TryParse_improved2.cs

public static bool TryParse(object number, out int result)
{
    switch (number)
    {
        case int i:
            result = i;
            return true;
        case string s:
            bool ParseString(string str, out int res)
            {
                res = 0;
                for (var i = 0; i < str.Length; i++)
                {
                    var charCode = (int)str[i];
                    if (charCode < MinDigit || charCode > MaxDigit)
                    {
                        return false;
                    }

                    res = res * 10 + (charCode - MinDigit);
                }

                return true;
            }

            return ParseString(s, out result);
        default:
            throw new NotSupportedException($"{nameof(IntegerParser)} only accepts strings ans integers, type '{number.GetType()}' is not supported");
    }
}