获得奇/偶与LINQ序列的一部分 [英] Getting odd/even part of a sequence with LINQ

问题描述

说我有所有的项目的名单，而我组他们通过类别是这样的：

Say I have a list of all Projects, and that I group them by Category like this:

var projectsByCat = from p in Projects
                    group p by p.Category into g
                    orderby g.Count() descending
                    select new { Category = g.Key, Projects = g };

现在我想显示这是一个网页列表，其中第一个创建左侧DIV，其次右侧股利。我对中的每个类别项目的数量订购显示类别最项目之上 - 所以我想分裂 projectsByCat 两种 - 如果我把所有在奇数类别左侧和偶数关于正确分类，我想我会得到一个合理健全的看法。

Now I want to display this as a list in a web page, where first I create the left side div, secondly the right side div. I am ordering by number of Projects in each Category to show the Categories with the most Projects on top - thus I would like to split projectsByCat in two - if I put all the "odd numbered" Categories on the left and the "even numbered" categories on the right, I think I will get a reasonably sane view.

所以我想我能做到这一点获得的奇数和偶数成员 projectsByCat ：

So I thought I could do this to get the odd and even members of projectsByCat:

var oddCategories = projectsByCat.Where((cat, index) => index % 2 != 0);

var evenCategories = projectsByCat.Where((cat, index) => index % 2 == 0);

和它编译 - 但是，当我运行它，我得到一个异常，如这样的：

And it compiles - however, when I run it, I get an exception such as this:

用于查询运营商不支持重载'在哪里'。

Unsupported overload used for query operator 'Where'.

和我想我是安全的，因为它摆在首位编译..;）

And I thought I was safe since it compiled in the first place.. ;)

有一种优雅的方式来做到这一点？而且，有一个优雅的解释，为什么我的创作使用其中（）将无法正常工作？

Is there an elegant way to do this? And also, is there an elegant explanation for why my creative use of Where() won't work?

在此先感谢！

推荐答案

如果你使用LINQ to SQL或LINQ到实体应首先完全实现结果到内存：

If you're using LINQ to SQL or LINQ to Entities you should first fully materialize the results into memory:

var oddCategories  = projectsByCat.ToList().Where((c,i) => i % 2 != 0);
var evenCategories = projectsByCat.ToList().Where((c,i) => i % 2 == 0);

这是不可能通过与索引数据库的结果，而不使用游标，这无论是ORM框架做的不可以做的。

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