键唯一为何静态类Define初始化时抛出重复键添加异常？

问题 #

调用Define类的任意内容时，总会触发System.TypeInitializationException异常，内部错误为ArgumentException: "An item with the same key has already been added. Key: Key1"。明明Pair1和Pair2的键是唯一的，也没有代码修改字典，却必然抛出重复键的异常，请问问题原因是什么？

代码示例 #

public static class Define
{
    public static readonly Dictionary<DefineChoices, string> Definitions = new([Pair1, Pair2]);

    public enum DefineChoices
    {
        Key1,
        Key2
    }

    public static readonly KeyValuePair<DefineChoices, string> Pair1 = new(DefineChoices.Key1, "string1");
    public static readonly KeyValuePair<DefineChoices, string> Pair2 = new(DefineChoices.Key2, "string2");
}

堆栈跟踪 #

Unhandled exception. System.TypeInitializationException: The type initializer for 'Program.Namespace.Define' threw an exception.
 ---> System.ArgumentException: "An item with the same key has already been added. Key: Key1"
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
   at System.Collections.Generic.Dictionary`2.AddRange(IEnumerable`1 enumerable)
   at System.Collections.Generic.Dictionary`2..ctor(IEnumerable`1 collection, IEqualityComparer`1 comparer)
   at System.Collections.Generic.Dictionary`2..ctor(IEnumerable`1 collection)
   at Program.Namespace.Define..cctor() in FilePath\Define.cs:line 2
   --- End of inner exception stack trace ---

原因分析 #

问题出在静态字段的初始化顺序上：

• C#规定，类的静态字段会按照它们在代码中声明的先后顺序完成初始化，而非使用顺序。
• 这里Definitions字段声明在Pair1、Pair2和枚举DefineChoices之前，所以初始化Definitions时，后三者还未被赋值，会使用默认值。
• KeyValuePair<TKey, TValue>是值类型，默认初始化时：枚举DefineChoices的默认值为0（对应Key1），字符串默认值为null。此时Pair1和Pair2的实际值都是new KeyValuePair<DefineChoices, string>(DefineChoices.Key1, null)。
• 用这两个键重复的键值对初始化字典，必然触发重复键的ArgumentException，进而导致类型初始化失败，抛出TypeInitializationException。

修复方案 #

调整静态字段的声明顺序，将枚举和键值对字段放在字典之前：

public static class Define
{
    public enum DefineChoices
    {
        Key1,
        Key2
    }

    public static readonly KeyValuePair<DefineChoices, string> Pair1 = new(DefineChoices.Key1, "string1");
    public static readonly KeyValuePair<DefineChoices, string> Pair2 = new(DefineChoices.Key2, "string2");

    public static readonly Dictionary<DefineChoices, string> Definitions = new([Pair1, Pair2]);
}

内容的提问来源于stack exchange，提问作者user21161268