Task

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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.

await Result
asynchronous wait blocking wait
"yield" the current thread block the current thread
(re-)raise the exception wrap the exception in an AggregateException

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));
Récupérée de « http://wiki.bananeatomic.fr/index.php?title=Task&oldid=31343 »