C# 列表推导式简介
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Introduction to C# list comprehensions
提问by William Hutchen
How can I perform list comprehensions in C#?
如何在 C# 中执行列表推导式？
回答by Ian P
While this isn't a tutorial, here's some code that illustrates the concept:
虽然这不是教程，但这里有一些代码来说明这个概念：
public List<string> ValidUsers(List<User> users) {
List<string> names = new List<string>();
foreach(User user in users) {
if(user.Valid) {
names.Add(user.Name);
}
}
return names;
}
回答by brock.holum
You can use LINQ to make expressions that are similar to list comprehensions. Here's a site explaining it a little:
您可以使用 LINQ 来创建类似于列表推导式的表达式。这是一个解释它的网站：
List Comprehension in C# with LINQ
回答by Nescio
@Ian P
@伊恩P
return (from user in users
where user.Valid
select user.Name).ToArray();
回答by Justin Bozonier
Found this when I was looking up how to do list comprehensions in C#...
当我在查找如何在 C# 中执行列表推导式时发现了这个...
When someone says list comprehensions I immediately think about Python. The below code generates a list that looks like this:
当有人说列表推导式时，我立即想到了 Python。下面的代码生成一个如下所示的列表：
[0,2,4,6,8,10,12,14,16,18]
The Python way is like this:
Python的方式是这样的：
list = [2*number for number in range(0,10)]
In C#:
在 C# 中：
var list2 = from number in Enumerable.Range(0, 10) select 2*number;
Both methods are lazily evaluated.
这两种方法都是惰性评估的。
回答by λ Jonas Gorauskas
A List Comprehension is a type of set notation in which the programmer can describe the properties that the members of a set must meet. It is usually used to create a set based on other, already existing, set or sets by applying some type of combination, transform or reduction function to the existing set(s).
List Comprehension 是一种集合表示法，程序员可以在其中描述集合成员必须满足的属性。它通常用于通过将某种类型的组合、变换或归约函数应用于现有集合来创建基于其他、已经存在的一个或多个集合的集合。
Consider the following problem: You have a sequence of 10 numbers from 0 to 9 and you need to extract all the even numbers from that sequence. In a language such a C# version 1.1, you were pretty much confined to the following code to solve this problem:
考虑以下问题：您有一个从 0 到 9 的 10 个数字的序列，您需要从该序列中提取所有偶数。在诸如 C# 1.1 版这样的语言中，您几乎只能使用以下代码来解决此问题：
ArrayList evens = new ArrayList();
ArrayList numbers = Range(10);
int size = numbers.Count;
int i = 0;
while (i < size)
{
if (i % 2 == 0)
{
evens.Add(i);
}
i++;
}
The code above does not show the implementation of the Range function, which is available in the full code listing below. With the advent of C# 3.0 and the .NET Framework 3.5, a List Comprehension notation based on Linq is now available to C# programmers. The above C# 1.1 code can be ported to C# 3.0 like so:
上面的代码没有显示 Range 函数的实现，它可以在下面的完整代码列表中找到。随着 C# 3.0 和 .NET Framework 3.5 的出现，C# 程序员现在可以使用基于 Linq 的 List Comprehension 表示法。上面的 C# 1.1 代码可以像这样移植到 C# 3.0：
IEnumerable<int> numbers = Enumerable.Range(0, 10);
var evens = from num in numbers where num % 2 == 0 select num;
And technically speaking, the C# 3.0 code above could be written as a one-liner by moving the call to Enumarable.Rangeto the Linq expression that generates the evenssequence. In the C# List Comprehension I am reducing the set numbersby applying a function (the modulo 2) to that sequence. This produces the evenssequence in a much more concise manner and avoid the use of loop syntax. Now, you may ask yourself: Is this purely syntax sugar? I don't know, but I will definitelly investigate, and maybe even ask the question myself here. I suspect that this is not just syntax sugar and that there are some true optimizations that can be done by utilizing the underlying monads.
和技术上来说，在C＃3.0以上代码可以通过移动呼叫要被写入作为一衬垫Enumarable.Range到生成的LINQ表达式找齐序列。在 C# List Comprehension 中，我通过将函数（模 2）应用于该序列来减少集合数。这会以更简洁的方式生成evens序列，并避免使用循环语法。现在，您可能会问自己：这纯粹是语法糖吗？我不知道，但我一定会调查，甚至可能会在这里自己问这个问题。我怀疑这不仅仅是语法糖，还有一些真正的优化可以通过利用底层 monad 来完成。
The full code listing is available here.
完整的代码清单可在此处获得。
