A very simple QuickSort implemented with LINQ.
Since we use IEnumerable<int> it doesn't matter if you work with a List<int> or an int[] array.

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

namespace ConsoleAppQSortLinq
{
    class Program
    {
        static void Main(string[] args)
        {
            var dataSource = Enumerable.Range(0, 1000).Reverse().ToList();  // 999, 998, ... 0
            var dataSorted = QuickSortLinq(dataSource);  // 0, 1,  ... 999

            foreach(var n in dataSorted)
            {
                Console.WriteLine(n);
            }

            Console.ReadKey();
        }

        public static IEnumerable<int> QuickSortLinq(IEnumerable<int> source)
        {
            if (!source.Any())
                return source;

            int first = source.First();

            return source
                .GroupBy(i => i.CompareTo(first))
                .OrderBy(g => g.Key)
                .SelectMany(g => g.Key == 0 ? g : QuickSortLinq(g));
        }
    }
}

Notice: While the performance is not slow, the use of OrderBy in conjunction with IComparer is definitely faster.