ToLookUp

相当于ToDictionary只能一对一，ToLookUp可以一对多，所以Dictionary<string,string[]>其实应该用LookUp<string,string>

            using (var context = new KTStoreModel())
            {
                context.Database.Log = Console.WriteLine;

                var products = from product in context.ProductSet select product;
                Console.WriteLine("执行");
                ILookup<int, ProductSet> look = products.ToLookup(key => key.CategoryID);
                foreach (IGrouping<int, ProductSet> group in look)
                {
                    Console.WriteLine(group.Key);
                    foreach (ProductSet product in group)
                    {
                        Console.WriteLine(product.Name);
                    }
                }
            }