CODEWITHSUNDEEP
c#.netvb.netdictionaryreference-type
Jules
Jules

Reputation: 4339

Quick question about a reference type key in a generic dictionary in .Net

I have a mutable class that I'm using as a key to a generic dictionary. Two keys should be equal only if their references are equal. From what I've read, in this case, I don't need to override Equals, GetHashCode , or implement IEqualityComparer.

Is this correct?

Upvotes: 7

Views: 4174

Answers (6)

Tony Edgecombe
Tony Edgecombe

Reputation: 3915

For anybody using .Net 5 or later it comes with a ReferenceEqualityComparer class that you can pass to the dictionary's constructor. This means you don't need to worry about someone overriding GetHashCode and Equals in the future.

Upvotes: 0

Chris
Chris

Reputation: 27384

Yes you are correct doing a == comparison (or .Equals) on two objects compares their references if no other overload is specified.

String s = "a";

object test1 = (object)s;
object test2 = (object)s;

Debug.Assert(test1.Equals(test2));

Upvotes: -1

Peter Oehlert
Peter Oehlert

Reputation: 16938

I'll add on to what everyone else has said here (yes) but with one more point that no one seems to have mentioned here.

When using generic collections (Dictionary, List, etc) you can override IEquatable to provide a type specific version that can do your comparison without boxing or up/down casting. These generic collections will use this overload when present to do comparisons and it can be a bit more efficient.

As noted in the docs, when implementing IEquatable you still need to override Equals/Hashcode from Object.

Upvotes: 1

Pat
Pat

Reputation: 16891

As everyone else pointed out already, yes, you are correct. In fact, you definitely do not want to override the equality members if your type is mutable (it has setters). But, if you want to have equality checking which uses values in your type, you can make your type immutable (like String) by ensuring that there are no setters (only the constructor sets values). Or use a struct.

Upvotes: 0

Nick Craver
Nick Craver

Reputation: 630459

Yes, this is correct. As long as you don't override, reference is the default comparison.

Upvotes: 1

Reed Copsey
Reed Copsey

Reputation: 564451

Yes. The default comparison operation in System.Object uses reference equality. If this behavior is what you want, the defaults should work fine.

Upvotes: 16

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