定义运算符==而不定义Equals()或GetHashCode()有什么问题? [英] What's wrong with defining operator == but not defining Equals() or GetHashCode()?

问题描述

对于下面的代码

public struct Person
{
    public int ID;
    public static bool operator ==(Person a, Person b) { return  a.Equals(b); }
    public static bool operator !=(Person a, Person b) { return !a.Equals(b); }
}

为什么编译器会给我这些警告?
不定义下面的方法怎么办?

Why does the compiler give me these warnings?
What's wrong with not defining the methods below?

warning CS0660: 'Person' defines operator == or operator != but
    does not override Object.Equals(object o)

warning CS0661: 'Person' defines operator == or operator != but
    does not override Object.GetHashCode()

推荐答案

EDIT :此答案已得到纠正，除其他事项外，请注意用户定义的值类型不会生成== ，并提及 ValueType.Equals 的性能问题.

EDIT: This answer has been corrected, among other things to note that user-defined value types don't generate ==, and to mention the performance issues with ValueType.Equals.

通常，覆盖其中一个(但不是全部)会造成混淆.用户希望这两个语义都不会被覆盖，或者两者都被覆盖.

In general, overridding one, but not all, is confusing. The user expects neither to be overridden, or both to be, with the same semantics.

Microsoft针对此状态的建议(除其他事项外) ):

Microsoft's recommendations for this state (among other things):

• 每当实现Equals方法时，都要执行GetHashCode方法.这样可以使Equals和GetHashCode保持同步.

• Implement the GetHashCode method whenever you implement the Equals method. This keeps Equals and GetHashCode synchronized.

每当实现相等运算符(==)时都重写Equals方法，并使它们执行相同的操作.

Override the Equals method whenever you implement the equality operator (==), and make them do the same thing.

在您的情况下，您有充分的理由服从Equals(编译器不会自动实现==)并仅覆盖这两个(==/!=).但是，由于ValueType.Equals使用反射，因此仍然存在性能问题:

In your case, you have a valid reason to defer to Equals (the compiler doesn't automatically implement ==) and override only those two (==/!=). However, there's still a performance issue, since ValueType.Equals uses reflection:

重写特定类型的Equals方法以改善 该方法的性能并更紧密地代表了该概念 类型相等."

"Override the Equals method for a particular type to improve the performance of the method and more closely represent the concept of equality for the type."

因此，仍然建议最后覆盖所有(==/!=/Equals).当然，对于这种琐碎的结构，性能可能并不重要.

Thus, it's still recommended to override all (==/!=/Equals) in the end. Of course, performance may not matter for this trivial struct.

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