c# - 为什么 System.Random.Sample 会抛出 IndexOutOfRangeException？

Question

我正在 Unity (2017.1.1f1) 中的 .NET System.Random 实例上调用 Next()。它从 protected 函数 Sample() 中抛出 IndexOutOfRangeException ，它不接受任何参数并返回一个介于 0 和 1 之间的 double 。可能导致此行为的原因是什么？

这里是异常的详细信息

System.IndexOutOfRangeException: Array index is out of range.

at System.Random.Sample () [0x0003e] in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System/Random.cs:91

at System.Random.Next (Int32 maxValue) [0x00017] in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System/Random.cs:112

at Quicksilver.SysIRand.RandInt (Int32 max_exclusive) [0x00008] in F:\SVNHome\gemrush\GemRush\Assets\Source\Shared\Utility\IRand.cs:38

at Quicksilver.IEnumerableExt.SelectRandom[Skill] (IEnumerable`1 collection, IRand rand, Int32 count) [0x00070] in F:\SVNHome\gemrush\GemRush\Assets\Source\Shared\Utility\IEnumerableExt.cs:61

(上面还有13层调用栈)

这是一个多线程环境，但每个线程都有自己专用的 System.Random 实例。从下面的代码中可以看出，传递给 Next() 的参数必须为 1 或更高。

这个错误在一个复杂的自动化测试脚本中被抛出大约 1 小时，因此运行多次来测试理论是昂贵的，并且任何改变 RNG 调用方式的修改都将阻止重现。 (如果错误以某种方式涉及线程之间的意外交互，那么它可能根本无法重现。)

由于它在测试脚本中花费了一个小时，因此绝大部分调用此方法都不会引发错误。

直接使用随机数的函数在这里:

    // Chooses count items at random from the enumeration and returns them in an array
    // The order of selected items within the array is also random
    // If the collection is smaller than count, the entire collection is returned (in random order)
    public static T[] SelectRandom<T>(this IEnumerable<T> collection, IRand rand, int count = 1)
    {
        if (count <= 0) return new T[0];    // Optimization for trivial case

        T[] keep = new T[count];
        int found = 0;
        foreach (T item in collection)
        {
            if (found < count)
            {
                // Take the first #count items, in case that's all there are

                // Move a random item of those found so far (including the new one)
                // to the end of the array, and insert the new one in its place
                int r = rand.RandInt(found + 1);
                keep[found++] = keep[r];
                keep[r] = item;
            }
            else
            {
                // Random chance to replace one of our previously-selected elements
                ++found;
                if (rand.RandInt(found) < count)    // probability desired/found
                {
                    // Replace a random previously-selected element
                    int r = rand.RandInt(count);
                    keep[r] = item;
                }
            }
        }
        if (found < count)
        {
            // The collection was too small to get everything requested;
            // Make a new, smaller array containing everything in the collection
            T[] all = new T[found];
            Array.Copy(keep, all, found);
            return all;
        }
        return keep;
    }

此行抛出错误:

                if (rand.RandInt(found) < count)    // probability desired/found

IRand 是一个非常薄的 System.Random 包装器的接口(interface)； IRand.RandInt() 仅返回 Random.Next()。

编辑

以下是 Random 实例的创建和分发方式:

   private void Start()
    {
        SysIRand[] rngs = new SysIRand[parallelTesters];
        if (parallelTesters > 0) rngs[0] = new SysIRand(new System.Random(548913));
        if (parallelTesters > 1) rngs[1] = new SysIRand(new System.Random(138498));
        if (parallelTesters > 2) rngs[2] = new SysIRand(new System.Random(976336));
        if (parallelTesters > 3) rngs[3] = new SysIRand(new System.Random(793461));
        if (parallelTesters > 4) rngs[4] = new SysIRand(new System.Random(648791));
        if (parallelTesters > 5) rngs[5] = new SysIRand(new System.Random(348916));
        if (parallelTesters > 6) rngs[6] = new SysIRand(new System.Random(8467168));
        if (parallelTesters > 7) rngs[7] = new SysIRand(new System.Random(6183569));
        for (int i = 8; i < parallelTesters; ++i)
        {
            rngs[i] = new SysIRand(new System.Random(7 * i * i + 8961 * i + 129786));
        }

        for (int t = 0; t < parallelTesters; ++t)
        {
            SysIRand rand = rngs[t];

            IBot bot = BotFactory.DrawBot(rand);

            BotTester tester = new BotTester(rand, bot);
            tester.testerID = t + 1;
            tester.OnMessage += (str) => UponMessage(tester.testerID + " ~ " + str);
            tester.OnError += (str) => UponError(tester.testerID + " ~ " + str);
            tester.OnGameAborted += UponGameAborted;
            tester.OnDebugMoment += UponDebugMoment;

            testers.Add(tester);
        }

(在这次运行中，parallelTesters 的值为 3)

每个 BotTester 只使用传递给其构造函数的 Random 实例。每个线程都是私有(private)的 BotTester，从 BotTester.RunGame() 开始:

    public bool RunGame(GameMode mode, int maxGames = 1, long maxMilliSeconds = 100000000, int maxRetries = 5000)
    {
        if (threadRunning) return false;
        thread = new Thread(() => ThreadedRunGame(mode, maxGames, maxMilliSeconds, maxRetries));
        thread.Start();
        return true;
    }

Answer 1

唯一有意义的解释是你认为你正在访问Random()实例线程安全的，用你自己的话来说，每个线程都有自己的Random()实例，但看起来您正在访问相同的 Random() 实例，而它仍在计算过程中。这是Unity中的实现。 Sample() 只是调用 InternalSample()

private int InternalSample()
{
  int inext = this.inext;
  int inextp = this.inextp;
  int index1;
  if ((index1 = inext + 1) >= 56)
    index1 = 1;
  int index2;
  if ((index2 = inextp + 1) >= 56)
    index2 = 1;
  int num = this.SeedArray[index1] - this.SeedArray[index2];
  if (num < 0)
    num += int.MaxValue;
  this.SeedArray[index1] = num;
  this.inext = index1;
  this.inextp = index2;
  return num;
}

如您所见，您可以获得 IndexOutOfRangeException 的地方是有限的，即当您访问 this.SeedArray 时。这是 SeedArray 的定义。

public class Random
{
    private int[] SeedArray = new int[56];
....
}

它已经分配给了 56 个元素，在 InternalSample 方法的实现中你可以看到每次调用 index1 和 index2 总是限制为最多 55 除非多次调用 InternalSample 方法。