为什么运行时间这么短？

Question

我在C#中实现了一个餐饮哲学家问题的解决方案(一段时间前，我不记得什么时候，什么地方不记得)，最近我重新打开它，并添加了一些定时输出。当实际的测试运行需要很多很多秒时，测试似乎只运行几毫秒。事实上，我可以强迫它运行几分钟，尽管测试仍然显示不到500毫秒。

简而言之，我正在创建一个任务集合，让它们每个运行几秒钟(打开和关闭)，在主执行中循环，直到它们完成，并写出开始和结束时间之间的差异。

下面是用于运行解决方案的NUnit单元测试：

[Test]
public void DoesEveryOneEat_WaitsForAllToFinish()
{
    // arrange
    var start = DateTime.Now;

    // act
    foreach (var philosopher in Thinkers)
    {
        philosopher.StartEating();
    }

    // wait
    bool someoneIsHungry = true;

    while (someoneIsHungry)
    {
        someoneIsHungry = false;

        foreach (var philosopher in Thinkers)
        {
            if (!someoneIsHungry && philosopher.IsHungry)
                someoneIsHungry = true;
        }
    }

    Console.WriteLine(DateTime.Now.Subtract(start).Milliseconds);

    // assert
    Assert.AreEqual(false, Thinkers[0].IsHungry, "Philosopher 0 is hungry and ate for only " + Thinkers[0].AteForMillis + " milliseconds.");
    Assert.AreEqual(false, Thinkers[1].IsHungry, "Philosopher 1 is hungry and ate for only " + Thinkers[1].AteForMillis + " milliseconds.");
    Assert.AreEqual(false, Thinkers[2].IsHungry, "Philosopher 2 is hungry and ate for only " + Thinkers[2].AteForMillis + " milliseconds.");
    Assert.AreEqual(false, Thinkers[3].IsHungry, "Philosopher 3 is hungry and ate for only " + Thinkers[3].AteForMillis + " milliseconds.");
    Assert.AreEqual(false, Thinkers[4].IsHungry, "Philosopher 4 is hungry and ate for only " + Thinkers[4].AteForMillis + " milliseconds.");
}

philosopher.StartEating();启动一个任务，直到一个令人满意的结果到达，然后退出：

public async void StartEating()
{
    await Task.Factory.StartNew(Run);
}

public void Run()
{
    while (_totalRunTime < MaxEatMillis)
    {
        if (Monitor.TryEnter(Left))
        {
            if (Monitor.TryEnter(Right))
            {
                Eat();
                Monitor.Exit(Right);
            }
            Monitor.Exit(Left);
        }
    }
}

虽然我欢迎对这段代码进行建设性的评论，但我的问题是:为什么单元测试控制台只输出582毫秒，而测试本身在9秒或更长时间内不容易完成？

(我推测这是因为实际单元测试代码运行所需的时间仅为582毫秒，但NUnit库不允许断言在测试启动的所有任务完成之前运行。然而，这在我的脑海中并不存在，就好像编写依赖于这个事实的代码会失败一样。)

完整清单：

public class ChopStick
{
    public int Index { get; set; }
}

public class Philosopher
{
    protected ChopStick Left { get; set; }
    protected ChopStick Right { get; set; }

    private int _totalRunTime = 0;
    private readonly int MaxEatMillis = 3000;
    private readonly int MaxRunMillis = 1000;

    public Philosopher(ChopStick left, ChopStick right)
    {
        Left = left;
        Right = right;
    }

    public async void StartEating()
    {
        await Task.Factory.StartNew(Run);
    }

    public void Run()
    {
        while (_totalRunTime < MaxEatMillis)
        {
            if (Monitor.TryEnter(Left))
            {
                if (Monitor.TryEnter(Right))
                {
                    Eat();
                    Monitor.Exit(Right);
                }
                Monitor.Exit(Left);
            }
        }
    }

    private void Eat()
    {
        var eatTime = new Random().Next(1, MaxRunMillis);
        Thread.Sleep(eatTime);
        _totalRunTime += eatTime;
    }

    public bool IsHungry => _totalRunTime < MaxEatMillis;

    public int AteForMillis => _totalRunTime;
}

[Test]
public void DoesEveryOneEat_WaitsForAllToFinish()
{
    // arrange
    var start = DateTime.Now;

    Console.WriteLine(DateTime.Now.Subtract(start).Milliseconds);

    // act
    foreach (var philosopher in Thinkers)
    {
        philosopher.StartEating();
    }

    // wait
    bool someoneIsHungry = true;

    while (someoneIsHungry)
    {
        someoneIsHungry = false;

        foreach (var philosopher in Thinkers)
        {
            if (!someoneIsHungry && philosopher.IsHungry)
                someoneIsHungry = true;
        }
    }

    Console.WriteLine(DateTime.Now.Subtract(start).Milliseconds);

    // assert
    Assert.AreEqual(false, Thinkers[0].IsHungry, "Philosopher 0 is hungry and ate for only " + Thinkers[0].AteForMillis + " milliseconds.");
    Assert.AreEqual(false, Thinkers[1].IsHungry, "Philosopher 1 is hungry and ate for only " + Thinkers[1].AteForMillis + " milliseconds.");
    Assert.AreEqual(false, Thinkers[2].IsHungry, "Philosopher 2 is hungry and ate for only " + Thinkers[2].AteForMillis + " milliseconds.");
    Assert.AreEqual(false, Thinkers[3].IsHungry, "Philosopher 3 is hungry and ate for only " + Thinkers[3].AteForMillis + " milliseconds.");
    Assert.AreEqual(false, Thinkers[4].IsHungry, "Philosopher 4 is hungry and ate for only " + Thinkers[4].AteForMillis + " milliseconds.");

    Console.WriteLine(DateTime.Now.Subtract(start).Milliseconds);
}

Answer 1

在计时代码中，应该使用TotalMilliseconds而不是Milliseconds。

或者直接输出TimeSpan：

Console.WriteLine(DateTime.Now.Subtract(start));

顺便提一下，Stopwatch通常用于计时，而不是DateTime.Now。