Detailed explanation of examples of dynamic Lambda expressions generated by SqlDataReader in ASP.NET

This article mainly introduces the dynamic Lambda expression generated by SqlDataReader. Friends who need it can refer to it

The previous article uses dynamic lambda expression to convert DataTable into entities. It is much faster than using reflection directly. few. The main reason is that the commission is dynamically generated when the first line is converted.

The subsequent conversions all call the delegate directly, eliminating the performance loss caused by multiple reflections.

Today we are processing the stream object SqlDataReader returned by SqlServer, and also converting entities by dynamically generating Lambda expressions.

First version of the code

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();
  }
 }
}

Based on the previous article, the extension method of SqlDataReader is added

The following code is called

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 &#39;ID&#39;,name &#39;Name&#39; 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; }
 }
}

Currently only Ability to process objects returned by sqlserver and other databases. Originally, I wanted to add the extension method of DbDataReader, but I found an error in dynamically generating lambda expressions, so I recorded the current

solution first.

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