Question

我在代码中看到一个奇怪的行为，这是一个使用苹果和人的类似例子，但代码基本相同：

List<Apple> apples = ...
var selectableApples = apples.Select(a => new SelectableApple { SelectedByPerson = null, Apple = a });

foreach (Person person in persons)
{
    foreach (var unselectedApple in selectableApples.Where(aa => aa.SelectedByPerson == null))
    {
        if (/*the person satisfies some conditions*/)
        {
            // This gets executed like 100 times:
            unselectedApple.SelectedByPerson = person;
        }
    }
}

foreach (var selectedApple in selectableApples.Where(aa => aa.SelectedByPerson != null))
{
    Unreachable code - the collection is empty... WTF???
}

SelectableApple类只是一个没有逻辑的普通C＃类，以及所有属性的公共getter和setter。

为什么会这样？

提前致谢！

Answer 1

selectedApples不是包含对象的集合，它是一个动态创建集合的表达式。这意味着您对对象所做的更改将被丢弃，当您再次循环selectedApples时，它将从头开始重新创建。

使用ToList方法将其设为集合：

var selectableApples = apples.Select(a => new SelectableApple { SelectedByPerson = null, Apple = a }).ToList();

Answer 2

这里有几个问题。第一个是Where语句不生成对象列表。这是一个表达陈述。

表达式语句在运行时进行评估，因此每次运行语句时都会丢弃对生成的对象的更改。信不信由你，这是一个理想的结果。这允许您以更高效和更优雅的方式处理复杂的嵌套for语句。

回答问题的最佳方法是分析您撰写的内容并重新编写一些代码，以便向您展示更好的方法。

在您的代码中：

List<Apple> apples = ...
var selectableApples = apples.Select(a => new SelectableApple { SelectedByPerson = null, Apple = a });

foreach (Person person in persons)
{
    foreach (var unselectedApple in selectableApples.Where(aa => aa.SelectedByPerson == null))
    {
        // This will ideally give all apples to the first person who
        // meets the conditions. As such this if condition can be moved
        // out side of the above the foreach loop.
        if (/*the person satisfies some conditions*/)
        {
            // This gets executed like 100 times:
            unselectedApple.SelectedByPerson = person;
        }
    }
}

foreach (var selectedApple in selectableApples.Where(aa => aa.SelectedByPerson != null))
{
    Unreachable code - the collection is empty... WTF???
}

因此，如果我们重写这段代码，那么if语句就在内循环的一边。您的代码将执行相同的逻辑操作。请注意，这还没有解决问题，但会让您更近一步。以下是代码的外观：

List<Apple> apples = ...
var selectableApples = apples.Select(a => new SelectableApple { SelectedByPerson = null, Apple = a });

foreach (Person person in persons)
{
    // Now we can see that since this will all apples to the first person
    // who satisfies the below conditions we are still doing to much. And it
    // still does not work.
    if (/*the person satisfies some conditions*/)
    {
        foreach (var unselectedApple in selectableApples.Where(aa => aa.SelectedByPerson == null))
        {
            // This gets executed like 100 times:
            unselectedApple.SelectedByPerson = person;
        }
    }
}

foreach (var selectedApple in selectableApples.Where(aa => aa.SelectedByPerson != null))
{
    Unreachable code - the collection is empty... WTF???
}

现在我们已经开始对事物进行分组，以便能够看到更简单的答案。因为if语句意味着只有满足条件的第一个人才能获得所有苹果。因此，让我们摆脱外部的foreach循环并将其浓缩为LINQ。

List<Apple> apples = ...
var selectableApples = apples.Select(a => new SelectableApple { SelectedByPerson = null, Apple = a }); 
var selectedPerson = persons.Where(p => /*the person satisfies some conditions*/).First()

if(selectedPerson != null)
{
    foreach (var unselectedApple in selectableApples.Where(aa => aa.SelectedByPerson == null))
    {
        // This gets executed like 100 times:
        unselectedApple.SelectedByPerson = person;
    }
}

foreach (var selectedApple in selectableApples.Where(aa => aa.SelectedByPerson != null))
{
    Unreachable code - the collection is empty... WTF???
}

查看上面的代码，我们现在可以看到内部循环只是对原始选择的修改。让我们来看看：

List<Apple> apples = ...
var selectedPerson = persons.Where(p => /*the person satisfies some conditions*/).First()
var selectableApples = apples.Select(a => new SelectableApple { SelectedByPerson = selectedPerson, Apple = a }); 


foreach (var selectedApple in selectableApples.Where(aa => aa.SelectedByPerson != null))
{
    // This should now run provided that some person passes the condition.
}

现在您的代码将按照需要运行，您可以利用LINQ中提供的延迟加载和循环优化。

LINQ to Objects的奇怪行为

2 个答案: