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c#iequatablesymmetric

Should I Allow Asymmetric Equals?

I have a C# type for which it makes logical sense to compare for equality with int. Call this type Number.

However, I can't make Equals symmetric, because I can't change Int32.Equals(object other). So, should I permit the asymmetric equality, like this:

Number numberThree = new Number(3);
int intThree = 3;

bool equal;
equal = numberThree.Equals(intThree); // equal is true
equal = intThree.Equals(numberThree); // equal is false

Or should I just only allow equality if the types match as well as the numeric value?

Solution

  • No. It violates the contract of Object.Equals:

    The following statements must be true for all implementations of the Equals(Object) method. In the list, x, y, and z represent object references that are not null.

    • ...
    • x.Equals(y) returns the same value as y.Equals(x).

    Anything using Equals should be able to depend on that - so violating it could generate some really odd effects. Imagine if Dictionary<,>.ContainsKey called key.Equals(candidateEntry.Key) but the indexer called candidateEntry.Key.Equals(key)... you might see a key as existing, but not be able to fetch the entry... or vice versa. Or maybe it will work in one implementation of Dictionary<,>, but not in a subsequent release (or not on Mono, etc).