初识三层架构

             复杂项目不能把SQL语句直接写到程序里，不模块化、难以维护，应该采用三层架构。Web开发中的三层架构也是同样的结构。 模型层Model；数据访问层DAL（Data Access Layer）；业务逻辑层BLL（business logic layer ）。实体类就是Model；对数据进行操作的代码写在DAL中，一般就是SQL语句，DAL只有对数据的操作；BLL调用DAL中的代码进行逻辑操作SQL语句一般只应该出现在DAL中。 三层：UI（界面，User Interface）、BLL、DAL。Model是在三层之间进行数据传递的。UI层调用BLL、BLL调用DAL，数据用Model传递，UI不能直接调用DAL。

using System;
using System.Collections.Generic;
using System.Text;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

namespace 三层架构.DAL
{
    class SQLHelper
    {
        public static readonly string conStr = ConfigurationManager.ConnectionStrings["conStr"].ConnectionString;
        /// <summary>
        /// 执行非查询语句
        /// </summary>
        /// <param name="sql">SQL语句</param>
        /// <param name="parameters">传入变量</param>
        /// <returns>影响行数</returns>
        public static int ExecuteNonQuery(string sql, params SqlParameter[] parameters)
        {
            using (SqlConnection con = new SqlConnection(conStr))
            {
                con.Open();
                using (SqlCommand cmd = con.CreateCommand())
                {
                    cmd.CommandText = sql;
                    if (parameters != null)
                    {
                        foreach (SqlParameter param in parameters)
                        {
                            cmd.Parameters.Add(param);
                        }
                    }
                    return cmd.ExecuteNonQuery();
                }
            }
        }
        /// <summary>
        /// 执行非查询语句
        /// </summary>
        /// <param name="sql">SQL语句</param>
        /// <param name="parameters">传入变量</param>
        /// <returns>返回查询结果中的第一行第一列的值</returns>
        public static object ExecuteScalar(string sql, params SqlParameter[] parameters)
        {
            using (SqlConnection con = new SqlConnection(conStr))
            {
                con.Open();
                using (SqlCommand cmd = con.CreateCommand())
                {
                    cmd.CommandText = sql;
                    if (parameters != null)
                    {
                        foreach (SqlParameter param in parameters)
                        {
                            cmd.Parameters.Add(param);
                        }
                    }
                    return cmd.ExecuteScalar();
                }
            }
        }
        public static DataTable ExecuteDataTable(string sql, params SqlParameter[] parameters)
        {
            using (SqlConnection con = new SqlConnection(conStr))
            {
                con.Open();
                using (SqlCommand cmd = con.CreateCommand())
                {
                    cmd.CommandText = sql;
                    if (parameters != null)
                    {
                        foreach (SqlParameter param in parameters)
                        {
                            cmd.Parameters.Add(param);
                        }
                    }
                    DataTable dt = new DataTable();
                    SqlDataAdapter da = new SqlDataAdapter(cmd);
                    da.Fill(dt);
                    return dt;
                }
            }
        }
    }
}

using System;
using 三层架构.Model;
using System.Data.SqlClient;
using System.Data;
using System.Collections.Generic;

namespace 三层架构.DAL
{
    class PersonDAL
    {
        //增加字段
        public static int AddNew(Person model)//返回新添加字段的主键
        {
         object obj= SQLHelper.ExecuteScalar("insert into T_person(Age,Name) output inserted.id values(@Age,@Name)",new SqlParameter("Age",model.Age),new SqlParameter("Name",model.Name));
         return Convert.ToInt32(obj);
        }
        //删除操作
        public static int Delete(int id)
        {
            return SQLHelper.ExecuteNonQuery("delete from T_person where id=@id", new SqlParameter("id", id));
        }
        //更新数据
        public static int UpDate(Person model)
        {
            return SQLHelper.ExecuteNonQuery("update T_Person set age=@age,name=@name where id=@id",new SqlParameter("age",model.Age), new SqlParameter("name",model.Name),new SqlParameter("id",model.Id));
        }   
        //查询数据
        public static Person GetPerson(int id)
        {
         DataTable dt=SQLHelper.ExecuteDataTable("select * from T_Person where id=@id",new SqlParameter("id",id));
         if (dt.Rows.Count <= 0)
         {
             return null;
         }
         else if (dt.Rows.Count ==1)
         {
             DataRow row =dt.Rows[0];
             Person person = new Person();
             person.Id = (int)row["Id"];
             person.Name = (string)row["Name"];
             person.Age =Convert.ToInt32( row["Age"]);
             return person;
         }
         else
         { 
             throw new Exception("未知数据错误！");
         }
       }
        public static IEnumerable<Person> GetAllPerson()
        {
            DataTable dt = SQLHelper.ExecuteDataTable("select * from T_Person");
            List<Person> list = new List<Person>();
            foreach (DataRow row in dt.Rows)
            {
                Person person = new Person();
                person.Id = (int)row["Id"];
                person.Name = (string)row["Name"];
                person.Age = Convert.ToInt32(row["Age"]);
                list.Add(person);
            }
            return list;
        }
    }
}

using 三层架构.Model;
using 三层架构.DAL;
using System;
using System.Collections;
using System.Collections.Generic;

namespace 三层架构.BLL
{
    class PersonBLL
    {
        //增加字段
        public static int AddNew(Person model)//返回新添加字段的主键
        {
            return PersonDAL.AddNew(model);
        }
        //删除操作
        public static int Delete(int id)
        {
            return PersonDAL.Delete(id);
        }
        //更新数据
        public static int UpDate(Person model)
        {
            if (model.Age < 0)
            {
                throw new Exception("年龄不能为负数！");
            }
            return PersonDAL.UpDate(model);
        }
        //查询一条数据
        public static Person GetPerson(int id)
        {
            return PersonDAL.GetPerson(id);
        }
        //查询所有数据
        public static IEnumerable<Person> GetAllPerson()
        {
            return PersonDAL.GetAllPerson();
        }
    }
}

namespace 三层架构.Model
{
    class Person
    {
        public int Id { get; set; }
        public int Age { get; set;}
        public string Name { get; set; }
    }
}