using System;

using System.Collections.Generic;

using System.Linq;

​

public class ExternalIdInformation

{

    public string ProductId {get;set;}

}

​

public static class Extensions

{

​

 public static IEnumerable<TSource> DistinctBy<TSource, TKey>

            (this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)

        {

            HashSet<TKey> seenKeys = new HashSet<TKey>();

            foreach (TSource element in source)

            {

                if (seenKeys.Add(keySelector(element)))

                {

                    yield return element;

                }

            }

        }

}

​

public class Program

{

    public static void Main()

    {

        List<ExternalIdInformation> modifiedCatalogItems = new List<ExternalIdInformation>();

        List<ExternalIdInformation> changedInventoryItems = new List<ExternalIdInformation>();

        modifiedCatalogItems.Add(new ExternalIdInformation {ProductId = "1"});

        modifiedCatalogItems.Add(new ExternalIdInformation {ProductId = "2"});

        modifiedCatalogItems.Add(new ExternalIdInformation {ProductId = ""});

        changedInventoryItems.Add(new ExternalIdInformation {ProductId = "1"});

        changedInventoryItems.Add(new ExternalIdInformation {ProductId = "2"});

        changedInventoryItems.Add(new ExternalIdInformation {ProductId = "3"});

        var results = CombineLists(modifiedCatalogItems, changedInventoryItems);

        foreach(var result in results)

        {

            Console.WriteLine(result.ProductId == "" ? "Emtpty" : result.ProductId);

        }

    }

    private static List<ExternalIdInformation> CombineLists(List<ExternalIdInformation> modifiedCatalogItems, List<ExternalIdInformation> changedInventoryItems)

        {

            IEnumerable<ExternalIdInformation> modifiedNonProducts = modifiedCatalogItems

                .Where(p => string.IsNullOrEmpty(p.ProductId));

            IEnumerable<ExternalIdInformation> modifiedProducts = modifiedCatalogItems

                .Where(p => !string.IsNullOrEmpty(p.ProductId))

                .Union(changedInventoryItems)

                .DistinctBy(p => p.ProductId);

            return modifiedNonProducts.Union(modifiedProducts).ToList();

        }

}

​

​