MyEnumerableExtensionsWithExample1.cs

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

namespace Tester
{
    static void Main()
    {
        var nameOccurrences =
            new[]
            {
                new MyNamedItem { Name = "Dan" },
                new MyNamedItem { Name = "Dan" },
                new MyNamedItem { Name = "Ted" },
                new MyNamedItem { Name = "Dan" },
                new MyNamedItem { Name = "Dan" }
            }
            .MyGroupBy(value => value.Name, null)
            .Select(group => new { Name = group.Key, Count = group.Count() });

        Console.WriteLine(string.Join(", ", nameOccurrences.Select(name => name.Name + ": " + name.Count)));
        Console.ReadLine();
    }
    
    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();
        }
    }
}