When Do You Need To Implement "GetHashCod e?"

[Dad]

Hi,

A number of interfaces, e.g., IEqualityCompar er, include GetHashCode.
When does it need to be implemented? I usually just return 0 or 1.

Thanks,
Gary

[Marc Gravell]

In short, any time you expect to use hash-logic on the item - for
example, use it as the key in a lookup.

GetHashCode() is used as a mechanism for doing a hash-match equality
test, in particular for building hash-tables, such as in
Dictionary<TKey , TValueetc. In this case, the hash-code (or a
modulo) is used to split the items into different buckets; it is
further used to to hash-match equality tests.

The rules are that if the hash match of 2 items is different they
*can't* be the same; if it is the same, then check the actual equality
test (which mihgt be more expensive).

By returning a constant value, you are a: forcing everything into the
same bucket, and b: forcing the system to check every item for
equality, rather than being able to just look at those with matching
hash values. Ideally, the hash-code would be generated from something
immutable, or which you *expect* to be immutable while the item is
used as a key. Otherwise chaos breaks out, as it can fail to find the
item you know is there...

If none of that makes sense, post back...

Marc