How to serialize async/await?

Question

Let\'s suppose I have this simple snippet:

async void button_Click(object sender, RoutedEventArgs e)
{
    await Task.Factory.StartNew(() =>
    {
                


        

Answer 1

I might be missing something, but I don't think SemaphoreSlim is needed for the OP's scenario. I'd do it the following way. Basically, the code just await the previous pending instance of the task before continuing (no exception handling for clarity):

// the current pending task (initially a completed stub)
Task _pendingTask = Task.FromResult(true);

async void button_Click(object sender, RoutedEventArgs e)
{
    var previousTask = _pendingTask;

    _pendingTask = Task.Run(async () =>
    {
        await previousTask;

        Console.WriteLine("start");
        Thread.Sleep(5000);
        Console.WriteLine("end");
    });

    // the following "await" is optional, 
    // you only need it if you have other things to do 
    // inside "button_Click" when "_pendingTask" is completed
    await _pendingTask;
}

[UPDATE] To address the comment, here's a thread-safe version, when button_Click can be called concurrently:

Task _pendingTask = Task.FromResult(true);
object _pendingTaskLock = new Object();

async void button_Click(object sender, RoutedEventArgs e)
{
    Task thisTask;

    lock (_pendingTaskLock)
    {
        var previousTask = _pendingTask;

        // note the "Task.Run" lambda doesn't stay in the lock
        thisTask = Task.Run(async () =>
        {
            await previousTask;

            Console.WriteLine("start");
            Thread.Sleep(5000);
            Console.WriteLine("end");
        });

        _pendingTask = thisTask;
    }

    await thisTask;
}