CancellationTokenSource source = new CancellationTokenSource();
        CancellationToken token = source.Token;
        Console.WriteLine("Основной поток запущен");
        for (int i = 0; i < 300; i++) {
            task[i] = new Task( () =>
            {
                MyTask();

            },token);
            task[i].Start();
        }

        Task.WaitAll(task);

        Console.WriteLine("Основной поток завершен");
        Console.ReadLine();
	
using (var cts = new CancellationTokenSource())
{
    var ct = cts.Token;
    tasks =
        Enumerable.Range(0, 300)
                  .Select(i => Task.Run(() => MyTask(ct), ct)
    cts.CancelAfter(TimeSpan.FromSeconds(30));
    await Task.WhenAll(tasks);
}