Group IEnumerable<T> by key, and get the first item from each group using a predicate

GroupExtensions.cs

public static IEnumerable<T> FirstGroupItems<T, TKey>(this IEnumerable<T> list, Func<T, TKey> keySelector, Func<IGrouping<TKey, T>, T> firstPredicate = null) where T : class
		{
			return list.GroupBy(keySelector).Select(a => firstPredicate == null ? a.First() : firstPredicate(a) ?? a.First());
		}