这两个链接涵盖了我的问题的一切。

1. Process asynchronous tasks as they complete
2. Cancel async tasks after a period of time
   有这样一个代码示例

static async Task Main()
{
    Console.WriteLine("Application started.");

    try
    {
        s_cts.CancelAfter(3500);

        await SumPageSizesAsync();
    }
    catch (OperationCanceledException)
    {
        Console.WriteLine("\nTasks cancelled: timed out.\n");
    }
    finally
    {
        s_cts.Dispose();
    }

    Console.WriteLine("Application ending.");
}

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