OrderBy/ThenBy循环-C#中的嵌套列表 [英] OrderBy / ThenBy looping - nested Lists in C#

问题描述

我有一个嵌套列表，

List<List<String>> intable;

我想对所有列进行排序.问题在于列数取决于用户输入.

where I would like to sort all the columns. The problem is that the number of columns depends on user input.

按这样对列表进行排序可以很好地工作(此示例假设4列)

Sorting the list like this works fine (assuming 4 columns for this example)

var tmp = intable.OrderBy(x => x[0]);
tmp = tmp.ThenBy(x => x[1]);
tmp = tmp.ThenBy(x => x[2]);
tmp = tmp.ThenBy(x => x[3]);
intable = tmp.ToList();

但是，当我将其放入循环中时，就像这样:

But, when I put it in a loop, like this:

var tmp = intable.OrderBy(x => x[0]);
for (int i = 1; i <= 3; i++)
{
        tmp = tmp.ThenBy(x => x[i]);
}
intable = tmp.ToList();

它不再正常工作，并且仅对第四列进行排序.

it no longer works correctly, and sorts only the fourth column.

推荐答案

这是访问修改后的闭包的情况.将代码更改为此，它将起作用:

This is a case of access to a modified closure. Change the code to this and it will work:

var tmp = intable.OrderBy(x => x[0]);
for (int i = 1; i <= 3; i++) {
    var thisI = i;
    tmp = tmp.ThenBy(x => x[thisI]);
}
intable = tmp.ToList();

埃里克·利珀特(Eric Lippert)已编写

Eric Lippert has written a two-part article describing the problem. The reason it doesn't work as you expect to is - in short - because LINQ is only using the last value of i when it is evaluated when you call ToList(). It's the same as if you had written:

var tmp = intable.OrderBy(x => x[0]);
tmp = tmp.ThenBy(x => x[3]);
tmp = tmp.ThenBy(x => x[3]);
tmp = tmp.ThenBy(x => x[3]);
intable = tmp.ToList();

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