Linq Distinct By implementation

Distinct By

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

namespace Testes
{
    class Program
    {
        static void Main(string[] args)
        {
            var list = new List<Foo> {
                new Foo{ Id = 1, Text = "Foo 1" },
                new Foo{ Id = 2, Text = "Foo 2" },
                new Foo{ Id = 1, Text = "Foo 1" },
            };

            var distinctList = list.Distinct();

            Console.WriteLine(distinctList.Count());
            Console.Read();

            distinctList = list.DistinctBy(x => x.Id);

            Console.WriteLine(distinctList.Count());
            Console.Read();
        }
    }

    public class Foo
    {
        public int Id { get; set; }

        public string Text { get; set; }
    }


    static class LinqExtensions
    {
        public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> predicate)
        {
            if (source == null) throw new ArgumentException("source not defined");
            if (predicate == null) throw new ArgumentException("predicate not defined");

            if (source != null && source.Count() > 0)
            {
                return source.GroupBy(predicate)
                                 .Select(group => group.First())
                                 .ToList();
            }

            return source;
        }
    }
}