SqlDataReader怎样生成动态Lambda表达式

这篇文章主要介绍SqlDataReader怎样生成动态Lambda表达式，文中介绍的非常详细，具有一定的参考价值，感兴趣的小伙伴们一定要看完！

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  /// 
  /// DataTable生成实体  /// 
  ///   ///   ///   public static List ToList(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 func = dataTable.Rows[0].ToExpression();   List collection = new List(dataTable.Rows.Count);   foreach (DataRow dr in dataTable.Rows)   {    collection.Add(func(dr));   }   return collection;  }  /// 
  /// 生成表达式  /// 
  ///   ///   ///   public static Func ToExpression(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 binds = new List();   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>(init, parameter).Compile();  }  #endregion  /// 
  /// 生成lambda表达式  /// 
  ///   ///   ///   public static Func ToExpression(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 binds = new List();   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>(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 func = null;   List usrs = new List();   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 = 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; } }}

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