SqlDataReader生成动态Lambda表达式

上一扁使用动态lambda表达式来将DataTable转换成实体，比直接用反射快了不少。主要是首行转换的时候动态生成了委托。

后面的转换都是直接调用委托，省去了多次用反射带来的性能损失。

今天在对SqlServer返回的流对象 SqlDataReader 进行处理，也采用动态生成Lambda表达式的方式转换实体。

先上一版代码

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Common;
using System.Data.SqlClient;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
using System.Text;
using System.Threading.Tasks;

namespace Demo1
{
    public static class EntityConverter
    {
        #region
        /// <summary>
        /// DataTable生成实体
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="dataTable"></param>
        /// <returns></returns>
        public static List<T> ToList<T>(this DataTable dataTable) where T : class, new()
        {
            if (dataTable == null || dataTable.Rows.Count <= 0) throw new ArgumentNullException("dataTable", "当前对象为null无法生成表达式树");
            Func<DataRow, T> func = dataTable.Rows[0].ToExpression<T>();
            List<T> collection = new List<T>(dataTable.Rows.Count);
            foreach (DataRow dr in dataTable.Rows)
            {
                collection.Add(func(dr));
            }
            return collection;
        }

      
        /// <summary>
        /// 生成表达式
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="dataRow"></param>
        /// <returns></returns>
        public static Func<DataRow, T> ToExpression<T>(this DataRow dataRow) where T : class, new()
        {
            if (dataRow == null) throw new ArgumentNullException("dataRow", "当前对象为null 无法转换成实体");
            ParameterExpression parameter = Expression.Parameter(typeof(DataRow), "dr");
            List<MemberBinding> binds = new List<MemberBinding>();
            for (int i = 0; i < dataRow.ItemArray.Length; i++)
            {
                String colName = dataRow.Table.Columns[i].ColumnName;
                PropertyInfo pInfo = typeof(T).GetProperty(colName);
                if (pInfo == null || !pInfo.CanWrite) continue;
                MethodInfo mInfo = typeof(DataRowExtensions).GetMethod("Field", new Type[] { typeof(DataRow), typeof(String) }).MakeGenericMethod(pInfo.PropertyType);
                MethodCallExpression call = Expression.Call(mInfo, parameter, Expression.Constant(colName, typeof(String)));
                MemberAssignment bind = Expression.Bind(pInfo, call);
                binds.Add(bind);
            }
            MemberInitExpression init = Expression.MemberInit(Expression.New(typeof(T)), binds.ToArray());
            return Expression.Lambda<Func<DataRow, T>>(init, parameter).Compile();
        }
        #endregion
        /// <summary>
        /// 生成lambda表达式
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="reader"></param>
        /// <returns></returns>
        public static Func<SqlDataReader, T> ToExpression<T>(this SqlDataReader reader) where T : class, new()
        {
            if (reader == null || reader.IsClosed || !reader.HasRows) throw new ArgumentException("reader", "当前对象无效");
            ParameterExpression parameter = Expression.Parameter(typeof(SqlDataReader), "reader");
            List<MemberBinding> binds = new List<MemberBinding>();
            for (int i = 0; i < reader.FieldCount; i++)
            {
                String colName = reader.GetName(i);
                PropertyInfo pInfo = typeof(T).GetProperty(colName);
                if (pInfo == null || !pInfo.CanWrite) continue;
                MethodInfo mInfo = reader.GetType().GetMethod("GetFieldValue").MakeGenericMethod(pInfo.PropertyType);
                MethodCallExpression call = Expression.Call(parameter, mInfo, Expression.Constant(i));
                MemberAssignment bind = Expression.Bind(pInfo, call);
                binds.Add(bind);
            }
            MemberInitExpression init = Expression.MemberInit(Expression.New(typeof(T)), binds.ToArray());
            return Expression.Lambda<Func<SqlDataReader, T>>(init, parameter).Compile();
        }

    }
}

在上一篇的基础上增加了 SqlDataReader 的扩展方法

以下代码是调用

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Common;
using System.Data.SqlClient;
using System.Diagnostics;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Threading.Tasks;

namespace Demo1
{
    class Program
    {
        static void Main(string[] args)
        {
            String conString = "Data Source=.; Initial Catalog=master; Integrated Security=true;";
            Func<SqlDataReader, Usr> func = null;
            List<Usr> usrs = new List<Usr>();
            using (SqlDataReader reader = GetReader(conString, "select object_id 'ID',name 'Name' from sys.objects", CommandType.Text, null))
            {
                while (reader.Read())
                {
                    if (func == null)
                    {
                        func = reader.ToExpression<Usr>();
                    }
                    Usr usr = func(reader);
                    usrs.Add(usr);
                }
            }
            usrs.Clear();
            Console.ReadKey();
        }

        public static SqlDataReader GetReader(String conString, String sql, CommandType type, params SqlParameter[] pms)
        {
            SqlConnection conn = new SqlConnection(conString);
            SqlCommand cmd = new SqlCommand(sql, conn);
            cmd.CommandType = type;
            if (pms != null && pms.Count() > 0)
            {
                cmd.Parameters.AddRange(pms);
            }
            conn.Open();
            return cmd.ExecuteReader(CommandBehavior.CloseConnection);
        }
    }
    class Usr
    {
        public Int32 ID { get; set; }
        public String Name { get; set; }
    }


}

目前只能处理sqlserver返回的对象，处理其它数据库本来是想增加 DbDataReader 的扩展方法，但发现动态生成lambda表达式的地方出错，所以先将现在的

方案记录下来。

文中如果有不清楚或者有问题的地方请园友指出。谢谢。

原创作品，转载请注明出处：http://www.cnblogs.com/lclblog/p/6711292.html