c# - Dictionary.GetKey 返回 False，即使 key 存在 - GetHashCode/Equals 已经被覆盖

Question

这是在 C# 中。我有一个问题，即 Dictionary.ContainsKey 返回 false，即使我知道其中的键。

不幸的是，我没有任何代码可以显示。代码不容易拼凑起来；它分布在多个类中，并通过事件等触发。我编写的快速单元测试没有重现问题。

这是调试 session 期间即时窗口的输出(添加注释并更改以保护细节):

// throws KeyNotFoundException
myDict[key]  

// throws KeyNotFoundException
myDict[new MyKey("SomeString .1", "SomeOtherString", SomeEnum.Foo)]

// Element [5] is the key
myDict.Keys
Count = 10
    [0]: {...}
    [1]: {...}
    [2]: {...}
    [3]: {...}
    [4]: {...}
    [5]: {Foo SomeOtherString SomeString  .1}
    [6]: {...}
    [7]: {...}
    [8]: {...}
    [9]: {...}

// Get key at element [5]   
enumerator.Current
{Foo SomeOtherString SomeString  .1}
    [My.Namespace.KeyType]: {Foo SomeOtherString SomeString  .1}
    SomeEnum: Foo
    SomeOtherStringProperty: "SomeOtherString"

// key used to do lookup
key
{Foo SomeOtherString SomeString  .1}
    [My.Namespace.KeyType]: {Foo SomeOtherString SomeString  .1}
    SomeEnum: Foo
    SomeOtherStringProperty: "SomeOtherString"

// hash codes of key in dictionary matches hash code of lookup key
enumerator.Current.GetHashCode()
193014103
key.GetHashCode()
193014103

一些额外的注意事项:

• 用作键的类型覆盖了 GetHashCode 和等于。
• 字典构造为 new Dictionary()没有额外的构造函数参数。
• 通过调试，我已经验证调用键类型中的 GetHashCode，但不调用 Equals(obj)
• 当应用程序运行时，只有一个具有 key 类型的 DLL 被加载，所以它可能不是不同版本的同一类型的情况同一个动态链接库

有谁知道为什么会发生这种情况？

感谢您的帮助 - 我的想法已经用完了。

Answer 1

The type used as the key has overridden methods for GetHashCode and Equals.

这是我要检查的第一件事。如果哈希码基于可变值，则肯定会导致此问题。

来自 MSDN :

In general, for mutable reference types, you should override GetHashCode only if:

• You can compute the hash code from fields that are not mutable; or

• You can ensure that the hash code of a mutable object does not change while the object is contained in a collection that relies on its hash code.

Otherwise, you might think that the mutable object is lost in the hash table. If you do choose to override GetHashCode for a mutable reference type, your documentation should make it clear that users of your type should not modify object values while the object is stored in a hash table.