Rising events without blocking and receiving events in the right order

Question

This required some explanation first. There is a working thread which has to rise some event:

Task.Run(() =>
{
    for(int i = 0; i < 123456789; i++)
    {
        ... // some job
        OnSomeEvent(i);
    }
});

Rising events synchronously will block the job until all event handlers have finished:

void OnSomeEvent(int i) => SomeEvent?.Invoke(this, new SomeEventArgs(i));

Asynchronous event rising will not block the job anymore (yay!)

void OnSomeEvent(int i) => Task.Run(() => SomeEvent?.Invoke(this, new SomeEventArgs(i)));

but now there is another problem: events are not received in the right order:

OnSomeEvent(1);
OnSomeEvent(2);
OnSomeEvent(3);
...

// event handler
SomeEvent += (s, e) => Console.WriteLine(e.I);

// possible output
1
3
2

Question: how to implement asynchronous events rising which occurs in the right order?

Recently I learned what Dispatcher.InvokeAsync uses queue. It looks like I have to do something similar. And if I have to then: 1) should it be a job of a caller or 2) should I keep rising events synchronously and receiver has to organize producer/consumer to prevent job from blocking? Or maybe there is another way?

P.S.: this has nothing to do with ContinueWhith.. unless storing list of tasks is a proper solution. My concern is how to implement fire-and-forget events where: a) caller is not blocked 2) events are received in same order.

P.P.S.: I don't know how to make MCVE to reproduce the problem. It appears in real project with heavy UI, lots of threads, etc.

Answer 1

You can use the following TaskQueue in order to add asynchronous operations to a queue so that each one is started when the previous item in the queue finishes:

public class TaskQueue
{
    private Task previous = Task.FromResult(false);
    private object key = new object();

    public Task<T> Enqueue<T>(Func<Task<T>> taskGenerator)
    {
        lock (key)
        {
            var next = previous.ContinueWith(t => taskGenerator()).Unwrap();
            previous = next;
            return next;
        }
    }
    public Task Enqueue(Func<Task> taskGenerator)
    {
        lock (key)
        {
            var next = previous.ContinueWith(t => taskGenerator()).Unwrap();
            previous = next;
            return next;
        }
    }
}

This allows you to write:

private TaskQueue taskQueue = new TaskQueue();
private void OnSomeEvent(int i) => 
    taskQueue.Enqueue(() => Task.Run(() => SomeEvent?.Invoke(this, new SomeEventArgs(i))));

Answer 2

You could use the ActionBlock queue from the TPL Dataflow to maintain a queue of events.

You would create the queue as follows:

queue = new ActionBlock<SomeEventArgs>(item => SomeEvent?.Invoke(item));

Then you would add events to the queue like so:

queue.Post(new SomeEventArgs(value));

When the queue is no longer needed you do this:

queue.Complete();

After that, if you need to wait until any items still in the queue are processed, you can do this:

queue.Completion.Wait();

However, note that queue.Completion is in fact a Task so you would often use it with await.

Here's a complete example showing one approach (which doesn't keep a thread alive all the time just to process the event queue):

using System;
using System.Threading;
using System.Threading.Tasks.Dataflow;

namespace Demo
{
    public class SomeEventArgs : EventArgs
    {
        public SomeEventArgs(int value)
        {
            Value = value;
        }

        public int Value { get; }
    }

    internal class Program
    {
        public delegate void SomeEventHandler(SomeEventArgs e);

        public event SomeEventHandler SomeEvent;

        ActionBlock<SomeEventArgs> queue;

        private void run()
        {
            queue = new ActionBlock<SomeEventArgs>(item => SomeEvent?.Invoke(item));

            // Subscribe to my own event (this just for demonstration purposes!)

            this.SomeEvent += Program_SomeEvent;

            // Raise 100 events.

            for (int i = 0; i < 100; ++i)
            {
                OnSomeEvent(i);
                Console.WriteLine("Raised event " + i);
            }

            Console.WriteLine("Signalling that queue is complete.");
            queue.Complete();

            Console.WriteLine("Waiting for queue to be processed.");
            queue.Completion.Wait();

            Console.WriteLine("Done.");
        }

        private void Program_SomeEvent(SomeEventArgs e)
        {
            Console.WriteLine("Handled " + e.Value);
            Thread.Sleep(1); // Simulate load.
        }

        private void OnSomeEvent(int value)
        {
            queue.Post(new SomeEventArgs(value));
        }

        private static void Main()
        {
            new Program().run();
        }
    }
}