可以将这两个LINQ查询可以互换使用？ [英] Can these two LINQ queries be used interchangeably?

问题描述

a）项下面两个查询产生相同的结果：

a) Would the following two queries produce the same results:

  var query1 = collection_1
            .SelectMany(c_1 => c_1.collection_2)
            .SelectMany(c_2 => c_2.collection_3)
            .Select(c_3 => c_3);

  var query2 = collection_1
            .SelectMany(c_1 => c_1.collection_2
            .SelectMany(c_2 => c_2.collection_3.Select(c_3 => c_3)));

二）我假定这两个查询不能总是交替使用？例如，如果我们想要输出的元素也包含值 C_1 和 C_2 ，那么我们只有做到这一点 QUERY2 ，但不能与 QUERY1 ：

b) I assume the two queries can't always be used interchangeably? For example, if we wanted the output elements to also contain values of c_1 and c_2, then we only achieve this with query2, but not with query1:

  var query2 = collection_1
            .SelectMany(c_1 => c_1.collection_2
            .SelectMany(c_2 => c_2.collection_3.Select(c_3 => new { c_1, c_2, c_3 } )));

?

感谢您

推荐答案

您已经给该片段似乎是无效的。 C_3 未在选择语句的范围界定，所以，除非我误解的东西，这亿韩元 ŧ编译。

The snippets you've given seem to be invalid. c_3 isn't defined in the scope of the Select statement, so unless I've misunderstood something, this won't compile.

这好像你要选择的 collection_3 的元素，但这样做隐含的SelectMany ，因此，在这两种情况下最终选择语句是多余的。 。带他们出去，两个查询是等效的。

It seems as though you're trying to select the elements of collection_3, but this is done implicitly by SelectMany, and so the final Select statements in both cases are redundant. Take them out, and the two queries are equivalent.

所有你需要的是这样的：

All you need is this:

var query = collection_1
           .SelectMany(c_1 => c_1.collection_2)
           .SelectMany(c_2 => c_2.collection_3);

---

更新： X => X 是标识映射，那么选择（X => X）永远是多余的，不管上下文。它只是意味着为序列中的每一个元素，选择该元素。

---

Update: x => x is the identity mapping, so Select(x => x) is always redundant, regardless of the context. It just means "for every element in the sequence, select the element".

第二个片段，当然是不同的，而的SelectMany 和选择语句确实需要嵌套，以选择所有三个元素， C_1 ， C_2 和 C_3 。

The second snippet is of course different, and the SelectMany and Select statements indeed need to be nested in order to select all three elements, c_1, c_2, and c_3.

像格特说，不过，你可能最好使用的查询语法的理解的。它更简洁，更容易精神分析查询的工作原理。

Like Gert, says, though, you're probably better off using query comprehension syntax. It's much more succinct and makes it easier to mentally parse the workings of a query.

这篇关于可以将这两个LINQ查询可以互换使用？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！