Binary Search - C#

Binary Search - C# (input array is assumed to be sorted)

Efficiency: O(log n) 

// input array is assumed to be sorted
        public int BinarySearch(int[] arr, int x)
        {
            if (arr.Length == 0)
                return -1;

            int mid = arr.Length / 2;

            if (arr[mid] == x)
                return mid;

            if (x < arr[mid])
                return BinarySearch(GetSubArray(arr, 0, mid - 1), x);
            else
            {
                int _indexFound = BinarySearch(GetSubArray(arr, mid + 1, arr.Length - 1), x);
                if (_indexFound == -1)
                    return -1;
                else
                    return mid + 1 + BinarySearch(GetSubArray(arr, mid + 1, arr.Length - 1), x);
            }
        }


        public int[] GetSubArray(int[] arr, int start, int end)
        {
            List _result = new List();
            for (int i = start; i <= end; i++)
            {
                _result.Add(arr[i]);
            }
            return _result.ToArray();
        }