Why async functions are called twice?

Question

I\'m using Threading timer to do some periodic job:

private static async void TimerCallback(object state)
{
        if (Interlocked.CompareExchange(ref curre         


        

Answer 1

When you use Task.WhenAll on a IEnumerable> it will return a T[] of the completed Tasks results. You need to save that variable and use it or else you will end up with the multiple enumerations like Henzi mentioned in his answer.

Here is a solution without the unnecessarily calling of .ToList()

private static async void TimerCallback(object state)
{
        if (Interlocked.CompareExchange(ref currentlyRunningTasksCount, 1, 0) != 0)
        {
            return;
        }

        var tasksRead = Enumerable.Range(3, 35).Select(i => ReadSensorsAsync(i));
        var finshedTasks = await Task.WhenAll(tasksRead);
        var tasksRecord = finshedTasks.Where(x => x != null).Select(x => RecordReadingAsync(x));
        await Task.WhenAll(tasksRecord);

        Interlocked.Decrement(ref currentlyRunningTasksCount);
}