Program.cs

using System;

class Program
{
    static void Main(string[] args)
    {
        foreach (var number in new object[] { 123, "2017", "-1", "1a2b3c", '5' })
        {
            try
            {
                int result;
                if (IntegerParser.TryParse(number, out result))
                {
                    Console.WriteLine($"Parsed {number.GetType()} {number}, got {result}");
                }
                else
                {
                    Console.WriteLine($"Could not parse {number.GetType()} {number}");
                }
            }
            catch (Exception e)
            {
                Console.WriteLine($"Error when parsing: {e.Message}");
            }
        }
    }
}

public static class IntegerParser
{
    private const int MinDigit = '0';
    private const int MaxDigit = '9';

    public static bool TryParse(object number, out int result)
    {
        if (number is int)
        {
            result = (int)number;
            return true;
        }

        if (number is string)
        {
            return ParseString((string)number, out result);
        }

        throw new NotSupportedException($"{nameof(IntegerParser)} only accepts strings ans integers, type '{number.GetType()}' is not supported");
    }

    private static bool ParseString(string number, out int result)
    {
        result = 0;
        for (var i = 0; i < number.Length; i++)
        {
            var charCode = (int)number[i];
            if (charCode < MinDigit || charCode > MaxDigit)
            {
                return false;
            }

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

        return true;
    }
}