GroupBy and IEqualityComparer<TKey> comparer

Question

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.

Answer 1

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:

1. It checks the hash code of the item (using GetHashCode) against the hash code of existing keys. If it does not equal any of those values it is added to a new group
2. If a matching hash code is found, it then checks for equality (using Equals). 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:

• If two items are equal, they must have the same hash code.
• The opposite is not true - two items that have the same hash code do not have to be equal.