; set; }
}
</kode>
</kode>


=== Solution: use the task ids ===
<kode lang='cs'>
<kode lang='cs'>
// lock sur une ressource statique
List<(string, Task<Job>)> jobNameWithCreateJobTasks = Enumerable.Range(1, 10).Select(x => (jobName: $"Job {x}", task: CreateJobAsync($"Job {x}"))).ToList();
public static class MyStaticResource
Dictionary<int, string> jobNameFromTaskId = jobNameWithCreateJobTasks.ToDictionary(x => x.task.Id, x => x.jobName);
List<Task<Job>> createJobTasks = jobNameWithCreateJobTasks.Select(x => x.task).ToList();
 
List<(string JobName, Job? Job, string ErrorMessage)> results;
var results = await TaskExtension.WhenAll(createJobTasks);
 
results.Select(x => new
    {
        JobName = x.Result?.JobName ?? jobNameFromTaskId[x.taskId],
        Job = x.Result,
        GlobalErrorMessage = x.Exception?.Message ?? string.Empty
    }));
</kode>
 
<filebox fn='TaskExtension.cs' collapsed>
public static async Task<IReadOnlyCollection<(int taskId, T? Result, AggregateException? Exception)>> WhenAll<T>(IReadOnlyCollection<Task<T>> tasks)
{
{
     static object _lock = new object();
     ArgumentNullException.ThrowIfNull(tasks);


     public static void DoSomething()
     List<(int taskId, T? Result, AggregateException? Exception)> results = new();
 
    try
     {
     {
         lock (_lock)
         await Task.WhenAll(tasks);
        { ... }
     }
     }
    catch
    {
        results.AddRange(tasks.Where(x => x.IsFaulted).Select(x => (x.Id, (T?)default, x.Exception)));
    }
    finally
    {
        results.AddRange(
            tasks.Where(x => x.IsCanceled).Select(x => (x.Id, (T?)default,
            (AggregateException?)new AggregateException(new[] { new TaskCanceledException(x) }))));
        results.AddRange(
            tasks.Where(x => x.IsCompletedSuccessfully).Select(x => (x.Id, (T?)x.Result,
            (AggregateException?)default)));
    }
    return results;
}
}
</kode>
</filebox>
[[Design_Patterns#Double-checked_locking|Double-checked locking]]
 
=== [https://stackoverflow.com/questions/55887028/is-it-possible-to-get-successful-results-from-a-task-whenall-when-one-of-the-tas Solution: use ContinueWith] ===
<kode lang='cs' collapsed>
var (jobs, exceptions) = await WhenAllWithExceptions(createJobTasks);


== Interlocked ==
static Task<(T[] Results, Exception[] Exceptions)> WhenAllWithExceptions<T>(IReadOnlyCollection<Task<T>> tasks)
Permet de réaliser des opérations atomiques. Évite qu'un autre thread s’insère entre la lecture et l'écriture d'une variable.
<kode lang='cs'>
var up = Task.Run(() =>
{
{
     for (int i = 0; i < 1_000_000; i++)
     ArgumentNullException.ThrowIfNull(tasks);
         Interlocked.Increment(ref n);
 
});
    return Task.WhenAll(tasks).ContinueWith(t =>
         {
            T[] results = tasks
                .Where(t => t.IsCompletedSuccessfully)
                .Select(t => t.Result)
                .ToArray();
 
            var aggregateExceptions = tasks
                .Where(t => t.IsFaulted)
                .Select(t => t.Exception!);


for (int i = 0; i < 1_000_000; i++)
            var exceptions = new AggregateException(aggregateExceptions)
    Interlocked.Decrement(ref n);
                .Flatten()
                .InnerExceptions
                .ToArray();


// assigne une nouvelle valeur et retourne l'ancienne valeure.
            // No exceptions and at least one task was canceled
int oldValue = Interlocked.Exchange(ref value, newValue);
            if (exceptions.Length == 0 && t.IsCanceled)
            {
                exceptions = new[] { new TaskCanceledException(t) };
            }


