c#3.0新特性使用lambda表达式创建表达式树
2009年8月18日 云飞扬 发表评论 阅读评论
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c#3.0新特性使用lambda表达式创建表达式树
题外话：这空间访问速度真慢,在我发布文章的时候，感觉花费九牛二虎之力。几篇已经准备好的文章花费1个多小时发布。
Using Lambda Expressions to Create Expression Trees 使用lambda表达式创建表达式树
1.使用前引入类
using System.Linq.Expressions;
2.加入main()
Expression<Func<int, bool>> filter = n => (n * 3) < 5;
BinaryExpression lt = (BinaryExpression)filter.Body;//{((n * 3) < 5)}
BinaryExpression mult = (BinaryExpression)lt.Left;//{(n * 3)}
ParameterExpression en = (ParameterExpression)mult.Left;// {(n)}
ConstantExpression three = (ConstantExpression)mult.Right;//{(3)}
ConstantExpression five = (ConstantExpression)lt.Right;//{(5)}
Console.WriteLine("({0} ({1} {2} {3}) {4})", lt.NodeType,
mult.NodeType, en.Name, three.Value, five.Value);//<LessThan <Multiply n 3>>
Console.ReadKey();
完整的程序
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Linq.Expressions;
namespace NewLanguageFeatures
{
public delegate bool KeyValueFilter<K, V>(K key, V value);
public static class Extensions
{
public static List<K> FilterBy<K, V>(
this Dictionary<K, V> items,
KeyValueFilter<K, V> filter)
{
var result = new List<K>();
foreach (KeyValuePair<K, V> element in items)
{
if (filter(element.Key, element.Value))
result.Add(element.Key);
}
return result;
}
public static List<T> Append<T>(this List<T> a, List<T> b)
{
var newList = new List<T>(a);
newList.AddRange(b);
return newList;
}
public static bool Compare(this Customer customer1, Customer customer2)
{
if (customer1.CustomerId == customer2.CustomerId &&
customer1.Name == customer2.Name &&
customer1.City == customer2.City)
{
return true;
}
return false;
}
}
public class Customer
{
public int CustomerId { get; private set; }
public string Name { get; set; }
public string City { get; set; }
public Customer(int Id)
{
CustomerId = Id;
}
public override string ToString()
{
return Name + "/t" + City + "/t" + CustomerId;
}
}
class Program
{
static void Main(string[] args)
{
Expression<Func<int, bool>> filter = n => (n * 3) < 5;
BinaryExpression lt = (BinaryExpression)filter.Body;//{((n * 3) < 5)}
BinaryExpression mult = (BinaryExpression)lt.Left;//{(n * 3)}
ParameterExpression en = (ParameterExpression)mult.Left;// {(n)}
ConstantExpression three = (ConstantExpression)mult.Right;//{(3)}
ConstantExpression five = (ConstantExpression)lt.Right;//{(5)}
Console.WriteLine("({0} ({1} {2} {3}) {4})", lt.NodeType,
mult.NodeType, en.Name, three.Value, five.Value);//<LessThan <Multiply n 3>>
Console.ReadKey();
//去掉注释的可以调试一下
////------2----------------------------------------
//Func<int, int> addOne = n => n + 1;
//Console.WriteLine("Result: {0}", addOne(5));
////---------------------------------------3-------------
//Func<int, int> addOne = n => n + 1;
//Console.WriteLine("Result: {0}", addOne(5));
//Expression<Func<int, int>> addOneExpression = n => n + 1;
//var addOneFunc = addOneExpression.Compile();
//Console.WriteLine("Result: {0}", addOneFunc(5));
}
public static List<Customer> FindCustomersByCity(
List<Customer> customers,
string city)
{
return customers.FindAll(c => c.City == city);
}
static List<Customer> CreateCustomers()
{
return new List<Customer>
{
new Customer(1) { Name = "Alex Roland", City = "Berlin" },
new Customer(2) { Name = "Oliver Cox", City = "Marseille" },
new Customer(3) { Name = "Maurice Taylor", City = "London" },
new Customer(4) { Name = "Phil Gibbins", City = "London" },
new Customer(5) { Name = "Tony Madigan", City = "Torino" },
new Customer(6) { Name = "Elizabeth A. Andersen", City = "Portland" },
new Customer(7) { Name = "Justin Thorp", City = "London" },
new Customer(8) { Name = "Bryn Paul Dunton", City = "Portland" }
};
}
}
}
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