Recursive implementation of QuickSort in C#. It's really slow and absolutely not recommended to use it!! It's only meant for demonstration purposes.

QuickSort.cs

static class Program
{
    static void Main(string[] args)
    {
        var list = new List<int> {4, 5, 4, 7, 9, 1, 6, 1, 0, -99, 10000, 3, 2};

        var sorted = list.QuickSort();

        Console.WriteLine(String.Join(", ", sorted));
        // output: -99, 0, 1, 1, 2, 3, 4, 4, 5, 6, 7, 9, 10000
    }

    /// <summary>
    /// Recursive implementation of QuickSort. It's really slow and absolutely not recommended to use it!! It's only meant for demonstration purposes.
    /// </summary>
    /// <param name="list"></param>
    /// <returns></returns>
    public static List<int> QuickSort(this List<int> list)
    {
        return !list.Any()
            ? list
            : list.Skip(1)
                .Where(x => x <= list.First())
                .ToList()
                .QuickSort()
                .Concat(new List<int> { list.First() })
                .Concat(list.Skip(1)
                    .Where(x => x > list.First())
                    .ToList()
                    .QuickSort())
                .ToList();
    }
}