// si value == compareTo alors value = newValue
            return (results, exceptions);
Interlocked.CompareExchange(ref value, newValue, compareTo);
        },
        default,
        TaskContinuationOptions.DenyChildAttach | TaskContinuationOptions.ExecuteSynchronously,
        TaskScheduler.Default);
}
</kode>
</kode>


= Parallel =
=== [https://thesharperdev.com/csharps-whenall-and-exception-handling/ Solution: wrapping the task into a TaskResult] ===
Utile si le code n'est pas séquentiel.
Instead of having a unique try/catch for all the tasks, have it for each task.
<kode lang='cs'>
<kode lang='cs' collapsed>
Parallel.For(0, 10, i =>
// the created jobs are wrapped into TaskResult to handle Exception
{
IEnumerable<Task<TaskResult<Job>>> createJobTasks = Enumerable.Range(1, 10).Select(x => CreateJobAsync($"Job {x}").ToTaskResultAsync());
    Thread.Sleep(1000);
var jobs = await Task.WhenAll(createJobTasks);
    Console.WriteLine(i);
 
});
// if the TaskResult is a succes then access to the Result otherwise access to the Exception ErrorMessage
var writeTasks = jobs.Select(x => Task.Run(() => Console.WriteLine(x.Success ? x.Result.Name : x.ErrorMessage)));
await Task.WhenAll(writeTasks);
</kode>


var numbers = Enumerable.Range(10, 20);
<filebox fn='TaskExtension.cs' collapsed>
Parallel.ForEach(numbers, (int i, ParallelLoopState loopState) =>
public static class TaskExtension
{
{
     Thread.Sleep(1000);
     public static async Task<TaskResult<T>> ToTaskResultAsync<T>(this Task<T> task)
    Console.WriteLine(i);
    {
        try
        {
            return new TaskResult<T> { Result = await task };
        }
        catch (Exception e)
        {
            return new TaskResult<T> { Exception = e };
        }
    }


     // break loop
     public class TaskResult<T>
     loopState.Break();
     {
});
        public T? Result;
</kode>
        public Exception? Exception { get; set; }
        public string ErrorMessage => Exception?.InnerException?.Message ?? Exception?.Message ?? string.Empty;
        public bool Success => Exception is null;
    }
}
</filebox>


== PLINQ ==
== PLINQ ==
{{boxx|AsParallel}} analyses the query to see if it is suitable for parallelization. This analysis adds overhead.<br>
If it is unsafe or faster to run sequentially then it won't be run in parallel.
<kode lang='cs'>
<kode lang='cs'>
var numbers = Enumerable.Range(0, 100_000_000);
var numbers = Enumerable.Range(0, 100_000_000);


var parallelResult = numbers.AsParallel().AsOrdered()
var parallelResult = numbers.AsParallel()
  .Where(i => i % 2 == 0);
                            .WithDegreeOfParallelism(2)
                            .WithCancellation(token)
                            .WithExecutionMode(ParallelExecutionMode.ForceParallelism)
                            .WithMergeOptions(ParallelMergeOptions.Default)
                            .AsOrdered() // add overhead
                            .Where(i => i % 2 == 0);


// parcourt d'itération en mode parallèle, l'ordre est perdu.
// parcourt d'itération en mode parallèle, l'ordre est perdu.
// le parcourt commence même si parallelResult n'est pas au complet
// le parcourt commence même si parallelResult n'est pas au complet
parallelResult.ForAll(e => Console.WriteLine(e));
parallelResult.ForAll(e => Console.WriteLine(e));
</kode>
== [https://blogs.msdn.microsoft.com/pfxteam/2012/03/05/implementing-a-simple-foreachasync-part-2/ ForEachAsync] ==
<kode lang='cs'>
// extension de méthode
public static Task ForEachAsync<T>(this IEnumerable<T> source, int dop, Func<T, Task> body)
{
    return Task.WhenAll(
        from partition in Partitioner.Create(source).GetPartitions(dop)
        select Task.Run(async delegate
        {
            using (partition)
                while (partition.MoveNext())
                    await body(partition.Current);
        }));
}
await myItems.ForEachAsync(100, async (myItem) =>
{
    myItem.MyProperty = await MyMethodAsync();
});
</kode>
</kode>

