空集合初始化为空列表属性结果

问题描述

当我，它不初始化 ThisIsAList 来一个空的集合作为我期待......而不是 ThisIsAList 为null。

When I run this code, it doesn't initialize ThisIsAList to an empty collection as I was expecting... instead ThisIsAList was null.

void Main()
{
    var thing = new Thing
    {
        ThisIsAList = {}
    };

    Console.WriteLine(thing.ThisIsAList == null); // prints "True"
}

public class Thing
{
    public List<string> ThisIsAList { get; set; }
}

为什么不是这样的编译错误？为什么空？

我在想，如果可能的结果有一个隐式转换会在这里，但下面的尝试产生编译错误：

I was wondering if maybe there was an implicit conversion going on here, but the following attempts produced compile errors:

thing.ThisIsAList = Enumerable.Empty<string>().ToArray();
List<int> integers = { 0, 1, 2, 3 };

据上的的，这听起来像一个集合初始化基本上只是处理电话添加（）为你。所以，
我寻找可能的过载到，但没有发现任何东西，我认为也适用。

According to MSDN documenation on collection initializers, it sounds like a collection initializer basically just handles calling Add() for you. SoI looked for possible overloads to List.Add, but didn't find anything that I think would apply.

有人能解释？什么是从C＃规范怎么回事

Can someone explain what's going on here from the C# spec?

推荐答案

在部分C＃5.0规范的7.6.10.2：

In section 7.6.10.2 of the C# 5.0 specs:

这是指定集合初始化后等号成员初始化是嵌入集合的初始化。 除了到外地或财产分配一个新的集合，在初始化中给出的元素添加到由外地或财产引用的集合。该字段或属性必须满足在§7.6.10.3规定的要求的集合类型。

（重点煤矿）

所以，因为你的集合初始化嵌套在另一个对象/集合初始化器内的行为是它解决它initialzing一个值的成员，然后将相关项目。在这种情况下，该属性是空，使空价值得到解决，并在初始化所有零项目添加。如果你真的想增加一个项目，它会抛出一个NRE，因为你会试图将项目添加到空对象。

So since your collection initializer is nested inside of another object/collection initializer the behavior is that it resolves the member it is initialzing to a value, and then adds the relevant items. In this case, the property is null, so that null value is resolved, and all zero items in your initializer are added. If you actually tried to add an item, it'd throw a NRE since you'd be trying to add an item to a null object.

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