Sample Linq Test, C#

Sample Linq Test

//Title of this code
//Rextester.Program.Main is the entry point for your code. Don't change it.

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

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            List<Person> persons;
            persons[0] = new Person { PersonID = 1, car = "C1-1" }; 
            persons[2] = new Person { PersonID = 2, car = "C2-1"    }; 
            persons[2] = new Person { PersonID = 2, car = "C2-2"    }; 
            persons[1] = new Person { PersonID = 1, car = "C1-2"     }; 
            persons[2] = new Person { PersonID = 2, car = "C2-3"    }; 
            persons[2] = new Person { PersonID = 3, car = "C3-1"    }; 
            persons[2] = new Person { PersonID = 3, car = "C3-2"    }; 
            persons[2] = new Person { PersonID = 4, car = "C4-1"    }; 
            persons[2] = new Person { PersonID = 4, car = "C4-2"    }; 
            persons[2] = new Person { PersonID = 2, car = "C2-4"    }; 
            persons[2] = new Person { PersonID = 5, car = "C5-1"    }; 
            persons[2] = new Person { PersonID = 6, car = "C6-1"    }; 
            persons[2] = new Person { PersonID = 6, car = "C6-2"    }; 
            persons[2] = new Person { PersonID = 6, car = "C6-3"    }; 
            persons[2] = new Person { PersonID = 7, car = "C7-1"    }; 
            
            // Understanding of the following code piece (non C#-dev.)
            persons = persons.GroupBy(id => id.PersonID).Select(group => group.First()).ToList();
            
            Console.WriteLine("Hello, world!");
        }
    }
    
   public class Person { 
        internal int PersonID; 
        internal string car  ; 
    }
}


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