SqlDataReader生成动态Lambda表达式

221 阅读 0 评论 146 点赞

我是靠谱客的博主曾经书包，最近开发中收集的这篇文章主要介绍SqlDataReader生成动态Lambda表达式，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

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。