List<string> StringList = new List<string>();
foreach (var item in StringList)
{
    Console.WriteLine(item);
}

namespace PegasusZero.Extensions
{
    public static class ListExtension
    {
        public static void PrintMember(this List<string> listInstance) {
            foreach (var item in listInstance)
            {
                Console.WriteLine(item);
            }
        }
    }
}

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

namespace PegasusZero.Demo.ExtensionMethod
{
    class Program
    {
        static void Main(string[] args)
        {
            List<string> StringList = new List<string>();
            StringList.Add("Copyright (c)2013 PegasusZero");
            StringList.Add("Item 1");
            StringList.Add("Item 2");
            StringList.Add("Item 3");
            StringList.Add("Item 4");
            StringList.Add("Item 5");

            StringList.PrintMember();
        }
    }
}