三層的組成就是DAL,BLL,UI層和Model層。其中的DAL層是與資料庫連結，需要引用Model層，進行對資料的操作，但我們一般在此層不進行資料的處理。BLL層負責引用DAL和MODEL層，在此層對從資料層所得到的資料進行處理，例如，利用ExecuteReader得到的SqlDataReader物件，傳到BLL層，我們就需要在此層對其進行遍歷。UI層負責介面，引用BLL層和MODEL層，將資料展示或者向BLL層傳送使用者操作。這就是三層的各層之間的作用。

 
首先，我們先對DAL曾編寫，先建立一個空的解決方案，向其中加入一個類庫，命名為DAL，現在建立完DAL層，我們就需要對DAL層進行編寫了。那麼需要實現的常見方法有如下幾個（1）查詢所有（2）登陸驗證（3）查詢單個（4）更改資料（5）新增資料

資料庫的兩個表結構如下

usermoney表的useridm是userMes的USERID的外來鍵，查詢不涉及兩表查詢。下邊是DAL查詢的方法，有的地方沒有在BLL處理，查詢時我直接在DAL層將資料處理，返回。

string con = "Data Source=.;Initial Catalog=FirstBig;Integrated Security=True";//宣告連結字串

刪除一條資料，傳入的引數為Id
        public int deleteOne(int id)
        {
            string sql = "delete from userMes where

[email protected]";
            using (SqlConnection conn=new SqlConnection(con))
            {
                using (SqlCommand cmd=new SqlCommand(sql,conn))
                {
                    conn.Open();
                    SqlParameter p1 = new SqlParameter("@userid", id);
                    cmd.Parameters.Add(p1);
                    int res= cmd.ExecuteNonQuery();
                    return res;
                }

            }
        }

//新增一個使用者 引數為使用者名稱和密碼
        public int addone(string username,string userpass)
        {
            string insert = "insert into userMes(username,userpass) values(@username,@userpass)";
            using (SqlConnection conn=new SqlConnection(con))
            {
                using (SqlCommand cmd=new SqlCommand(insert,conn))
                {
                    conn.Open();
                    SqlParameter sql1 = new SqlParameter("@username", username);
                    SqlParameter sql2 = new SqlParameter("@userpass", userpass);
                    cmd.Parameters.Add(sql1);
                    cmd.Parameters.Add(sql2);
                    int res= cmd.ExecuteNonQuery();
                    return res;
                }
            }
        }

//修改一個使用者
        public int changeOnew(userMes li,int id)
        {
            string sql="update userMes set [email protected], [email protected] where [email protected] ";
            using (SqlConnection conn=new SqlConnection(con))
            {
                using (SqlCommand cmd=new SqlCommand(sql,conn))
                {
                    SqlParameter[] sq = new SqlParameter[]
                    {
                        new SqlParameter("@username",System.Data.SqlDbType.NVarChar,50)
                        {
                            Value=li.username
                        },
                        new SqlParameter("@usersex",System.Data.SqlDbType.NVarChar,50)
                        {
                            Value=li.userSex
                        },
                        new SqlParameter("@userid",System.Data.SqlDbType.NVarChar,50)
                        {
                            Value=id
                        }
                    };
                    conn.Open();
                    cmd.Parameters.AddRange(sq);
                   int res= cmd.ExecuteNonQuery();
                   return res;
                }
            }
        
        }

//查詢頁數
        public int selectCount()
        {
            string sql = "select COUNT(*) from userMes";
            using (SqlConnection conn=new SqlConnection(con))
            {
                using (SqlCommand cmd=new SqlCommand(sql,conn))
                {
                    conn.Open();
                   int res= cmd.ExecuteNonQuery();
                   return res;
                }
            }
        }

//查詢所有資料
        public List<userMes> selectAll()
        {
            List<userMes> li = new List<userMes>();
            string sql="select * from userMes";
            using (SqlConnection conn=new SqlConnection(con))
            {
                using (SqlCommand cmd =new SqlCommand(sql,conn))
                {
                    conn.Open();
                   SqlDataReader sqr= cmd.ExecuteReader();
                   userMes u = null;
                   if (sqr.HasRows)
                   {
                       while (sqr.Read())
                       {
                           u=new userMes();
                           u.id =Convert.ToInt32(sqr["userid"]);
                           u.username =sqr["username"].ToString();
                           u.userSex = sqr["userSex"].ToString();
                           li.Add(u);
                       }
                   }   
                   return li;
                }
            }
        }

//這個暫時沒用上，還沒有時間去做分頁

 public List<userMes> selectMes(int pageCount, int pageSize)
        {
            string sql = "select top(@pageSize) * from userMes where userid not in (select top(5*(@pageCount-1)) UserId from userMes order by UserId) order by UserId";
            using (SqlConnection conn=new SqlConnection(con))
            {
                using (SqlCommand cmd=new SqlCommand(sql,conn))
                {
                    conn.Open();
                    SqlParameter sq1 = new SqlParameter("@pageSize", pageSize);
                    SqlParameter sq2 = new SqlParameter("@pageCount", pageCount);
                    cmd.Parameters.Add(sq1);
                    cmd.Parameters.Add(sq2);
                    SqlDataReader sqr= cmd.ExecuteReader();
                    userMes u = null;
                    List<userMes> li = new List<userMes>();
                    if (sqr.HasRows)
                    {
                        while (sqr.Read())
                        {
                            u = new userMes();
                            u.id = Convert.ToInt32(sqr["id"]);
                            u.username = sqr["username"].ToString();
                            u.userSex = sqr["userSex"].ToString();
                            li.Add(u);
                        }
                    }
                    return li;
                }
            }
        }

以上為DAL層中的程式碼，其實可以去建立一個sqlHelper類，呼叫這個類進行操作，下一篇文章會寫這個類的具體程式碼。

Model層的程式碼很簡單，對應資料庫裡的列名去建立就好。這個是對應於第一個表的一個Model

  public class userMes
    {
        public int id { get; set; }
        public string username { get; set; }
        public string userSex { get; set; }
    }