using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var list1 = new List<string> { "item1", "item2" };      

        var list2 = new List<string> { "item2", "item3" }; 

​

        var unionResult = list1.Union(list2);

        Console.WriteLine(string.Join(", ", unionResult));

    }

}