MyEnumerableExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace Tester
{
    public static class MyEnumerableExtensions
    {
        public static IEnumerable<IGrouping<TKey, TSource>> MyGroupBy<TSource, TKey>(
            this IEnumerable<TSource> source,
            Func<TSource, TKey> keySelector,
            IEqualityComparer<TKey> comparer)
        {
            throw new NotImplementedException();
        }
    }
}