C# ORM学习笔记：使用特性+反射实现简单ORM

    一、原理与环境

    在生成数据表的实体类时，利用自定义特性，给它打上表及字段的特性，然后使用反射原理，将自定义特性拼接成增、删、改、查对应的SQL，即可完成一个简单的ORM。

    本示例的执行环境：

    1）数据库：SQL Server。（可根据自己的需要，建立不同的数据库工厂。）

    2）数据表：需使用自增类型(identity)作为数据表的主键。主键名字可以随便起，如ID。

    3）实体类：实体类需提供无参构造函数。

    二、演示数据表

    Person表，包含主键（ID）、姓名（Name）、年龄（Age）、性别（Gender）。

CREATE TABLE [dbo].[Person](
    [ID] [BIGINT] IDENTITY(1,1) NOT NULL,
    [Name] [NVARCHAR](50) NULL,
    [Age] [INT] NULL,
    [Gender] [NVARCHAR](10) NULL,
 CONSTRAINT [PK_Person] PRIMARY KEY CLUSTERED 
(
    [ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

    二、自定义特性

    定义两个自定义特性：

    2.1、DataTableAttribute

    此为数据表特性，包含表名（TableName）、主键（Key）。

    [Serializable]
    public class DataTableAttribute : Attribute
    {
        /// <summary>
        /// 数据表
        /// </summary>
        public string TableName { get; }

        /// <summary>
        /// 主键
        /// </summary>
        public string Key { get; }

        /// <summary>
        /// 构造函数
        /// </summary>
        /// <param name="tableName"></param>
        /// <param name="key"></param>
        public DataTableAttribute(string tableName, string key)
        {
            TableName = tableName;
            Key = key;
        }
    }

    2.2、DataFieldAttribute

    此为字段特性，包含字段名（FieldName）、字段类型（FieldType）、长度（Length）、是否自增（IsIdentity）。

    [Serializable]
    public class DataFieldAttribute : Attribute
    {
        public string FieldName { get; set; }
        public string FieldType { get; set; }
        public int Length { get; set; }
        public bool IsIdentity { get; set; }

        /// <summary>
        /// 构造函数
        /// </summary>
        /// <param name="fieldName">字段名</param>
        /// <param name="fieldType">字段类型</param>
        /// <param name="length">长度</param>
        /// <param name="isIdentity">是否自增长</param>
        public DataFieldAttribute(string fieldName, string fieldType, int length, bool isIdentity)
        {
            FieldName = fieldName;
            FieldType = fieldType;
            Length = length;
            IsIdentity = isIdentity;
        }

        /// <summary>
        /// 构造函数
        /// </summary>
        /// <param name="fieldName"></param>
        /// <param name="length"></param>
        /// <param name="isIdentity"></param>
        public DataFieldAttribute(string fieldName, string fieldType, int length) : this(fieldName, fieldType, length, false)
        { }

        /// <summary>
        /// 构造函数
        /// </summary>
        /// <param name="fieldName"></param>
        /// <param name="fieldtype"></param>
        public DataFieldAttribute(string fieldName, string fieldType) : this(fieldName, fieldType, 0, false)
        { }

        /// <summary>
        /// 构造函数
        /// </summary>
        /// <param name="fieldName"></param>
        /// <param name="isIdentity"></param>
        public DataFieldAttribute(string fieldName, bool isIdentity) : this(fieldName, "", 0, isIdentity)
        { }

        /// <summary>
        /// 构造函数
        /// </summary>
        /// <param name="fieldName"></param>
        public DataFieldAttribute(string fieldName) : this(fieldName, false)
        { }
    }

    三、生成实体类

    3.1、实体类样式

    依照前面的规划，Person表需要生成下面这个样子：

using System;
using System.Collections.Generic;
using System.Text;
using LinkTo.ORM.CustomAttribute;

namespace LinkTo.ORM.Model
{
    [DataTable("Person","ID")]
    [Serializable]
    public class Person
    {
        public Person() { }

        [DataField("ID","bigint",19,true)]
        public long? ID {get; set;}

        [DataField("Name","nvarchar",50,false)]
        public string Name {get; set;}

        [DataField("Age","int",10,false)]
        public int? Age {get; set;}

        [DataField("Gender","nvarchar",10,false)]
        public string Gender {get; set;}
    }
}

