Links

Task

A Task represents an asynchronous operation.

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

Cancel a task

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);
}

Fire and forget

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

_ = MyMethodAsync().ConfigureAwait(false);  // ConfigureAwait to avoid deadlock
MyMethodAsync().Forget();

async Task MyMethodAsync() {}

static class TaskExtension
{
    public static void Forget(this Task _)
    {
    }
}

Propriété WPF

Nuget:

Nito.Mvvm.Async prerelease
FontAwesome.WPF

MainWindow.xaml

MainVM.cs

Parallel

Utile si le code n'est pas séquentiel.

// 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

// 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.

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

private static 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.

int sum = 0; // shared variable, updated by threads
Parallel.For(0, 100, i =>
{
    Interlocked.Add(ref sum, 1);
});

AsyncLocal

Allow to have a different variable for each async task.

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.

BlockingCollection<string> col = new BlockingCollection<string>();
col.Add("text");
string s = col.Take();

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

Tasks

// 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
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);
}

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.

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));