HOw to return values list in c#

public static void my_insert(List <string > li)
   {
     li.Add("foo");
     li.Add("boo");
     li.Add("goo");
   }
   public static void my_delete(List<string> li)
   {// deletes all but the very first element
     if (li.Count > 0)
     {
       string s = li[0];
       li.Clear();
       li.Add(s);
     }
   }

   public static void Main()
   {
     List<string> li = new List<string>();
     my_insert(li);
     my_delete(li);
     foreach (string s in li)
     {
       Console.WriteLine(s);
     }
   }