Dernière version du 18 septembre 2024 à 12:27

Links

Task

A Task represents an asynchronous operation.

Cs.svg
// create and run a task in a new thread
var result = await Task.Run(async () => 
{
    await Task.Delay(4000);
    return 0;
});

await vs Result

await task; will "yield" the current thread, it is an asynchronous wait.
task.Result will block the current thread, it is a blocking wait.

Cancel a task

Cs.svg
var cancellationTokenSource = new CancellationTokenSource(5000); // cancel after 5s
CancellationToken token = cancellationTokenSource.Token;

try
{
    await Task.Run(async () =>
    {
        var i = 1;
        while (!token.IsCancellationRequested) // break the loop if cancellation is requested
        {
            Console.Write($"{i++} ");
            await Task.Delay(1000);
        }

        token.ThrowIfCancellationRequested(); // or throw an OperationCanceledException
    }, token);
}
catch (OperationCanceledException e)
{
    Console.WriteLine(e.Message);
}

cancellationTokenSource.Token.Register(() => {
    /* what to when the token is cancelled */
});

Fire and forget

Call an async method without waiting for the response. Exceptions will be lost.

Cs.svg
// without await, the method is called and the following lines of code are executed without waiting the end of MyMethodAsync
MyMethodAsync();

// without await, the code is executed in background
// you may need to create a scope if you want to consume scoped services
Task.Run(async () =>
{
    Method1();
    await Method2Async(); // wait the end of Method2Async before calling Method3
    Method3();
});

MyMethodAsync().Forget();

async Task MyMethodAsync() {}

public static class TaskExtension
{
    public static void Forget(this Task task)
    {
        if (!task.IsCompleted || task.IsFaulted)
        {
            _ = ForgetAwaited(task);
        }

        async static Task ForgetAwaited(Task task)
        {
            await task.ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);
        }
    }

    // simple version: returns void
    public static async void Forget(this Task task)
    {
        await task.ConfigureAwait(false);  // ConfigureAwait to avoid deadlock
    }
}

Propriété WPF

Nuget:

  • Nito.Mvvm.Async prerelease
  • FontAwesome.WPF
MainWindow.xaml
<Window xmlns:Controls="clr-namespace:System.Windows.Controls;assembly=PresentationFramework"
        xmlns:fa="http://schemas.fontawesome.io/icons/">
    <Window.Resources>
        <Controls:BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter" />
    </Window.Resources>

    <TextBox Text="{Binding Query, UpdateSourceTrigger=PropertyChanged}" />
    <TextBlock Text="{Binding ResultTask.Result}" />

    <Label Content="Loading..." 
           Visibility="{Binding ResultTask.IsNotCompleted, 
                                Converter={StaticResource BooleanToVisibilityConverter}, 
                                FallbackValue=Collapsed}"/>
    <fa:ImageAwesome Icon="Refresh" Spin="True" Height="16" Width="16" 
                     Visibility="{Binding ResultTask.IsNotCompleted, 
                     Converter={StaticResource BooleanToVisibilityConverter}, 
                     FallbackValue=Collapsed}" />

