Friday, December 21, 2018

Sonar Source rule 3898 Value types should implement "IEquatable"

This is another post discussing a static analysis rule that made me learn something new. SonarSource Rule 3898 says: If you're using a struct, it is likely because you're interested in performance. But by failing to implement IEquatable<T> you're loosing performance when comparisons are made because without IEquatable<T>, boxing and reflection are used to make comparisons.

There is a StackOverflow entry that discusses just that and the answer to this particular problem is not actually the accepted one. In pure StackOverflow fashion I will quote the relevant bit of the answer, just in case the site will go offline in the future: I'm amazed that the most important reason is not mentioned here. IEquatable<> was introduced mainly for structs for two reasons:
  1. For value types (read structs) the non-generic Equals(object) requires boxing. IEquatable<> lets a structure implement a strongly typed Equals method so that no boxing is required.
  2. For structs, the default implementation of Object.Equals(Object) (which is the overridden version in System.ValueType) performs a value equality check by using reflection to compare the values of every field in the type. When an implementer overrides the virtual Equals method in a struct, the purpose is to provide a more efficient means of performing the value equality check and optionally to base the comparison on some subset of the struct's field or properties.

I thought this was worth mentioning, for those performance critical struct equality scenarios.

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