using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string[] arr1 =

        {

            "item-1",

            "item-2",

            "item-3",

            "item-777",

            "item-4"

        };

​

        string[] arr2 =

        {

            "item-1",

            "item-2",

            "item-3",

            "item-4"

        };

        IEnumerable<(string, string)> result =  from a1 in arr1

                                                join a2 in arr2 on a1 equals a2 into j

                                                from a2item in j.DefaultIfEmpty()

                                                select (a1,a2item ?? "empty");

​

        foreach (var item in result)

        {

          Console.WriteLine($"{item.Item1} - {item.Item2}");

        }

    }

}