C# 嵌套初始化奇怪

Question

在这些初始化语句编译通过的前提下

List<int> l = new List<int> { 1, 2, 3 };
Dictionary<int, int> d = new Dictionary<int, int> { [1] = 11, [2] = 22 };
Foo f = new Foo { Bar = new List<int>() };

这不会

List<int> l = { 1, 2, 3 };
Dictionary<int, int> d = { [1] = 11, [2] = 22 };
Foo f = { Bar = new List<int>() };

我对嵌套初始化有疑问。鉴于以下 class

public class Foo {
    public List<int> Bar { get; set; } = new List<int>();
    public Dictionary<int, Foo> Baz { get; set; } = new Dictionary<int, Foo>();
}

我偶然发现你实际上可以这样做：

Foo f = new Foo {
    Bar = { 1, 2, 3 },
    Baz = {
        [1] = {
            Bar = { 4, 5, 6 }
        }
    }
};

虽然它编译它抛出 KeyNotFoundException。所以我将属性更改为

public List<int> Bar { get; set; } = new List<int> { 4, 5, 6 };
public Dictionary<int, Foo> Baz { get; set; }
    = new Dictionary<int, Foo> { [1] = new Foo { Bar = new List<int>() { 1, 2, 3 } } };

假设这是替换现有成员的一些不寻常的符号。现在初始化抛出一个 WhosebugException.

所以我的问题是，为什么表达式甚至可以编译？它应该做什么？我觉得我一定错过了一些非常明显的东西。

Answer 1

So my question is, why does the expression even compile?

它是一个带有集合初始化器值的对象初始化器。来自 C# 规范部分 7.6.10.2:

A member initializer that specifies a collection initializer after the equals sign is an initialization of an embedded collection. Instead of assigning a new collection to the field or property, the elements given in the initializer are added to the collection referenced by the field or property.

所以你的code代码大致相当于：

Foo tmp = new Foo();
tmp.Bar.Add(1);
tmp.Bar.Add(2);
tmp.Bar.Add(3);
tmp.Baz[1].Bar.Add(4); // This will throw KeyNotFoundException if Baz is empty
tmp.Baz[1].Bar.Add(5);
tmp.Baz[1].Bar.Add(6);
Foo f = tmp;

您的初始化版本将抛出 WhosebugException，因为 Foo 的初始化程序需要创建 Foo 的新实例，而后者需要创建 [=12= 的新实例]等

C# 嵌套初始化奇怪

C# Nested initialization strangeness

c#

syntax

semantics

c#-6.0