Question

If object.ReferenceEquals returns true, should instance.Equals always return true?

Would you think its unexpected behaviour where the output below was pass/fail?

Assert.True(object.ReferenceEquals(obj,obj));
Assert.True(obj.Equals(obj));

Personally I think its strange, and cant really think of a good reason, where an instance should not be equal to itself.

Answer 1

There are two ways to answer your question.

• It’s unexpected behavior, yes. A properly-designed class should always return true from .Equals if it returns true from .ReferenceEquals. That’s codified in MSDN’s Design Guidelines for Developing Class Libraries, Implementing the Equals Method article:

  Follow the contract defined on the Object.Equals Method as follows:
       x.Equals(x) returns true.

• But that requirement is not enforced by the language or runtime. The designer of the class in question is perfectly free to define Equals as { return false; }. Perverse, but possible.