为什么IEnumerable.Count()重新评估查询? [英] Why is IEnumerable.Count() reevaluating the query?

问题描述

当我期望"1 1 1 1"时，以下代码将显示"2 2 2 2".为什么"Count()"会重新评估查询?

The following code prints "2 2 2 2" when I would expect "1 1 1 1". Why does "Count()" reevaluates the query?

class Class1
{
    static int GlobalTag = 0;

    public Class1()
    {
        tag = (++GlobalTag);
    }

    public int tag;

    public int calls = 0;

    public int Do()
    {
        calls++;
        return tag;
    }
}

class Program
{
    static void Main(string[] args)
    {
        Class1[] cls = new Class1[] { new Class1(), new Class1(), new Class1(), new Class1() };

        var result = cls.Where(c => (c.Do() % 2) == 0);
        if (result.Count() <= 10)
        {
            if (result.Count() <= 10)
            {
                foreach (var c in cls)
                {
                    Console.WriteLine(c.calls);
                }
            }
        }
    }
}

推荐答案

还可以如何工作?您希望Count()做什么以缓存值?

How else could it work? What would you expect Count() to do in order to cache the values?

LINQ to Objects通常会延迟执行，仅在需要时才实际评估查询-例如对元素进行计数.因此，对Where的调用根本不评估序列；它只是记住谓词和序列，以便它可以在需要时对其进行评估.

LINQ to Objects generally executes lazily, only actually evaluating a query when it needs to - such as to count the elements. So the call to Where isn't evaluating the sequence at all; it just remembers the predicate and the sequence so that it can evaluate it when it needs to.

有关LINQ to Objects如何工作的更多详细信息，建议您阅读我的 Edulinq博客系列.它相当长(并且还没有完成)，但是它将使您对LINQ to Objects的工作原理有更多的了解.

For a lot more details about how LINQ to Objects works, I suggest you read my Edulinq blog series. It's rather long (and not quite finished) but it'll give you a lot more insight into how LINQ to Objects works.

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