Best way to do this?

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You can do something like following:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
  class Program
  {
    static void Main(string[] args)
    {
      string s1 = "99998";
      string s2 = "19";
      string s3 = "9999";
      Console.WriteLine(Do(s1));
      Console.WriteLine(Do(s2));
      Console.WriteLine(Do(s3));
      Console.ReadKey();
    }
    static string Do(string input)
    {
      string temp = input;
      if (temp.Length < 10)
        temp += new string('9', 10 - temp.Length);
      return (temp == "9999999999" ? temp : input);
    }
  }
}

Sincerely,

Yasser.