    <Label Content="{Binding NotifyValuesTask.ErrorMessage}" 
           Visibility="{Binding ResultTask.IsFaulted, 
                                Converter={StaticResource BooleanToVisibilityConverter}}"/>
MainVM.cs
private string _query;
public string Query
{
    get { return _query; }
    set
    {
        Set(() => Query, ref _query, value, true);
        ResultTask = NotifyTask.Create(GetResultAsync(_query, MyCallback));
    }
}

private void MyCallback() { /* ... */ }

private NotifyTask<string> _resultTask;
public NotifyTask<string> ResultTask
{
    get
    {
        return _resultTask;
    }
    set
    {
        Set(() => ResultTask, ref _resultTask, value, true);
    }
}

public async Task<string> GetResultAsync(string query, Action callback)
{
    var url = $"http://localhost:57157/api/v1/test/result/{query}";
    var responseMessage = await _client.GetAsync(url);
    if (responseMessage.IsSuccessStatusCode)
    {
        return await responseMessage.Content.ReadAsStringAsync();
    }
    else
    {
        return await Task.FromResult($"{responseMessage.StatusCode}: {responseMessage.ReasonPhrase}");
    }

    callback();
}

Parallel

Utile si le code n'est pas séquentiel.

Cs.svg
// for loop
Parallel.For(1, 20, i =>
{
    Console.WriteLine(i);
    Thread.Sleep(1000);
});

// for each loop
Parallel.ForEach(Enumerable.Range(1, 20), i =>
{
    Console.WriteLine(i);
    Thread.Sleep(1000);
});

// invoke actions
Parallel.Invoke(
    () => {
        Console.WriteLine(1);
        Thread.Sleep(1000);
    },
    () => {
        Console.WriteLine(2);
        Thread.Sleep(1000);
    }
);

Parallel options

Cs.svg
// after 4s throw an OperationCanceledException
// no further operations will start but don't stop currently executing operations
var cancellationTokenSource = new CancellationTokenSource(4000);

var parallelOptions = new ParallelOptions
{
    MaxDegreeOfParallelism = 12, // by default use as much computer power as possible
    TaskScheduler = null,
    CancellationToken = cancellationTokenSource.Token
}

Parallel.ForEach(
    numbers,
    parallelOptions,
    (int i, ParallelLoopState loopState) =>
{
    if (loopState.ShouldExitCurrentIteration) // check if another iteration has requested to break
    {
        loopState.Break(); // break loop
    }

    if (!cancellationTokenSource.Token.IsCancellationRequested) { /* next operation step */ } // useful for long operation to break
});

Handling exceptions

All the exceptions are catched and when all the tasks have been executed then an AggregateException is thrown if any.

Cs.svg
try
{
    Parallel.Invoke(
        () =>
        {
            var waitTime = DateTime.UtcNow.AddSeconds(4);
            while (DateTime.UtcNow < waitTime) { }
        },
        () =>
        {
            throw new Exception("MyException");
        }
    );
}
catch (AggregateException ex)
{
    ex.InnerExceptions; // ReadOnlyCollection<Exception>
}

Shared variable

Lock

Cs.svg
private readonly object sumLock = new();

var sum = 0m; // shared variable, updated by threads
Parallel.For(0, 100, i =>
{
    lock(sumLock) // only 1 thread at a time can access
    {
        sum += 0.5m; // code inside the lock should take as little time as possible
    }
});
To avoid deadlocks:
  • use 1 lock object for each shared resource
  • avoid nested locks
  • use a new object

Interlocked

Create thread-safe atomic operations.

Faster than lock, but Interlocked only works with integers.
Cs.svg
int sum = 0; // shared variable, updated by threads
Parallel.For(0, 100, i =>
{
    Interlocked.Increment(ref sum); // add 1 to sum and return sum + 1
    Interlocked.Add(ref sum, 2);    // add 2 to sum and return sum + 2
});

AsyncLocal

Allow to have a different variable for each async task.

