一步步實現本身的ORM（三）

時間 2019-11-17

標籤步步實現本身 orm 欄目 Java 简体版

原文原文鏈接

章節列表：html

經過前面兩篇文章，咱們大體瞭解了ORM的基本原理，是經過Attribute+反射獲取表的基本信息，再用表名和字段名拼接成SQL語句。而前面咱們只是完成了CRUD部分的CUD，還沒完成讀取(Retrieve)操做，今天就來完成這個R操做。咱們先看下平時寫實體類轉換代碼是怎麼寫的：優化

        static List<User> ToObject1()
        {
            SqlDataReader reader = null;
            List<User> list = new List<User>();
            while (reader.Read())
            {
                User user = new User();
                if (reader["UserId"] != null)
                    user.UserId = (int)reader["UserId"];

                if (reader["Email"] != null)
                    user.Email = (string)reader["Email"];

                if (reader["CreatedTime"] != null)
                    user.CreatedTime = (DateTime)reader["CreatedTime"];

                list.Add(user);
            }
            return list;
        }

常常寫DataReader對象轉換成實體類的童鞋們，WHILE循環裏的內容是根據實體類的不一樣代碼而有所不一樣，循環外的代碼則是固定不變的，如今咱們要把變化的代碼抽離出來，咱們仍是請出明星--反射。ui

在前面章節咱們都是用反射讀取實體類裏的，好比typeof(User).GetProperty("UserId").GetValue(obj,null)，有GetValue確定也有SetValue，咱們看下SetValue的定義：spa

public virtual void SetValue(
    Object obj,
    Object value,
    Object[] index
)

obj：將設置其屬性值的對象。3d

value：新的屬性值。code

index : 索引化屬性的可選索引值。對於非索引化屬性，該值應爲 null。 xml

示例代碼：htm

            User user = new User();
            typeof(User).GetProperty("UserId").SetValue(user, 10, null);
            Console.WriteLine(user.UserId);

輸出結果我就不貼圖了，是「10」。

HOHO，咱們開始構建代碼，步驟以下：

　　1. new 實體類對象

　　2. 反射類的屬性，而後用SetValue給實體類屬性賦值

具體代碼以下：

            string sql = "select * from tb_users";
            SqlConnection conn = new SqlConnection(EntityHelper.connectionString);
            conn.Open();

           var cmd =  conn.CreateCommand();
           cmd.CommandText = sql;

            SqlDataReader reader = cmd.ExecuteReader();
            List<User> list = new List<User>();
            var type = typeof(User);
            var properties = type.GetProperties();
            while (reader.Read())
            {
                User user = new User();
                for (int i = 0; i < properties.Length; i++)
                {
                    var pi = properties[i];
                    if (reader[pi.Name] != null) //等同於 if (reader["UserId"] != null)這樣的語句
                        pi.SetValue(user, reader[pi.Name], null); //等同於 user.UserId = (int)reader["UserId"];
                }

                list.Add(user);
            }

            foreach (var item in list)
            {
                Console.WriteLine("UserId:{0}，Email：{1}，CreatedTime：{2}", item.UserId, item.Email, item.CreatedTime);
            }

View Code

運行結果：

下面一個問題來了，咱們看這一行代碼

User user = new User();

難道咱們每次都要這麼寫這樣的代碼嗎，能不能作一個通用的嗎？這須要咱們用反射實例化一個類，咱們請Activator.CreateInstance幫忙建立實例，代碼以下：

                var type = typeof(User);
                var user = Activator.CreateInstance(type);

建立實例和給屬性賦值都動態完成，我稍稍修改下代碼，只要傳遞SQL語句就能夠返回集合。

        static List<T> GetEntityList<T>(string sql)
        {
            SqlConnection conn = new SqlConnection(EntityHelper.connectionString);
            conn.Open();

            var cmd = conn.CreateCommand();
            cmd.CommandText = sql;

