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I have a CustomObject object which overrides GetHashCode(). I have a HashSet, and I am able to call add with two distinct object having the same hash code. Both get added and later on I end up with some database insertion troubles (primary key duplicates)... The purpose of using a hashSet was connected to these database insertions (avoiding key collisions).

Am I possibly missing out on some properties of HashSet ? Even when I try checking (.Contains) before adding (.Add), I end up adding hashCode duplicates...

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Because HashSet<T> membership is based on object equality, not hash code equality. It's perfectly legal for every member of a HashSet<T> to have the same hash code as long as the members are different according to Equals. The role that hash codes play in HashSet<T> is for rapid testing of membership. If you have an object and its hash code is not in the HashSet<T>, then you know that the object is not in the HashSet<T>. If you have an object and its hash code is in the HashSet<T>, then you have to walk the chain of objects with that same hash code testing for equality using Equals to see if the object is actually in the HashSet<T> or not. That's why a balanced hash code distribution is important. But it is not the case that unique hash codes are necessary.

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Overriding GetHashCode is not enough. You need to override Equals function as well.

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Do not use hashsets to try to avoid duplicate values. Use them to balance hash tables!

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