三层架构

一、三层架构分为：BLL层：业务逻辑层，DAL层：数据访问层，UI层：界面   Model不是一层

UI层是调用BLL层的，BLL调用DAL层，每层都必须调用Model层

Model层：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Model
{
   public class ProductInfo
    {
        public string PId { get; set; }
        public string PName { get; set; }
        public decimal Price { get; set; }
        public int Amount { get; set; }
    }
}

DAL层如下：用来存放方法层

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Model;
using System.Data.SqlClient;


namespace DAL
{
    public class ProductInfoService
    {
        //查询所有商品信息
        public static List<ProductInfo> SelectProductsAll()
        {
            List<ProductInfo> pros = new List<ProductInfo>();
            string sql = "select * from ProductInfo";
            SqlDataReader sdr = DBHelper.ExecuteReader(sql);
            while (sdr.Read())
            {
                ProductInfo pro = new ProductInfo();
                pro.PId = sdr["PId"] + "";
                pro.PName = sdr["PName"] + "";
                pro.Price = Convert.ToDecimal(sdr["Price"]);
                pro.Amount = (int)sdr["Amount"];
                pros.Add(pro);
            }
            sdr.Close();
            DBHelper.CloseCon();
            
            return pros;
        }
        //根据商品ID查询商品信息
        public static List<ProductInfo> SelectProductsByPId(string pId)
        {
            List<ProductInfo> pros = new List<ProductInfo>();
            string sql = "select * from ProductInfo where PId like '%" + pId+"%'";
            SqlDataReader sdr = DBHelper.ExecuteReader(sql);
            while (sdr.Read())
            {
                ProductInfo pro = new ProductInfo();
                pro.PId = sdr["PId"] + "";
                pro.PName = sdr["PName"] + "";
                pro.Price = Convert.ToDecimal(sdr["Price"]);
                pro.Amount = (int)sdr["Amount"];
                pros.Add(pro);
            }
            sdr.Close();
            DBHelper.CloseCon();
            return pros;
        }
        //添加商品
        public static int InsertProduct(ProductInfo pi)
        {
            int n = 0;
            string sql = string.Format("insert ProductInfo values('{0}','{1}','{2}','{3}')", pi.PId, pi.PName, pi.Price, pi.Amount);
            n = DBHelper.ExecuteNonQuery(sql);
            return n;
        }
        //修改商品
        public static int UpdateProduct(ProductInfo pi)
        {
            int n = 0;
            string sql = string.Format("update ProductInfo set PName='{0}',Price='{1}',Amount='{2}' where PId='{3}'", pi.PName, pi.Price, pi.Amount, pi.PId);
            n = DBHelper.ExecuteNonQuery(sql);
            return n;
        }
        //删除商品
        public static int DeleteProduct(string pid)
        {
            int n = 0;
            string sql = "Delete  from ProductInfo where PId='" + pid+"'";
            n = DBHelper.ExecuteNonQuery(sql);
            return n;
        }

      
    }
}

BLL层如下：相当于桥梁

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using DAL;
using Model;
using System.Data.SqlClient;


namespace BLL
{
    public class ProductManager
    {
        //查询所有商品信息
        public static List<ProductInfo> SelectProductsAll()
        {
            return ProductInfoService.SelectProductsAll();
        }

        //根据商品ID查询商品信息
        public static List<ProductInfo> SelectProductsByPId(string pId)
        {
            return ProductInfoService.SelectProductsByPId(pId);
        }

        //删除商品
        public static int DeleteProduct(string pid)
        {
            return ProductInfoService.DeleteProduct(pid);
        }

        public static int InsertProduct(ProductInfo pi)
        {
            return ProductInfoService.InsertProduct(pi);
        }

        //修改商品
        public static int UpdateProduct(ProductInfo pi)
        {
            return ProductInfoService.UpdateProduct(pi);
        }

        
    }
}

UI层直接调用即可。