How to Ensure Sequential Task Execution and Re-entrancy in Asynchronous Operations?

Task Sequencing and Re-Entrancy

Issue:

Consider a scenario where tasks can be executed either synchronously or asynchronously. If a new task is received while a previous task is still pending, the new task should be queued and executed sequentially. Additionally, each task's result may depend on the result of the previous task. The challenge is to implement this logic while also supporting re-entrancy, where a task may be executed multiple times consecutively.

Solution:

To enforce sequential task execution and prevent breaking the logic due to re-entrancy, it is necessary to manually construct tasks using the Task constructor and start them later. The Task.Unwrap() method can be used to obtain the actual task that will be executed.

The modified AsyncOp class below implements this approach:

class AsyncOp<T>
{
    Task<T> _pending = Task.FromResult(default(T));

    public Task<T> CurrentTask { get { return _pending; } }

    public Task<T> RunAsync(Func<Task<T>> handler, bool useSynchronizationContext = false)
    {
        var pending = _pending;
        Func<Task<T>> wrapper = async () =>
        {
            // await the prev task
            var prevResult = await pending;
            Console.WriteLine("\nprev task result:  " + prevResult);
            // start and await the handler
            return await handler();
        };

        var task = new Task<Task<T>>(wrapper);
        var inner = task.Unwrap();
        _pending = inner;

        task.RunSynchronously(useSynchronizationContext ?
            TaskScheduler.FromCurrentSynchronizationContext() :
            TaskScheduler.Current);

        return inner;
    }
}

By using this modified AsyncOp class, the sequential task execution and re-entrancy are ensured. The modified code produces the desired output:

Test #1...

prev task result:  0
this task arg: 1000

prev task result:  1000
this task arg: 900

prev task result:  900
this task arg: 800

Press any key to continue to test #2...


prev task result:  800
this task arg: 100

prev task result:  100
this task arg: 200

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