using System;

using System.Collections.Generic;

using System.Linq;

​

public class Product {

    public List<Sale> Sales {get;set;}

    public int ID {get;set;}

    public string Name {get;set;}

}

​

public class Sale {

    public int ID {get;set;}

}

public class Program

{

    public static void Main()

    {

        var stuff = new List<Product>{

            new Product{ID = 1, Name = "Prod1", Sales = new List<Sale>(){}},

            new Product{ID = 2, Name = "Prod2", Sales = new List<Sale>(){}},

            new Product{ID = 3, Name = "Prod3", Sales = new List<Sale>(){ new Sale{ID = 1}}},

            new Product{ID = 4, Name = "Prod4", Sales = new List<Sale>(){}},

            new Product{ID = 5, Name = "Prod5", Sales = new List<Sale>(){ new Sale{ID = 2}, new Sale{ID =3}}}

        };

        var result = stuff.Where(p => p.Sales.Count() == 0).ToList();

        result.ForEach(p => Console.WriteLine("ID: " + p.ID + " Sales: " + p.Sales.Count()));// Console.WriteLine(result);

    }

}