[英]To Order By Descending sub-elements using LINQ or Lambda Expression
I had a problem to order sub-elements in List and found the topic to discuss List<T> vs IEnumerable<T> in foreach .I wonder how to order child List by using LINQ or Lambda Expression.我在对 List 中的子元素进行排序时遇到了问题,并找到了在 foreach 中讨论List<T> 与 IEnumerable<T>的主题。我想知道如何使用 LINQ 或 Lambda 表达式对子 List 进行排序。 Can someone recommend me?
有人可以推荐我吗? the code example as follows:
代码示例如下:
public class Parent
{
// Other properties...
public IList<Child> Children { get; set; }
}
public IEnumerable<Parent> DoStuff()
{
var result = DoOtherStuff() // Returns IEnumerable<Parent>
.OrderByDescending(SomePredicate)
.ThenBy(AnotherPredicate); // This sorting works as expected in the return value.
foreach (Parent parent in result)
{
parent.Children = parent.Children.OrderBy(YetAnotherPredicate).ToList();
// When I look at parent.Children here in the debugger, it's sorted properly.
}
return result;
// When I look at the return value, the Children are not sorted.
}
You enumerate result
each time that you enumerate the return IEnumerable
of DoStuff
, plus one additional time inside the loop in DoStuff
itself.每次枚举
DoStuff
的返回IEnumerable
时都会枚举result
,再加上DoStuff
本身的循环内的额外时间。 Modifications you make inside the loop do not stay, though, because of deferred execution : next time you enumerate DoStuff()
there is another call to DoOtherStuff
etc.但是,由于延迟执行,您在循环内所做的修改不会保留:下次您枚举
DoStuff()
,会再次调用DoOtherStuff
等。
You can fix this in several ways:您可以通过多种方式解决此问题:
Convert result
to list before sorting children, ie在排序孩子之前将
result
转换为列表,即
DoOtherStuff() // Returns IEnumerable<Parent> .OrderByDescending(SomePredicate) .ThenBy(AnotherPredicate) .ToList();
Add child sorting in a Select
:在
Select
添加子排序:
DoOtherStuff() // Returns IEnumerable<Parent> .Select(parent => { parent.Children = parent.Children.OrderBy(YetAnotherPredicate).ToList(); return parent; }) .OrderByDescending(SomePredicate) .ThenBy(AnotherPredicate) .ToList();
Use yield return result
in the loop (this is a variation of the Select
solution).在循环中使用
yield return result
(这是Select
解决方案的变体)。
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