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

This:

var tasksRead = Enumerable.Range(3, 35).Select(i => ReadSensorsAsync(i));

creates a lazily evaluated IEnumerable which maps numbers to method invocation results. ReadSensorsAsync is not invoked here, it will be invoked during evaluation.

This IEnumerable is evaluated twice. Here:

await Task.WhenAll(tasksRead);

and here:

// Here, another lazy IEnumerable is created based on tasksRead.
var tasksRecord = tasksRead.Where(...).Select(...);  
await Task.WhenAll(tasksRecord);  // Here, it is evaluated.

Thus, ReadSensorsAsync is invoked twice.

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As csharpfolk suggested in the comments, materializing the IEnumerable should fix this:

var tasksRead = Enumerable.Range(3, 35).Select(i => ReadSensorsAsync(i)).ToList();