检查一下:

    var a = Double.NaN;

    Console.WriteLine(a == a);
    Console.ReadKey();

打印“假”

    var a = Double.NaN;

    Console.WriteLine(a.Equals(a));
    Console.ReadKey();

打印“真”!

为什么打印“True”？由于 float 规范，NaN 值不等于自身!所以似乎 Equals() 方法实现错误...... 我错过了什么吗？

最佳答案

我发现一篇文章解决了您的问题:.NET Security Blog: Why == and the Equals Method Return Different Results for Floating Point Values

According to IEC 60559:1989, two floating point numbers with values of NaN are never equal. However, according to the specification for the System.Object::Equals method, it's desirable to override this method to provide value equality semantics. [...]

So now we have two conflicting ideas of what Equals should mean. Object::Equals says that the BCL value types should override to provide value equality, and IEC 60559 says that NaN does not equal NaN. Partition I of the ECMA spec provides resolution for this conflict by making a note about this specific case in section 8.2.5.2 [below]

更新:第 8.2.5 节的全文来自 the CLI spec (ECMA-335)对此有更多的了解。我已经在此处复制了相关位:

8.2.5 Identity and equality of values

There are two binary operators defined on all pairs of values: identity and equality. They return a Boolean result, and are mathematical equivalence operators; that is, they are:

• Reflexive – a op a is true.
• Symmetric – a op b is true if and only if b op a is true.
• Transitive – if a op b is true and b op c is true, then a op c is true.

In addition, while identity always implies equality, the reverse is not true. [...]

8.2.5.1 Identity

The identity operator is defined by the CTS as follows.

• If the values have different exact types, then they are not identical.
• Otherwise, if their exact type is a value type, then they are identical if and only if the bit sequences of the values are the same, bit by bit.
• Otherwise, if their exact type is a reference type, then they are identical if and only if the locations of the values are the same.

Identity is implemented on System.Object via the ReferenceEquals method.

8.2.5.2 Equality

For value types, the equality operator is part of the definition of the exact type. Definitions of equality should obey the following rules:

• Equality should be an equivalence operator, as defined above.
• Identity should imply equality, as stated earlier.
• If either (or both) operand is a boxed value, [...]

Equality is implemented on System.Object via the Equals method.

[Note: Although two floating point NaNs are defined by IEC 60559:1989 to always compare as unequal, the contract for System.Object.Equals requires that overrides must satisfy the requirements for an equivalence operator. Therefore, System.Double.Equals and System.Single.Equals return True when comparing two NaNs, while the equality operator returns False in that case, as required by the IEC standard. end note]

上面根本没有指定==运算符的属性(最后的注释除外)；它主要定义 ReferenceEquals 和 Equals 的行为。对于 == 运算符的行为，the C# language spec (ECMA-334) (第 14.9.2 节)清楚如何处理 NaN 值:

If either operand [to operator ==] is NaN, the result is false

关于c# NaN 比较 Equals() 和 == 的区别，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/4933769/