默认 == 运算符实现是否在 C# 内部调用 ReferenceEquals？

Question

I know that == operator implentation only do reference check by default, and it doesn't call Object.Equals virtual method. Since Object class also have a ReferenceEquals method that does reference check, so does == operator default implentation calls Object.ReferenceEquals method internally?

Answer 1

The default implementation of == for a class (not a struct) that has no == operator overload defined, is to do a straight reference comparison with a ceq instruction. It does not call ReferenceEquals (which cannot be overridden as it is static ).

The code for ReferenceEquals is quite simple:

public static bool ReferenceEquals (Object objA, Object objB) {
        return objA == objB;
    }

So it's exactly the same thing.

Answer 2

1. == checks for equality
2. ReferenceEquals checks for identity
3. Identity assumes equality, equality does not assume identity
4. == and Equals on a reference type defaults to a reference check, howevercan be overridden by its actual type, ReferenceEquals cannot

Going further via the CLI ECMA specs

Identity

• If the values have different exact types (cat and dog), then they are not identical.
• 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)
• If their exact type is a reference type, then they are identical if and only if the locations of the values are the same in memory.

Equality

• Equality is baked into value types and part of its exact type
• Identity should imply equality
• If either (or both) operand is a boxed value, equality should be computed by
  • first unboxing any boxed operand(s), and then
  • applying the usual rules for equality on the resulting values.

Identity is implemented via the ReferenceEquals . Equality is implemented via the Equals method.

Some examples

• The types Cat and Dog if not in a hierarchical relationship can never be identical.
• If Cat and Dog are separate instances, they can't be identical.
• Casting a reference type Cat to Mammal , preserves the identity and are identical
• (byte)4 and (int)4 are not identical however they are equivalent
• (byte)4 and (byte)4 are identical and therefore equivalent.
• Casting a value type may not preserve identity, however though through coercion may still be equivalent