Is result of Task.WhenAll order guaranteed?

Question

From the following test we can see the current version of framework guarantees the output order is the same that of as the input tasks.

async Task<string> GetString1()
{
    await Task.Delay(2000);
    return "1";
}

async Task<string> GetString2()
{
    await Task.Delay(1000);
    return "2";
}

var results = await Task.WhenAll(GetString1(), GetString2());
//now we have results[0] == "1" results[1] == "2"

However, from the documentation I can't find anything about this behavior, which means it's not document-guaranteed. From the opinions of answers in this question:

Do I need to put "order flags" in the output? e.g. change the example code into following:

class OrderTaskResult<T>
{
    public OrderTaskResult(int order, T value)
    {
        this.Order = order;
        this.Value = value;
    }
    public int Order { get; private set; }
    public T Value { get; private set; }
}

async Task<OrderTaskResult<string>> GetString1()
{
    await Task.Delay(2000);
    return new OrderTaskResult<string>(1, "1");
}

Answer 1

You're looking at the documentation for the wrong overload.

If you look at the overload that actually returns the results, you'll see:

The Task<TResult>.Result property of the returned task will be set to an array containing all of the results of the supplied tasks in the same order as they were provided