using System;

using System.Text.RegularExpressions;

using System.Linq;

using System.Numerics;

public class Program

{

    public static void Main()

    {

        //https://gist.github.com/fernandezja/91ca7bd840d504650260585092f75250

        var longNumberString = "123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789";

        var isNumberA = int.TryParse(longNumberString, out _);

        Console.WriteLine($"isNumberA = {isNumberA}");

        var isNumberB = long.TryParse(longNumberString, out _);

        Console.WriteLine($"isNumberB = {isNumberB}");

        var isNumberC = UInt64.TryParse(longNumberString, out _);

        Console.WriteLine($"isNumberC = {isNumberC}");

        //Option 1: BigInteger.TryParse

        var isNumberD = BigInteger.TryParse(longNumberString, out _);

        Console.WriteLine($"isNumberD = {isNumberD}");

        Console.WriteLine();

        //Option 2: Regex

        var isNumber1 = Regex.IsMatch(longNumberString, @"^\d+$");

        Console.WriteLine($"isNumber1 = {isNumber1}");

        //Option 3: Linq > All with IsDigit method

        var isNumber2 = longNumberString.All(char.IsDigit);

        Console.WriteLine($"isNumber2 = {isNumber2}");

    }

}