S3897 is an unsafe suggestion in my opinion

[Mike Barry]

Hi all,

I wanted to discuss: S3897 (https://sonarcloud.io/organizations/default/rules#rule_key=csharpsquid%3AS3897)

I think it is an unsafe suggestion on it's surface if the class is not sealed. Especially as a major. The reason is fairly well described here: http://blog.mischel.com/2013/01/05/inheritance-and-iequatable-do-not-mix/

But the important quote: A class that implements IEquatable<T> is saying, “I know how to compare two instances of type T or any type derived from T for equality.” After all, if type B derives from A, then B is-a A. That’s what inheritance means! The problem with implementing IEquatable<T> on a non-sealed type is that a base class cannot know how to compare instances of derived types. Trying to implement IEquatable<T> on a non-sealed class ends up producing inconsistent results when comparing instances of derived types.

Here is an example.

class Base : IEquatable<Base> {

  bool Equals(Base b) => return true;

}

class A : Base, IEquatable<A> {

  bool Equals(A a) => return false;

}

class B : Base, IEquatable<B> {

  bool Equals(B b) => return false;

}

A a = new A();

B b = new B();

Console.WriteLine(a.Equals(b)); // This should obviously return false as they are different classes however because of the existance of Base:Equals(Base b) it will return true.

Thanks,

Mike

[Amaury Leve]

Further discussion is going to be handled on the GitHub issue thread: https://github.com/SonarSource/sonar-csharp/issues/420

Cheers,

Amaury

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Amaury Levé | SonarSource

Software Developer - .Net Team

http://sonarsource.com

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