SqlDataReader生成动态Lambda表达式

Posted 2020-09-10 ssms

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

 1 using System;
 2 using System.Collections.Generic;
 3 using System.Data;
 4 using System.Data.Common;
 5 using System.Data.SqlClient;
 6 using System.Linq;
 7 using System.Linq.Expressions;
 8 using System.Reflection;
 9 using System.Text;
10 using System.Threading.Tasks;
11 
12 namespace Demo1
13 {
14     public static class EntityConverter
15     {
16         #region
17         /// <summary>
18         /// DataTable生成实体
19         /// </summary>
20         /// <typeparam name="T"></typeparam>
21         /// <param name="dataTable"></param>
22         /// <returns></returns>
23         public static List<T> ToList<T>(this DataTable dataTable) where T : class, new()
24         {
25             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");
26             Func<DataRow, T> func = dataTable.Rows[0].ToExpression<T>();
27             List<T> collection = new List<T>(dataTable.Rows.Count);
28             foreach (DataRow dr in dataTable.Rows)
29             {
30                 collection.Add(func(dr));
31             }
32             return collection;
33         }
34 
35       
36         /// <summary>
37         /// 生成表达式
38         /// </summary>
39         /// <typeparam name="T"></typeparam>
40         /// <param name="dataRow"></param>
41         /// <returns></returns>
42         public static Func<DataRow, T> ToExpression<T>(this DataRow dataRow) where T : class, new()
43         {
44             if (dataRow == null) throw new ArgumentNullException("dataRow", "当前对象为null 无法转换成实体");
45             ParameterExpression parameter = Expression.Parameter(typeof(DataRow), "dr");
46             List<MemberBinding> binds = new List<MemberBinding>();
47             for (int i = 0; i < dataRow.ItemArray.Length; i++)
48             {
49                 String colName = dataRow.Table.Columns[i].ColumnName;
50                 PropertyInfo pInfo = typeof(T).GetProperty(colName);
51                 if (pInfo == null || !pInfo.CanWrite) continue;
52                 MethodInfo mInfo = typeof(DataRowExtensions).GetMethod("Field", new Type[] { typeof(DataRow), typeof(String) }).MakeGenericMethod(pInfo.PropertyType);
53                 MethodCallExpression call = Expression.Call(mInfo, parameter, Expression.Constant(colName, typeof(String)));
54                 MemberAssignment bind = Expression.Bind(pInfo, call);
55                 binds.Add(bind);
56             }
57             MemberInitExpression init = Expression.MemberInit(Expression.New(typeof(T)), binds.ToArray());
58             return Expression.Lambda<Func<DataRow, T>>(init, parameter).Compile();
59         }
60         #endregion
61         /// <summary>
62         /// 生成lambda表达式
63         /// </summary>
64         /// <typeparam name="T"></typeparam>
65         /// <param name="reader"></param>
66         /// <returns></returns>
67         public static Func<SqlDataReader, T> ToExpression<T>(this SqlDataReader reader) where T : class, new()
68         {
69             if (reader == null || reader.IsClosed || !reader.HasRows) throw new ArgumentException("reader", "当前对象无效");
70             ParameterExpression parameter = Expression.Parameter(typeof(SqlDataReader), "reader");
71             List<MemberBinding> binds = new List<MemberBinding>();
72             for (int i = 0; i < reader.FieldCount; i++)
73             {
74                 String colName = reader.GetName(i);
75                 PropertyInfo pInfo = typeof(T).GetProperty(colName);
76                 if (pInfo == null || !pInfo.CanWrite) continue;
77                 MethodInfo mInfo = reader.GetType().GetMethod("GetFieldValue").MakeGenericMethod(pInfo.PropertyType);
78                 MethodCallExpression call = Expression.Call(parameter, mInfo, Expression.Constant(i));
79                 MemberAssignment bind = Expression.Bind(pInfo, call);
80                 binds.Add(bind);
81             }
82             MemberInitExpression init = Expression.MemberInit(Expression.New(typeof(T)), binds.ToArray());
83             return Expression.Lambda<Func<SqlDataReader, T>>(init, parameter).Compile();
84         }
85 
86     }
87 }

在上一篇的基础上增加了 SqlDataReader 的扩展方法

以下代码是调用

 1 using System;
 2 using System.Collections.Generic;
 3 using System.Data;
 4 using System.Data.Common;
 5 using System.Data.SqlClient;
 6 using System.Diagnostics;
 7 using System.Linq;
 8 using System.Reflection;
 9 using System.Text;
10 using System.Threading.Tasks;
11 
12 namespace Demo1
13 {
14     class Program
15     {
16         static void Main(string[] args)
17         {
18             String conString = "Data Source=.; Initial Catalog=master; Integrated Security=true;";
19             Func<SqlDataReader, Usr> func = null;
20             List<Usr> usrs = new List<Usr>();
21             using (SqlDataReader reader = GetReader(conString, "select object_id \'ID\',name \'Name\' from sys.objects", CommandType.Text, null))
22             {
23                 while (reader.Read())
24                 {
25                     if (func == null)
26                     {
27                         func = reader.ToExpression<Usr>();
28                     }
29                     Usr usr = func(reader);
30                     usrs.Add(usr);
31                 }
32             }
33             usrs.Clear();
34             Console.ReadKey();
35         }
36 
37         public static SqlDataReader GetReader(String conString, String sql, CommandType type, params SqlParameter[] pms)
38         {
39             SqlConnection conn = new SqlConnection(conString);
40             SqlCommand cmd = new SqlCommand(sql, conn);
41             cmd.CommandType = type;
42             if (pms != null && pms.Count() > 0)
43             {
44                 cmd.Parameters.AddRange(pms);
45             }
46             conn.Open();
47             return cmd.ExecuteReader(CommandBehavior.CloseConnection);
48         }
49     }
50     class Usr
51     {
52         public Int32 ID { get; set; }
53         public String Name { get; set; }
54     }
55 
56 
57 }

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