問題

Task.WhenAllメソッドが返すTaskオブジェクトは、引数であたえられたコレクション内のTaskオブジェクトすべてが完了(あるいは例外、中断)すると、IsCompletedがtrueになる。
では、IsCanceled, IsFaultedプロパティはどうなるか。
複数あるタスクのうち、ひとつだけがキャンセルされたときはどうなるか。
このとき、例外をスローしたタスクが混ざっていた場合はどうか。

検証方法

以下のコードで調査をおこなった。

static void Main(string[] args)
{
	Console.WriteLine("Empty");
	Action(new Task[0]);
	Console.WriteLine("");

	Console.WriteLine("Completed task");
	Action(new[] { CreateNormalTask() });
	Console.WriteLine("");

	Console.WriteLine("Canceled task");
	Action(new[] { CreateCanceledTask() });
	Console.WriteLine("");

	Console.WriteLine("Faulted task");
	Action(new[] { CreateFaultedTask() });
	Console.WriteLine("");

	Console.WriteLine("Completed task & Canceled task");
	Action(new[] { CreateNormalTask(), CreateCanceledTask() });
	Console.WriteLine("");

	Console.WriteLine("Complated task & Fauleted task");
	Action(new[] { CreateNormalTask(), CreateFaultedTask() });
	Console.WriteLine("");

	Console.WriteLine("Canceled task & Fauleted task");
	Action(new[] { CreateCanceledTask(), CreateFaultedTask() });
	Console.WriteLine("");

	Console.WriteLine("Completed task & Canceled task & Fauleted task");
	Action(new[] { CreateNormalTask(), CreateCanceledTask(), CreateFaultedTask() });
	Console.WriteLine("");

	Console.ReadKey();
}

static void Action(Task[] tasks)
{
	var whenAllTask = Task.WhenAll(tasks);
	Console.WriteLine($"WhenAllTask: IsCompleted = {whenAllTask.IsCompleted}, IsCanceled = {whenAllTask.IsCanceled}, IsFaulted = {whenAllTask.IsFaulted}");

	foreach (var pair in tasks.Select((task, index) => new {task, index}))
		Console.WriteLine($"  {pair.index}: IsCompleted = {pair.task.IsCompleted}, IsCanceled = {pair.task.IsCanceled}, IsFaulted = {pair.task.IsFaulted}");
}

static Task CreateNormalTask() => Task.CompletedTask;
static Task CreateCanceledTask()
{
	var source = new TaskCompletionSource<int>();
	source.TrySetCanceled();
	return source.Task;
}
static Task CreateFaultedTask()
{
	var source = new TaskCompletionSource<int>();
	source.TrySetException(new Exception());
	return source.Task;
}

実行結果一覧

正常終了したタスク	キャンセルされたタスク	例外をスローしたタスク	IsCompleted	IsCanceled	IsFaulted
あってもなくても	なし	なし	true	false	false
あってもなくても	あり	なし	true	true	false
あってもなくても	あってもなくても	あり	true	false	true

詳細は以下のとおり。

Empty
WhenAllTask: IsCompleted = True, IsCanceled = False, IsFaulted = False

Completed task
WhenAllTask: IsCompleted = True, IsCanceled = False, IsFaulted = False
  0: IsCompleted = True, IsCanceled = False, IsFaulted = False

Canceled task
WhenAllTask: IsCompleted = True, IsCanceled = True, IsFaulted = False
  0: IsCompleted = True, IsCanceled = True, IsFaulted = False

Faulted task
WhenAllTask: IsCompleted = True, IsCanceled = False, IsFaulted = True
  0: IsCompleted = True, IsCanceled = False, IsFaulted = True

Completed task & Canceled task
WhenAllTask: IsCompleted = True, IsCanceled = True, IsFaulted = False
  0: IsCompleted = True, IsCanceled = False, IsFaulted = False
  1: IsCompleted = True, IsCanceled = True, IsFaulted = False

Complated task & Fauleted task
WhenAllTask: IsCompleted = True, IsCanceled = False, IsFaulted = True
  0: IsCompleted = True, IsCanceled = False, IsFaulted = False
  1: IsCompleted = True, IsCanceled = False, IsFaulted = True

Canceled task & Fauleted task
WhenAllTask: IsCompleted = True, IsCanceled = False, IsFaulted = True
  0: IsCompleted = True, IsCanceled = True, IsFaulted = False
  1: IsCompleted = True, IsCanceled = False, IsFaulted = True

Completed task & Canceled task & Fauleted task
WhenAllTask: IsCompleted = True, IsCanceled = False, IsFaulted = True
  0: IsCompleted = True, IsCanceled = False, IsFaulted = False
  1: IsCompleted = True, IsCanceled = True, IsFaulted = False
  2: IsCompleted = True, IsCanceled = False, IsFaulted = True

結論

Task.WhenAllが返すタスクについて、IsComplatedがtrueのとき、

IsCanceledの値では、キャンセルされたタスクがないことはわからない。例外をスローしたタスクがひとつでもあると、必ずfalseになる。
IsFaultedの値によって、例外をスローしたタスクがあるかどうかがわかる