OOC：ToList（）和强制转换为List< T>有什么区别？在.NET中？ [英] OOC: What is the difference between ToList() and casting to List<T> in .NET?

问题描述

OOC：出于好奇

因此，作为一次小运动，为了学习，我决定检查我是否能够实现非常基本的递归该函数将返回 List< int> ，但具有以下限制：

So, as a little exercise and for the sake of learning, I decided to check if I was able to implement a very basic recursive function that would return a List<int>, but with the following restrictions:

1-结果应为由函数本身返回（而不是作为参数传递给 void 函数）。

1- The result should be returned by the function itself (as opposed to passed as an argument to a void function).

2-否在函数的主体中声明的局部命名变量。

2 - No local "named" variables declared in the function's body.

我想出了以下解决方案（顺便说一句：这可以通过任何方式加以改进吗？）

I came up with the solution below (BTW: can this be improved in any way?)

在执行此操作时，我了解到 ToList（）与强制转换为 List< T> ; （请参见下面的示例）-可以解释幕后情况的人以及两者之间的区别是什么？

While doing this, I learnt that ToList() is not the same thing as casting to List<T> (see example below) - Anyone out there who can explain what happens under the hood and what the difference between the two is?

谢谢！

PS-我使用的是4.0版本（以防万一）。

PS - I'm using version 4.0 (in case it matters).

编辑：运行时错误是无法类型为'< ConcatIterator> d__71'1 [System.Int32]'的st对象，以类型为'System.Collections.Generic.List'1 [System.Int32]'

public static List<int> SomeIntegers(int min, int max)
{
    //assume max >= min for simplicity  
    if (min == max)
        return new List<int>() { min };

    // runtime error 
    //return (List<int>)(SomeIntegers(min, max - 1).Concat(new List<int>() { max }));   

    //works
    return (SomeIntegers(min, max - 1).Concat(new List<int>() { max })).ToList(); 
}

推荐答案

ToList与（广播）到列表。

ToList is not the same as (casting) to List.

ToList使用任何IEnumerable（列表，数组，字典，集合等）并将其转换为列表。

ToList takes any IEnumerable (lists, arrays, Dictionaries, Sets, etc) and turns it into a list.

投射到列表会使用已经是某种形式的列表的对象，并将其标记为列表。示例：

Casting to List takes an object that is already a list of some sort, and labels it as a list. example:

// fail -- arrays are not lists
var not_a_list = (List<int>)int[];
// success: arrays *are* IEnumerable, so you can convert them to a list.
var list_from_array = new [] { 1,2,3,4,5 }.ToList();
// success: WorkflowRoleCollection derives from List<WorkflowRole>
var derived_from_list = (List<WorkflowRole>) new WorkflowRoleCollection();

在您的情况下，Concat返回IEnumerable，而不是List。请记住，它必须支持生成器（它们是经过延迟评估的），因此将其像下面的列表一样毫无意义。

In your case, Concat returns an IEnumerable, and not a List. Remember that it has to support generators (which are lazy-evaluated), so it doesn't make sense for it to be anything like a list underneath.

Btw您查看了内置函数 Enumerable.Range ？

Btw, have you taken a look at the built-in function Enumerable.Range ?

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