LinqProductDifferences

using System;
using System.Collections.Generic;
using System.Linq;

namespace Client
{
    public class ProductInfo
    {
        public string ProductId { get; set; }

        public string ProductName { get; set; }

        public decimal Price { get; set; }
    }

    public class ProductIdPriceEqualityComparer : IEqualityComparer<ProductInfo>
    {
        public bool Equals(ProductInfo x, ProductInfo y)
        {
            if (ReferenceEquals(x, y)) return true;
            if (ReferenceEquals(x, null)) return false;
            if (ReferenceEquals(y, null)) return false;
            if (x.GetType() != y.GetType()) return false;
            return String.Equals(x.ProductId, y.ProductId) && x.Price == y.Price;
        }

        public int GetHashCode(ProductInfo obj)
        {
            unchecked
            {
                return ((obj.ProductId != null ? obj.ProductId.GetHashCode() : 0)*397) ^ obj.Price.GetHashCode();
            }
        }
    }

    public class ProductIdProductNameEqualityComparer : IEqualityComparer<ProductInfo>
    {
        public bool Equals(ProductInfo x, ProductInfo y)
        {
            if (ReferenceEquals(x, y)) return true;
            if (ReferenceEquals(x, null)) return false;
            if (ReferenceEquals(y, null)) return false;
            if (x.GetType() != y.GetType()) return false;
            return string.Equals(x.ProductId, y.ProductId) && string.Equals(x.ProductName, y.ProductName);
        }

        public int GetHashCode(ProductInfo obj)
        {
            unchecked
            {
                return ((obj.ProductId != null ? obj.ProductId.GetHashCode() : 0) * 397) ^ ((obj.ProductId != null ? obj.ProductName.GetHashCode() : 0));
            }
        }
    }

    public class ProductIdEqualityComparer : IEqualityComparer<ProductInfo>
    {

        public bool Equals(ProductInfo x, ProductInfo y)
        {
            return x.ProductId == y.ProductId;
        }

        public int GetHashCode(ProductInfo obj)
        {
            return obj.ProductId.GetHashCode();
        }
    }

    public class ProductNameChangeInfo
    {
        public string OldProductName { get; set; }
        public string NewProductName { get; set; }
        public string ProductId { get; set; }
    }

    public class PriceChangeInfo
    {
        public decimal OldPrice { get; set; }

        public decimal NewPrice { get; set; }

        public string ProductName { get; set; }

        public string ProductId { get; set; }

        public decimal PriceChange { get { return NewPrice - OldPrice; } }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var oldProducts = new List<ProductInfo>
            {
                new ProductInfo { ProductId = "1", ProductName = "Marathon", Price = 0.1m },
                new ProductInfo { ProductId = "2", ProductName = "Opal Fruits", Price = 0.15m},
                new ProductInfo { ProductId = "3", ProductName = "Walkers Ready Salted", Price = 0.12m},
                new ProductInfo { ProductId = "5", ProductName = "New Coca Cola", Price = 0.15m}
            };

            var newProducts = new List<ProductInfo>
            {
                new ProductInfo { ProductId = "1", ProductName = "Snickers", Price = 0.5m },
                new ProductInfo { ProductId = "2", ProductName = "Starburst", Price = 0.40m},
                new ProductInfo { ProductId = "3", ProductName = "Walkers Ready Salted", Price = 0.40m},
                new ProductInfo { ProductId = "4", ProductName = "Sour Skittles", Price = 0.55m}
            };

            var addedProducts = newProducts.Except(oldProducts, new ProductIdEqualityComparer()).ToList();
            var droppedProducts = oldProducts.Except(newProducts, new ProductIdEqualityComparer()).ToList();
            var productsWithDifferentName = newProducts.Except(oldProducts, new ProductIdProductNameEqualityComparer()).ToList();

            var productsNameChanges = productsWithDifferentName.Join(oldProducts,
                                                        op => op.ProductId,
                                                        np => np.ProductId,
                                                        (np, op) => new ProductNameChangeInfo
                                                        {
                                                            NewProductName = np.ProductName,
                                                            OldProductName = op.ProductName,
                                                            ProductId = op.ProductId
                                                        }).ToList();

            var productsWithDifferentPrices = newProducts.Except(oldProducts, new ProductIdPriceEqualityComparer()).ToList();
            
            var priceChanges = productsWithDifferentPrices.Join(oldProducts,
                np => np.ProductId,
                op => op.ProductId,
                (np, op) => new PriceChangeInfo
                {
                    NewPrice = np.Price,
                    OldPrice = op.Price,
                    ProductId = np.ProductId,
                    ProductName = np.ProductName
                }).ToList();

        }
    }
}