一步步实现本身的ORM（三）

时间 2019-11-17

标签步步实现本身 orm 栏目 Java 繁體版

原文原文链接

章节列表：html

经过前面两篇文章，咱们大体了解了ORM的基本原理，是经过Attribute+反射获取表的基本信息，再用表名和字段名拼接成SQL语句。而前面咱们只是完成了CRUD部分的CUD，还没完成读取(Retrieve)操做，今天就来完成这个R操做。咱们先看下平时写实体类转换代码是怎么写的：优化

        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;
        }

常常写DataReader对象转换成实体类的童鞋们，WHILE循环里的内容是根据实体类的不一样代码而有所不一样，循环外的代码则是固定不变的，如今咱们要把变化的代码抽离出来，咱们仍是请出明星--反射。ui

在前面章节咱们都是用反射读取实体类里的，好比typeof(User).GetProperty("UserId").GetValue(obj,null)，有GetValue确定也有SetValue，咱们看下SetValue的定义：spa

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

obj：将设置其属性值的对象。3d

value：新的属性值。code

index : 索引化属性的可选索引值。对于非索引化属性，该值应为 null。 xml

示例代码：htm

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

输出结果我就不贴图了，是“10”。

HOHO，咱们开始构建代码，步骤以下：

　　1. new 实体类对象

　　2. 反射类的属性，而后用SetValue给实体类属性赋值

具体代码以下：

            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) //等同于 if (reader["UserId"] != null)这样的语句
                        pi.SetValue(user, reader[pi.Name], null); //等同于 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

运行结果：

下面一个问题来了，咱们看这一行代码

User user = new User();

难道咱们每次都要这么写这样的代码吗，能不能作一个通用的吗？这须要咱们用反射实例化一个类，咱们请Activator.CreateInstance帮忙建立实例，代码以下：

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

建立实例和给属性赋值都动态完成，我稍稍修改下代码，只要传递SQL语句就能够返回集合。

        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) //等同于 if (reader["UserId"] != null)这样的语句
                        pi.SetValue(user, reader[pi.Name], null); //等同于 user.UserId = (int)reader["UserId"];
                }

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

View Code

最后咱们在增长以下方法：

Get(object[] values) 根据主键获取实体类

GetList<T>(string where,string orderBy) 按条件查询实体类

        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;
            }

            /*将全部的列放到集合里*/
            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("参数个数和主键数不一致");

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

            for (int i = 0; i < columns.Count; i++)
            {
                if (i > 0) //考虑到有多个主键
                    sql.Append(" AND ");

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

                /*参数*/
                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) //等同于 if (reader["UserId"] != null)这样的语句
                        pi.SetValue(user, reader[pi.Name], null); //等同于 user.UserId = (int)reader["UserId"];
                }

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

View Code

下载代码：http://files.cnblogs.com/files/sobaby/ORM03.zip

EntityHelper里重复代码不少，并且采用反射效率很低，下一章就开始优化代码。