    3.2、使用T4模板生成实体类

    3.2.1、T4Code文件夹的文本模板

<#@ assembly name="System.Core" #>
<#@ assembly name="System.Data" #>
<#@ import namespace="System.Linq" #>
<#@ import namespace="System.Text" #>
<#@ import namespace="System.Collections.Generic" #>
<#@ import namespace="System.Data"#>
<#@ import namespace="System.Data.SqlClient"#>
<#+
    #region T4Code
    /// <summary>
    /// 数据库架构接口
    /// </summary>
    public interface IDBSchema : IDisposable
    {
        List<string> GetTableList();
        DataTable GetTableMetadata(string tableName);
    }

    /// <summary>
    /// 数据库架构工厂
    /// </summary>
    public class DBSchemaFactory
    {
        static readonly string DatabaseType = "SqlServer";
        public static IDBSchema GetDBSchema()
        {
            IDBSchema dbSchema;
            switch (DatabaseType) 
            {
                case "SqlServer":
                    {
                        dbSchema =new SqlServerSchema();
                        break;
                    }
                default: 
                    {
                        throw new ArgumentException("The input argument of DatabaseType is invalid.");
                    }
            }
            return dbSchema;
        }
    }

    /// <summary>
    /// SqlServer
    /// </summary>
    public class SqlServerSchema : IDBSchema
    {
        public string ConnectionString = "Server=.;Database=Test;Uid=sa;Pwd=********;";
        public SqlConnection conn;

        public SqlServerSchema()
        {
            conn = new SqlConnection(ConnectionString);
            conn.Open();
        }

        public List<string> GetTableList()
        {
            List<string> list = new List<string>();
            string commandText = "SELECT NAME TABLE_NAME FROM SYSOBJECTS WHERE XTYPE='U' ORDER BY NAME";

            using(SqlCommand cmd = new SqlCommand(commandText, conn))
            {
                using (SqlDataReader dr = cmd.ExecuteReader(CommandBehavior.CloseConnection))
                {
                    while (dr.Read())
                    {
                        list.Add(dr["TABLE_NAME"].ToString());
                    }
                }
            }

            return list;
        }
        
        public DataTable GetTableMetadata(string tableName)
        {
            string commandText=string.Format
                (
                    "SELECT A.NAME TABLE_NAME,B.NAME FIELD_NAME,C.NAME DATATYPE,ISNULL(B.PREC,0) LENGTH, "+
                        "CONVERT(BIT,CASE WHEN NOT F.ID IS NULL THEN 1 ELSE 0 END) ISKEY, "+
                        "CONVERT(BIT,CASE WHEN COLUMNPROPERTY(B.ID,B.NAME,'ISIDENTITY') = 1 THEN 1 ELSE 0 END) AS ISIDENTITY, "+
                        "CONVERT(BIT,B.ISNULLABLE) ISNULLABLE "+
                    "FROM SYSOBJECTS A INNER JOIN SYSCOLUMNS B ON A.ID=B.ID INNER JOIN SYSTYPES C ON B.XTYPE=C.XUSERTYPE "+
                        "LEFT JOIN SYSOBJECTS D ON B.ID=D.PARENT_OBJ AND D.XTYPE='PK' "+
                        "LEFT JOIN SYSINDEXES E ON B.ID=E.ID AND D.NAME=E.NAME "+
                        "LEFT JOIN SYSINDEXKEYS F ON B.ID=F.ID AND B.COLID=F.COLID AND E.INDID=F.INDID "+
                    "WHERE A.XTYPE='U' AND A.NAME='{0}' "+
                    "ORDER BY A.NAME,B.COLORDER", tableName
                );

            using(SqlCommand cmd = new SqlCommand(commandText, conn))
            {
                SqlDataAdapter da = new SqlDataAdapter(cmd);
                DataSet ds = new DataSet();
                da.Fill(ds,"Schema");
                return ds.Tables[0];
            }
        }

        public void Dispose()
        {
            if (conn != null)
            {
                conn.Close();
            }
        }
    }
    #endregion
#>

DBSchema.ttinclude