GroupAdjacentBy method for .NET in C#

group-adjacent-by.cs

public static class LINQExtensions
{
    public static IEnumerable<IGrouping<TKey, TElement>> GroupAdjacentBy<TElement, TKey>(this IEnumerable<TElement> source, Func<TElement, TKey> keySelector, IEqualityComparer<TKey> comparer=null)
    {
        comparer = comparer ?? EqualityComparer<TKey>.Default;
        List<TElement> elements = null;
        TKey key = default(TKey);
        TKey lastKey = default(TKey);
        foreach (var x in source)
        {
            key = keySelector(x);
            if (elements != null && elements.Any() && !comparer.Equals(lastKey, key))
            {
                yield return new Grouping<TKey, TElement>(lastKey, elements);
                elements = null;
            }
            if (elements == null)
            {
                elements = new List<TElement>();
                lastKey = key;
            }
            elements.Add(x);
        }
        if (elements != null && elements.Any())
        {
            yield return new Grouping<TKey, TElement>(key, elements);
        }
    }
    
    public static IEnumerable<IGrouping<TElement, TElement>> GroupAdjacentBy<TElement>(this IEnumerable<TElement> source, IEqualityComparer<TElement> comparer=null)
    {
        return source.GroupAdjacentBy(keySelector:x=>x,comparer:comparer);
    }
    
    // implement IGrouping
    public class Grouping<TKey, TElement> : IGrouping<TKey, TElement>
    {
        public TKey Key { get; private set; }
        private List<TElement> Elements { get; set; }
        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
        {
            return ((IEnumerable<TElement>)this).GetEnumerator();
        }
        IEnumerator<TElement> IEnumerable<TElement>.GetEnumerator()
        {
            return ((IEnumerable<TElement>)Elements).GetEnumerator();
        }
        public Grouping(TKey key, List<TElement> elements)
        {
            Key = key;
            Elements = elements;
        }
    }
}