Task.WhenAll - When to use this

Question

I know Task.WaitAll(Task1,Task2) waits for all of the provided Task objects to complete execution.

What is Task.WhenAll used for?

Task.WhenAll in MSDN says

Creates a task that will complete when all of the supplied tasks have completed.

What is the real world scenario where once can apply Task.WhenAll()

Answer 1

WaitAll blocks the calling thread while WhenAll provides a promise task that you can use to asynchronously wait using async-await without wasting up a thread in the meantime:

async Task ProcessAsync()
{
    await Task.WhenAll(DownloadAsync(), IntializeParserAsync(),...);
}

You should use WhenAll wherever you want to wait without having to block a thread, which improves scalability. (read: almost anywhere you can)

Answer 2

WaitAll is a void function that lets your code wait for completion of multiple tasks right away, at the time when you call it.

WhenAll, on the other hand, produces a Task that will wait for other tasks when it is its time to run. For example, if you are building a Task that needs to initialize a couple of things, and then run some computation, you can do it like this:

var initThenRun = Task
    .WhenAll(initTask1, initTask2, initTask3)
    .ContinueWith(computationTask);

Now the initThenRun task will run the three initialization tasks, wait for all of them to complete, and only then proceed to the computationTask.