I was going through the GroupBy method in LINQ :
public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector,
IEqualityComparer<TKey> comparer
)
I understand how to use GroupBy and what it returns. I want to understand the significance of IEqualityComparer<TKey> comparer
and what is it actually used for in GroupBy.
The IEqualityComparer<TKey>
object will be used to perform a two-step check to see if a TKey
instance is "equal" to the key of an existing group and thus should be in that group:
GetHashCode
) against the hash code of existing keys. If it does not equal any of those values it is added to a new groupEquals
). If the item is "equal to" the group key, the item is added to that group. If you do not supply a comparer (either by passing null
or using one of the overloads that does not have that parameter), the "default" comparer is used, which uses the TKey
class itself if it implements IEquatable
or any applicable overrides of Equals
and GetHashCode
.
So this implies a few key relationships between Equals
and GetHashCode
:
You've provided a nonsensical equality comparer, so your results are going to be nonsensical. Your hash code is based on the reference to the comparer itself, which has nothing to do with anything in your Equals
method, and in your Equals
method you're saying that two objects are equal if the first object is as long or longer than the second string. This just makes no sense, it even violates basic properties of equality in that the order of the parameters should be irrelevant.
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