Merge two sorted lists

MergeSort.cs

public static class Utils
{
  [Flags]
  public enum MergeStatus
  {
    LeftOnly = 1,
    RightOnly = 2,
    Both = 3,
  }

  public static void MergeSorted<T, TKey>(this IList<T> left, IList<T> right,
    Func<T, TKey> keyGetter, Action<MergeStatus, T, T> callback) where TKey : IComparable
  {
    MergeSorted(left, right, keyGetter, callback, Comparer<TKey>.Default);
  }

  public static void MergeSorted<T, TKey>(this IList<T> left, IList<T> right,
    Func<T, TKey> keyGetter, Action<MergeStatus, T, T> callback, IComparer<TKey> comparer)
  {
    var leftPtr = 0;
    var rightPtr = 0;
    var leftKey = keyGetter(left[leftPtr]);
    var rightKey = keyGetter(right[rightPtr]);
    int status;

    while (leftPtr < left.Count && rightPtr < right.Count)
    {
      status = comparer.Compare(leftKey, rightKey);
      if (status < 0)
      {
        callback(MergeStatus.LeftOnly, left[leftPtr], default(T));
        leftKey = keyGetter(left[++leftPtr]);
      }
      else if (status > 0)
      {
        callback(MergeStatus.RightOnly, default(T), right[rightPtr]);
        rightKey = keyGetter(right[++rightPtr]);
      }
      else
      {
        callback(MergeStatus.Both, left[leftPtr], right[rightPtr]);
        leftKey = keyGetter(left[++leftPtr]);
        rightKey = keyGetter(right[++rightPtr]);
      }
    }
    while (leftPtr < left.Count)
    {
      callback(MergeStatus.LeftOnly, left[leftPtr], default(T));
      leftPtr++;
    }
    while (rightPtr < right.Count)
    {
      callback(MergeStatus.RightOnly, default(T), right[rightPtr]);
      rightPtr++;
    }
  }
}