An example console application that shows you how to create an extension method on a collection that prints its content to the console in a generic way.

Program.cs

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

namespace ConsoleApplication1
{
    public class Program
    {
        static void Main(string[] args)
        {
            var persons = new List<Person>{
                new Person{FirstName="Gilles",LastName="achternaam"},
                new Person{FirstName="Nog",LastName="Iemand anders"},
                new Person{FirstName="Delaatse",LastName="persoon"}
            };

            persons.LogCollection();
        }
    }
    public class Person
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }

        public override string ToString()
        {
            return FirstName + ' ' + LastName;
        }
    }
    public static class CollectionExtension
    {
        public static void LogCollection<T>(this IEnumerable<T> collection)
        {
            foreach(T item in collection)
            {
                System.Diagnostics.Debug.WriteLine(item);
            }
        }
    }
}