LINQ对对象的奇怪行为

本文关键字：对象 LINQ | 更新日期: 2023-09-27 18:11:09

我在我的代码中看到一个奇怪的行为，这里有一个类似的例子，使用苹果和人，但代码基本上是相同的:

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。

为什么会发生这种情况?

提前感谢!

LINQ对对象的奇怪行为

selectedApples不是包含对象的集合，它是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，并且当您再次循环selectedApples时，它将从头开始重新创建。

使用ToList方法使它成为一个集合:

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

这里有几个问题。第一个是Where语句不生成对象列表。它是一个表达式语句。

Expression语句动态求值，因此每次运行该语句时都会丢弃对所生成对象的更改。信不信由你，这是一个理想的结果。这允许你以一种更高效、更优雅的方式处理复杂的嵌套for语句。

回答你的问题的最好方法是分析你所写的代码，并重新编写一些代码，向你展示一个更好的方法。

在你的代码中:

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，如果我们重新编写这段代码，使if语句位于内循环的外部。您的代码将执行相同的逻辑操作。请注意，这还不能解决问题，但会让你更接近一步。下面是代码的样子:

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???
}

现在我们已经开始把事情分组，以便看到一个更简单的答案。因为if语句意味着只有第一个满足条件的人才会得到所有的苹果。因此，让我们摆脱外部foreach循环并将其压缩为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。我们来看一下:

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.
}

现在您的代码将按预期运行，并且您可以利用LINQ中提供的延迟加载和循环优化。

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