Question

我想比较两个列表。我想检查List2是否包含List1中的任何项目。我得到意想不到的结果请参阅下面的代码。

测试代码类

class Program
{
    static void Main(string[] args)
    {
        bool loop = true;
        int textcount = 1;
        while (loop)
        {

            var collection1 = GetCollection();
            var collection2 = GetCollection();

            Console.WriteLine("Test No " + textcount.ToString());


            Console.WriteLine("Collection 1 =" + String.Join(", ", collection1.ToArray()));

            Console.WriteLine("Collection 2 =" + String.Join(", ", collection2.ToArray()));

            System.Diagnostics.Stopwatch watch = new System.Diagnostics.Stopwatch();
            watch.Start();
            var hasitem = collection1.Any(item => collection2.Contains(item));

            watch.Stop();

            Console.WriteLine(hasitem.ToString() + "  Found in " + watch.ElapsedTicks.ToString());
            watch.Reset();

            watch.Start();
            var hasAtLeastOne = collection1.Intersect(collection2).Any();
            watch.Stop();
            Console.WriteLine(hasAtLeastOne.ToString() + "  With Intersect Found in " + watch.ElapsedTicks.ToString());
            textcount++;
            Console.ReadKey();
        }


    }
    static Random ran = new Random();
    private static IEnumerable<int> GetCollection()
    {
        for (int i = 0; i < 5; i++)
        {
            yield return ran.Next(i, 20);
        }
    }
}

结果非常烦人。看到最后4个结果。

Test No 1
Collection 1 =10, 8, 18, 6, 11
Collection 2 =3, 12, 18, 13, 6
True  Found in 3075
True  With Intersect Found in 15297
 Test No 2
Collection 1 =18, 13, 7, 18, 5
Collection 2 =12, 18, 8, 3, 5
True  Found in 22
True  With Intersect Found in 100
 Test No 3
Collection 1 =1, 6, 15, 7, 9
Collection 2 =16, 15, 14, 14, 12
True  Found in 21
True  With Intersect Found in 23
 Test No 4
Collection 1 =3, 16, 7, 4, 19
Collection 2 =6, 3, 15, 15, 9
True  Found in 21
True  With Intersect Found in 56
 Test No 5
Collection 1 =18, 18, 9, 17, 10
Collection 2 =17, 12, 4, 3, 11
True  Found in 25
True  With Intersect Found in 20
 Test No 6
Collection 1 =9, 9, 2, 17, 19
Collection 2 =17, 2, 18, 3, 15
False  Found in 109
False  With Intersect Found in 41
 Test No 7
Collection 1 =3, 15, 3, 5, 5
Collection 2 =2, 2, 11, 7, 6
True  Found in 22
False  With Intersect Found in 15
 Test No 8
Collection 1 =7, 14, 17, 14, 18
Collection 2 =18, 4, 7, 18, 16
False  Found in 28
True  With Intersect Found in 19
 Test No 9
Collection 1 =3, 9, 6, 18, 9
Collection 2 =10, 3, 17, 17, 18
True  Found in 28
True  With Intersect Found in 22
 Test No 10
Collection 1 =15, 18, 2, 9, 8
Collection 2 =10, 15, 3, 10, 19
False  Found in 135
True  With Intersect Found in 128
 Test No 11
Collection 1 =6, 2, 17, 18, 18
Collection 2 =14, 16, 14, 6, 4
False  Found in 20
False  With Intersect Found in 17

Answer 1

问题在于，您所谓的“集合”实际上是一个不稳定的项目序列，每次枚举时都会发生变化。这样做的原因是您实施GetCollection的方式。使用yield return基本上会返回关于如何创建序列的蓝图。它不会返回序列本身因此，每次枚举“蓝色打印”时，它都会用于创建新序列。您只需输出两次“集合”即可验证这一点。您将看到每次值都不同。

这就是为什么你的测试产生完全任意结果的原因：您枚举了每个集合三次：

将其输出到控制台时会发生第一次枚举
第二次枚举在Any和Contains的测试中发生。因为这会启动一个新的枚举，所以会生成新的随机数。
第三次枚举发生在Intersect测试中。这会创建另一组随机数。

要解决此问题，请在ToArray()的结果上调用GetCollection来创建稳定的序列。

Answer 2

这是你的问题：

private static IEnumerable<int> GetCollection()
{
    for (int i = 0; i < 5; i++)
    {
        yield return ran.Next(i, 20);
    }
}

进入这个：

private static List<int> GetCollection()
{
  return new List<int>
         {
           ran.Next(0, 20),
           ran.Next(1, 20),
           ran.Next(2, 20),
           ran.Next(3, 20),
           ran.Next(4, 20)
         };
}

你的问题就会消失。

长期的解释是，当你创建一个IEnumerable函数时，你可以期望它在各种LINQ调用上重复调用迭代器（毕竟，这就是IEnumerable的作用，对吧？）。由于您在每个迭代器调用上执行yield return <some random number>，因此可能会出现不稳定的结果。最好保存对.ToArray()结果的引用，或者只返回稳定列表。

比较两个列表时的结果很奇怪

2 个答案: