QuickSorting not working.

gistfile1.cs

namespace SortingHomework
{
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;

    public class Quicksorter<T> : ISorter<T> where T : IComparable<T>
    {
        public IList<T> Collection { get; set; }

        public void Sort(IList<T> collection)
        {
            if (collection == null)
            {
                throw new ArgumentNullException("collection", "Collection is null. Collection cannot be sorted!");
            }

            int length = collection.Count;
            if (length <= 1)
            {
                return;
            }

            this.Collection = collection;
            this.QuickSorting(0, length - 1);
            collection = this.Collection;
        }

        private void QuickSorting(int startIndex, int endIndex)
        {
            if (startIndex < endIndex)
            {           

                int pivotIndex = this.Collection.Count / 2;

                int pivotNewIndex = this.Partition(startIndex, endIndex, pivotIndex);

                // Recursively sort elements smaller than the pivot
                this.QuickSorting(startIndex, pivotNewIndex - 1);

                // Recursively sort elements at least as big as the pivot
                this.QuickSorting(pivotNewIndex + 1, endIndex);
            }
            
        }

        private int Partition(int startIndex, int endIndex, int pivotIndex)
        {
            T pivot = this.Collection[pivotIndex];

            ////swap, move pivot to end
            //T safe = collection[endIndex];
            //collection[endIndex] = pivot;
            //collection[pivotIndex] = safe;
            this.Swap(pivotIndex, endIndex);

            int storeIndex = startIndex;

            for (int i = startIndex; i < endIndex - 1; i++)
            {
                if (this.Collection[i].CompareTo(pivot) <= 0)
                {
                    ////swap collection[i] and collection[storeIndex]
                    //T swap = collection[i];
                    //collection[i] = collection[storeIndex];
                    //collection[storeIndex] = swap;
                    //storeIndex++;
                    this.Swap(i, storeIndex);
                }
            }

           //T change = collection[storeIndex];
           //collection[storeIndex] = collection[endIndex];
           //collection[endIndex] = change;
            this.Swap(storeIndex, endIndex);

            return storeIndex;
        }

        private void Swap(int index1, int index2)
        {
            T swapValue = this.Collection[index1];
            this.Collection[index1] = this.Collection[index2];
            this.Collection[index2] = swapValue;
        }
    }
}