Program.cs

var input = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 20, 21, 22, 23 };

var list = new List<int>(input);
foreach (var n in input)
{
    var actualIndex = list.IndexOf(n);
    var foundIndex = FindAt(input, n, 0, input.Length - 1);

    Console.WriteLine($"[{n}] Expected: {actualIndex} | Found: {foundIndex}");
}

return;

// Binary search algorithm assuming that the array is sorted in ascending order.
int FindAt(int[] numbers, int number, int startPosition, int endPosition)
{
    if (numbers[0] > number) return -1;
    if (numbers[^1] < number) return -1;

    var middlePos = startPosition + (endPosition - startPosition) / 2;
    if (numbers[middlePos] == number) return middlePos;

    if (numbers[middlePos] > number)
    {
        return FindAt(numbers, number, startPosition, middlePos - 1);
    }

    return FindAt(numbers, number, middlePos + 1, endPosition);
}