using System;

using System.Linq;

using System.Text.RegularExpressions;

​

​

public class Simple

{

    public static void Main()

    {

        //[0] "1", [1] "11", [2] ""

        string[] names = {"F.01,hello", "F.11,hello", "F01" };

        var x = names.Select(c => c.Length > 3 ? int.Parse(c.Replace('.',',').Split(',')[1]).ToString() : "empty_string").ToArray(); 

        foreach(var name in x){

            Console.WriteLine(name);

        }

    }

}