字典初始化中KeyNotFoundException的原因 [英] Reason for KeyNotFoundException in Dictionary initialization

问题描述

以下代码

new Dictionary<string, List<int>> {
    ["a"] = {1},
};

抛出运行时 KeyNotFoundException {1}是一个格式良好的数组（即 int [] a = {1,2,3,4} 是有效的代码）。将词典的 TValue 更改为 int [] ，抛出编译时 CS1061 ，但这不会（请注意添加的 new [] 数组分配）：

Throws a run-time KeyNotFoundException, albeit that {1} is a perfectly well-formed array (i.e. int[] a = {1,2,3,4} being valid code). Changing the TValue of the Dictionary to int[], throws a compile-time CS1061, but this does not (note the added new[] array-allocation):

new Dictionary<string, IEnumerable<int>> {
    ["a"] = new[]{1},
};

为什么会这样？

推荐答案

您的第一段代码使用的是集合初始值设定项，该初始值设定项不使用逻辑分配，而是用于调用在现有集合上添加。换句话说，这是

Your first piece of code is using a collection initializer, which doesn't use logical assignment, but instead is intended to call Add on an existing collection. In other words, this:

var x = new Dictionary<string, List<int>> {
    ["a"] = {1},
};

等同于：

var tmp = new Dictionary<string, List<int>>();
var list = tmp["a"];
list.Add(1);
var x = tmp;

希望从扩展的第二行会引发异常很明显。

Hopefully it's obvious from that why the second line of the expansion would throw an exception.

推理中的部分错误是：

尽管{1}非常好格式的数组

albeit that {1} is a perfectly well-formed array

不，不是。语法 {1} 在不同的上下文中表示不同的内容。在这种情况下，它是一个集合初始化程序。在语句中：

No, it's not. The syntax {1} means different things in different contexts. In this case, it's a collection initializer. In the statement:

int[] a = { 1, 2, 3, 4 };

它是一个数组初始值设定项。该语法 only 仅在数组声明中创建新数组，或者作为数组创建表达式的一部分，例如 new [] {1，2，3，4} 。

it's an array initializer. That syntax only creates a new array in an array declaration, or as part of an array creation expression, e.g. new[] { 1, 2, 3, 4 }.

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