Cs.svg
private static AsyncLocal<decimal?> asyncLocal = new();
Parallel.For(0, 100, async (i) =>
{
    asyncLocal.Value = 10; // the asyncLocal is not shared among async tasks
});

Concurrent collections

BlockingCollection<T> ajout et suppression thread-safe. Add, Take. FIFO par défaut.
ConcurrentBag<T> sans ordre, doublons autorisés. Add, TryTake, TryPeek.
ConcurrentDictionary<TKey,T> TryAdd, TryUpdate, AddOrUpdate, GetOrAdd.
ConcurrentQueue<T> FIFO. Enqueue, TryDequeue.
ConcurrentStack<T> LIFO. Push, TryPop.
Cs.svg
BlockingCollection<string> col = new BlockingCollection<string>();
col.Add("text");
string s = col.Take();

foreach (string v in col.GetConsumingEnumerable())
    Console.WriteLine(v);

Tasks

Cs.svg
// AsParallel PLINQ
var tasks = Enumerable.Range(1, 30).AsParallel().Select(x => MyTaskAsync(x));
await Task.WhenAll(tasks);

// ForEachAsync
await Parallel.ForEachAsync(Enumerable.Range(1, 30), async (x, token) => await MyTaskAsync(x));

// Task.Run run task in a new thread
// even without Task.Run, Task.WhenAll will run the tasks in parallel and then for all of them to be done
var tasks = Enumerable.Range(1, 30).Select(x => Task.Run(() => MyTaskAsync(x)));
await Task.WhenAll(tasks);

private async Task MyTaskAsync(int i)
{
    Console.WriteLine(i);
    await Task.Delay(4000);
}

Handle exceptions with Task.WhenAll

The problem

If at least one exception occurred among the tasks, it is not possible to get the result of the working tasks.

Cs.svg
// create 10 jobs in parallel, the jobs 5 and 7 will raise an Exception
IEnumerable<Task<Job>> createJobTasks = Enumerable.Range(1, 10).Select(x => CreateJobAsync($"Job {x}"));
Task<Job[]> mainTask = Task.WhenAll(createJobTasks);
try
{
    var jobs = await mainTask;
}
catch (Exception e)
{
    e.Message; // Error Job 5, only the first exception is catched
    // AggregateException
    mainTask.Exception.Message; // One or more errors occurred. (Error Job 5) (Error Job 7)
    // ReadOnlyCollection<Exception>
    mainTask.InnerExceptions; // [0] Error Job 5, [1] Error Job 7
} 

async Task<Job> CreateJobAsync(string name)
{
    await Task.Delay(1);

    if (name.EndsWith("5") || name.EndsWith("7") )
        throw new Exception($"Error {name}");

    return new Job { Name = name };
}

class Job
{
    public string Name { get; set; }
}

Solution: use the task ids

Cs.svg
List<(string, Task<Job>)> jobNameWithCreateJobTasks = Enumerable.Range(1, 10).Select(x => (jobName: $"Job {x}", task: CreateJobAsync($"Job {x}"))).ToList();
Dictionary<int, string> jobNameFromTaskId = jobNameWithCreateJobTasks.ToDictionary(x => x.task.Id, x => x.jobName);
List<Task<Job>> createJobTasks = jobNameWithCreateJobTasks.Select(x => x.task).ToList();

List<(string JobName, Job? Job, string ErrorMessage)> results;
var results = await TaskExtension.WhenAll(createJobTasks);

results.Select(x => new
    {
        JobName = x.Result?.JobName ?? jobNameFromTaskId[x.taskId],
        Job = x.Result,
        GlobalErrorMessage = x.Exception?.Message ?? string.Empty
    }));
TaskExtension.cs
public static async Task<IReadOnlyCollection<(int taskId, T? Result, AggregateException? Exception)>> WhenAll<T>(IReadOnlyCollection<Task<T>> tasks)
{
    ArgumentNullException.ThrowIfNull(tasks);

