using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        ILookup<Guid, Guid> productMappings = new Dictionary<Guid, Guid>()

        {

            { Guid.Parse("{431C825C-AA92-48EA-B2CD-1B19474FDF35}"), Guid.Parse("{748001F0-BF03-4D58-A67B-B6475BDFC8B6}") },

            { Guid.Parse("{A216D67B-D543-4514-92C8-12BBC7B285D9}"), Guid.Parse("{01238CC3-7020-4AEC-8276-C9FE570D8194}") }

        }.ToLookup(item => item.Key, item => item.Value);

        ICollection<Guid> productIdCollection = new Guid[]

        {

            Guid.Parse("{431C825C-AA92-48EA-B2CD-1B19474FDF35}"),

            Guid.Parse("{A216D67B-D543-4514-92C8-12BBC7B285D9}")

        };

        var productsWithPendingJobs = new HashSet<Guid>()

        {

            Guid.Parse("{A216D67B-D543-4514-92C8-12BBC7B285D9}")

        };

​

        var selectedProductIds = productIdCollection.Where(id => productMappings.Contains(id) && !productsWithPendingJobs.Contains(id));

        Console.WriteLine(String.Join(Environment.NewLine, selectedProductIds));

        Console.WriteLine("New Style");

        var linqSelected = productIdCollection

            .Intersect(productMappings.Select(item => item.Key)) // hier könntest du schauen ob du aus dem ILookup ein Dictionary machst und dann nur auf dictionary.Keys zugreifen musst

            .Except(productsWithPendingJobs);

        Console.WriteLine(String.Join(Environment.NewLine, linqSelected));

    }

}