async within a LINQ code - Clarification?

I don't understand why now async inside a LINQ statement - does work . Didn't we just say "don't think about using async within LINQ" ?

async mostly doesn't work with LINQ because IEnumerable<T> extensions don't always infer the delegate type properly and defer to Action<T>. They have no special understanding of the Task class. This means the actual async delegate becomes async void, which is bad. In the case of Enumerable.Select, we have an overload which returns a Func<T> (which in turn will be Func<Task> in our case), which is equivalent to async Task, hence it works fine for async use-cases.

When the control reaches the await t here — What is actually happen? Does the control leaves the ProcessTasksAsync method ?

No, it doesn't. Enumerable.Select is about projecting all elements in the sequence. This means that for each element in the collection, await t which will yield control back to the iterator, which will continue iterating all elements. That's why you later have to await Task.WhenAll, to ensure all elements have finished execution.

Question 1:

The difference is that each task is continued with additional processing which is: Trace.WriteLine(result);. In the link you pointed to, that code does not change anything, just creates overhead of awaiting and wrapping with another task.

Question 2:

When the control reaches the await t here — What is actually happen?

It awaits for the result of ProcessTasksAsync's task, then continue with Trace.WriteLine(result);. We can say that the control leaves the ProcessTasksAsync method when we have the result and the processing is still inside the anonymous method.

At the end, we have await Task.WhenAll(processingTasks); which will await for all tasks including the additional processing (Trace.WriteLine(result);) to complete before continuing but each task does not await for the others to continue executing: Trace.WriteLine(result);