    List<(int taskId, T? Result, AggregateException? Exception)> results = new();

    try
    {
        await Task.WhenAll(tasks);
    }
    catch
    {
        results.AddRange(tasks.Where(x => x.IsFaulted).Select(x => (x.Id, (T?)default, x.Exception)));
    }
    finally
    {
        results.AddRange(
            tasks.Where(x => x.IsCanceled).Select(x => (x.Id, (T?)default,
            (AggregateException?)new AggregateException(new[] { new TaskCanceledException(x) }))));
        results.AddRange(
            tasks.Where(x => x.IsCompletedSuccessfully).Select(x => (x.Id, (T?)x.Result,
            (AggregateException?)default)));
    }

    return results;
}

Solution: use ContinueWith

Cs.svg
var (jobs, exceptions) = await WhenAllWithExceptions(createJobTasks);

static Task<(T[] Results, Exception[] Exceptions)> WhenAllWithExceptions<T>(IReadOnlyCollection<Task<T>> tasks)
{
    ArgumentNullException.ThrowIfNull(tasks);

    return Task.WhenAll(tasks).ContinueWith(t =>
        {
            T[] results = tasks
                .Where(t => t.IsCompletedSuccessfully)
                .Select(t => t.Result)
                .ToArray();

            var aggregateExceptions = tasks
                .Where(t => t.IsFaulted)
                .Select(t => t.Exception!);

            var exceptions = new AggregateException(aggregateExceptions)
                .Flatten()
                .InnerExceptions
                .ToArray();

            // No exceptions and at least one task was canceled
            if (exceptions.Length == 0 && t.IsCanceled)
            {
                exceptions = new[] { new TaskCanceledException(t) };
            }

            return (results, exceptions);
        },
        default,
        TaskContinuationOptions.DenyChildAttach | TaskContinuationOptions.ExecuteSynchronously,
        TaskScheduler.Default);
}

Solution: wrapping the task into a TaskResult

Instead of having a unique try/catch for all the tasks, have it for each task.

Cs.svg
// the created jobs are wrapped into TaskResult to handle Exception
IEnumerable<Task<TaskResult<Job>>> createJobTasks = Enumerable.Range(1, 10).Select(x => CreateJobAsync($"Job {x}").ToTaskResultAsync());
var jobs = await Task.WhenAll(createJobTasks);

// if the TaskResult is a succes then access to the Result otherwise access to the Exception ErrorMessage
var writeTasks = jobs.Select(x => Task.Run(() => Console.WriteLine(x.Success ? x.Result.Name : x.ErrorMessage)));
await Task.WhenAll(writeTasks);
TaskExtension.cs
public static class TaskExtension
{
    public static async Task<TaskResult<T>> ToTaskResultAsync<T>(this Task<T> task)
    {
        try
        {
            return new TaskResult<T> { Result = await task };
        }
        catch (Exception e)
        {
            return new TaskResult<T> { Exception = e };
        }
    }

    public class TaskResult<T>
    {
        public T? Result;
        public Exception? Exception { get; set; }
        public string ErrorMessage => Exception?.InnerException?.Message ?? Exception?.Message ?? string.Empty;
        public bool Success => Exception is null;
    }
}

PLINQ

AsParallel analyses the query to see if it is suitable for parallelization. This analysis adds overhead.
If it is unsafe or faster to run sequentially then it won't be run in parallel.

Cs.svg
var numbers = Enumerable.Range(0, 100_000_000);

var parallelResult = numbers.AsParallel()
                            .WithDegreeOfParallelism(2)
                            .WithCancellation(token)
                            .WithExecutionMode(ParallelExecutionMode.ForceParallelism)
                            .WithMergeOptions(ParallelMergeOptions.Default)
                            .AsOrdered() // add overhead
                            .Where(i => i % 2 == 0);

// parcourt d'itération en mode parallèle, l'ordre est perdu.
// le parcourt commence même si parallelResult n'est pas au complet
parallelResult.ForAll(e => Console.WriteLine(e));