            SqlDataReader reader = cmd.ExecuteReader();
            List<T> list = new List<T>();
            var type = typeof(T);
            var properties = type.GetProperties();
            while (reader.Read())
            {
                var user = Activator.CreateInstance(type);
                for (int i = 0; i < properties.Length; i++)
                {
                    var pi = properties[i];
                    if (reader[pi.Name] != null) //等同於 if (reader["UserId"] != null)這樣的語句
                        pi.SetValue(user, reader[pi.Name], null); //等同於 user.UserId = (int)reader["UserId"];
                }

                list.Add((T)user);
            }
            return list;
        }

View Code

最後咱們在增長以下方法：

Get(object[] values) 根據主鍵獲取實體類

GetList<T>(string where,string orderBy) 按條件查詢實體類

        public static T Get<T>(object[] values)
        {
            var type = typeof(T);
            Dictionary<string, object> parameters = new Dictionary<string, object>();
            var properties = type.GetProperties();

            string tableName = string.Empty;
            TableAttribute[] tableAttrs = (TableAttribute[])type.GetCustomAttributes(typeof(TableAttribute), true);
            if (tableAttrs.Length > 0)
            {
                tableName = tableAttrs[0].Name;
            }
            else
            {
                tableName = type.Name;
            }

            /*將全部的列放到集合裏*/
            List<IdAttribute> columns = new List<IdAttribute>();
            for (int i = 0; i < properties.Length; i++)
            {
                var pi = properties[i];
                var attrs = (IdAttribute[])pi.GetCustomAttributes(typeof(IdAttribute), true);
                if (attrs.Length > 0)
                {
                    columns.Add(attrs[0]);
                }
            }
            if (columns.Count != values.Length)
                throw new ArgumentException("參數個數和主鍵數不一致");

            StringBuilder sql = new StringBuilder();
            sql.Append("SELECT * FROM [").Append(tableName).Append("] ").Append(" WHERE ");

            for (int i = 0; i < columns.Count; i++)
            {
                if (i > 0) //考慮到有多個主鍵
                    sql.Append(" AND ");

                sql.Append(columns[i].Name).Append("=").Append("@p").Append(i);

                /*參數*/
                parameters.Add("@p" + i, values[i]);
            }
            Console.WriteLine(sql);
            return GetEntityList<T>(sql.ToString(), parameters).FirstOrDefault();
        }
        public static List<T> GetList<T>(string where,string orderBy)
        {
            var type = typeof(T);
            Dictionary<string, object> parameters = new Dictionary<string, object>();
            var properties = type.GetProperties();

            string tableName = string.Empty;
            TableAttribute[] tableAttrs = (TableAttribute[])type.GetCustomAttributes(typeof(TableAttribute), true);
            if (tableAttrs.Length > 0)
            {
                tableName = tableAttrs[0].Name;
            }
            else
            {
                tableName = type.Name;
            }


            StringBuilder sql = new StringBuilder();
            sql.Append("SELECT * FROM [").Append(tableName).Append("] ");
            if (string.IsNullOrEmpty(where))
                sql.Append(" WHERE ").Append(where);

            if (string.IsNullOrEmpty(orderBy))
                sql.Append(" Order By ").Append(orderBy);


            Console.WriteLine(sql);
            return GetEntityList<T>(sql.ToString(), parameters);
        }

        public static List<T> GetEntityList<T>(string sql, Dictionary<string, object> parameters)
        {
            SqlConnection conn = new SqlConnection(EntityHelper.connectionString);
            conn.Open();

            var cmd = conn.CreateCommand();
            cmd.CommandText = sql;
            if (parameters != null && parameters.Count > 0)
            {
                foreach (var item in parameters)
                {
                    var pa = cmd.CreateParameter();
                    pa.ParameterName = item.Key;
                    pa.Value = item.Value ?? DBNull.Value;
                    cmd.Parameters.Add(pa);
                }
            }

            SqlDataReader reader = cmd.ExecuteReader();
            List<T> list = new List<T>();
            var type = typeof(T);
            var properties = type.GetProperties();
            while (reader.Read())
            {
                var user = Activator.CreateInstance(type);
                for (int i = 0; i < properties.Length; i++)
                {
                    var pi = properties[i];
                    if (reader[pi.Name] != null) //等同於 if (reader["UserId"] != null)這樣的語句
                        pi.SetValue(user, reader[pi.Name], null); //等同於 user.UserId = (int)reader["UserId"];
                }

                list.Add((T)user);
            }
            return list;
        }