An example of using a extension method to achieve a similar functionality to the ES6 spread operator (...)

SpreadExample.cs

using System.Collections.Generic;

namespace SpreadExample
{
    public class SpreadExample
    {
        //Normally, this would take more lines, like:
        /*
        public List<string> InsertList(List<string> secondList)
        {
            List<string> myList = new List<string>
            {
                "firstItem",
            };

            myList.AddRange(secondList);

            myList.Add("lastItem");

            return myList;
        }
        */

        public List<string> InsertList(List<string> secondList)
        {
            return new List<string>
            {
                "firstItem",
                secondList, //In ES6 you would put (...secondList)
                "lastItem",
            };
        }
    }

    public static class CollectionExtensions
    {
        public static void Add<T>(this ICollection<T> collection, IEnumerable<T> items)
        {
            foreach (T item in items)
            {
                collection.Add(item);
            }
        }
    }
}