C#字典哈希集

Question

我有这个查询，我必须删除ORDER BY p.ID

 string query = "SELECT p.Price, p.ID, p.Name, c.Name as CategoryName, p.CategoryID, c.ID AS CategoryPK, c.ParentID,"
            + " o.ID as OrderID,o.ProductID,o.ClientCompanyID,o.Quentity"
            + " FROM Products p LEFT JOIN Categories c ON p.CategoryID=c.ID LEFT JOIN Orders o ON o.ProductID=p.ID ORDER BY p.ID";

我有这样的代码，我应该删除if子句，在那里我必须比较数据库中的最后一个元素和当前元素。

        static List<Product> GetProducts(SqlCommand command)
    {
        using (SqlDataReader reader = command.ExecuteReader())
        {
            List<Product> listProducts = new List<Product>();
            while (reader.Read())
            {
                Product product = null;
                int productId = (int)reader["ID"];
                if (listProducts.Count >= 1 && listProducts[listProducts.Count - 1].ID == productId)
                {
                    product = listProducts[listProducts.Count - 1];
                }
                if (product == null)
                {
                    product = new Product();
                    product.ID = productId;
                    product.Name = (!reader.IsDBNull(reader.GetOrdinal("Name")) ? (string)reader["Name"] : null);
                    product.Price = reader.GetDecimal(reader.GetOrdinal("Price"));
                    product.CategoryID = reader.GetInt32(reader.GetOrdinal("CategoryID"));
                    Category newCat = new Category();
                    newCat.ID = reader.GetInt32(reader.GetOrdinal("CategoryPK"));
                    newCat.CategoryName = reader.GetString(reader.GetOrdinal("CategoryName"));
                    newCat.Parent = (!reader.IsDBNull(reader.GetOrdinal("ParentID")) ? reader.GetInt32(reader.GetOrdinal("ParentID")) : 0);
                    product.Category = newCat;
                }
                Order order = new Order();
                order.ID = (!reader.IsDBNull(reader.GetOrdinal("OrderID")) ? (int?)reader["OrderID"] : null);
                order.ClientCompanyID = (!reader.IsDBNull(reader.GetOrdinal("ClientCompanyID")) ? (int?)reader["ClientCompanyID"] : null);
                order.ProductID = (!reader.IsDBNull(reader.GetOrdinal("ProductID")) ? (int?)reader["ProductID"] : null);
                order.Quentity = (!reader.IsDBNull(reader.GetOrdinal("Quentity")) ? (int?)reader["Quentity"] : null);
                product.Orders.Add(order);
                listProducts.Add(product);
            }
            reader.Close();
            listProducts = listProducts.Distinct().ToList();
            return listProducts;
        }
    }

我必须删除查询中的Order By，然后使用方法中的Dictionary从数据库中获取这些数据，但我对Dictionary是新手。有人能帮我怎么做吗？谢谢！

Answer 1

如果您只需要返回Dictionary<int，Product>其中key是产品ID，然后将您的返回修改为：

   return listProducts.GroupBy(x => x.ID).
   ToDictionary(y => y.Key, z => z.ToList());