Overriding Equality Operators

Question

I've implemented a class that overloads the == and != operators.

This seems to work fine; however, I get the warning 'type' defines operator == or operator != but does not override Object.Equals(object o).

Okay, so I implemented Equals. But now I get the warning 'type' defines operator == or operator != but does not override Object.GetHashCode().

Does this end at some point? Or have I wandered into a never-ending trail of requirements just because I want to overload == and !=?

Quick terminology correction: you don't *override* operators - you *overload* them. — Jon Skeet, Jul 25 '11 at 21:59
When you override `Equals`, you need to override `GetHashCode` as well, as it will check equality based on a hash function. — Igoris Azanovas, Jul 25 '11 at 22:00
@IgorisAzanovas No, Equals will not check equality based on a hash function (unless you overload it and specifically code it that way). The hash function is there for supporting hashed collections and is not expected to generate a unique value, only one that ideally will be infrequently used. — Suncat2000, Feb 02 '17 at 13:45

score 11 · Accepted Answer · answered Jul 25 '11 at 21:57

11

Does this end at some point?

Yes, once you implement GetHashCode it will end. Eric Lippert has blogged about its importance. All I can do is suggest you to read and trust him :-)

answered Jul 25 '11 at 21:57

Darin Dimitrov

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Okay, so can anyone point to a good algorithm that takes converts an integer value to a reasonable hash value? – Jonathan Wood Jul 25 '11 at 22:30
1

@Darin why do we need to override object.equals() when we just overload "==" operator? – Sandeep Jul 26 '11 at 16:53
@Jonathon: if your only identifying information is an `int`, the built-in `int` type uses itself as its hash value. If it's good enough for them... – porges Aug 03 '11 at 10:51

score 1 · Answer 2 · answered Jul 25 '11 at 21:58

Yes, it will end when you override GetHashCode.

When implementing equality operators, and furthermore Equals, it is the responsibility of the programmer to provide an implementation to deliver a custom hash code for that type.

See this MSDN reference for details.

Overriding Equality Operators

2 Answers2