part01.03 委托与 Lambda 表达式（三）：Lambda 表达式
“Lambda 表达式”是一个匿名函数，它可以包含表达式和语句，用于创建委托或表达式树类型
A. 用Lambda表达式代替匿名方法，复杂冗长的形式
格式：( 显式类型参数列表 )=>{ 语句 }
样例：
// 带返回值的委托Func<int, double, double> a = (m, n) => { return m * n; };Console.WriteLine(a);Console.WriteLine(a(10, 25.2));// 不带返回值的委托Action<int> b = x => { Console.WriteLine(x); };b(100);B. 用简单表达式作为主体：用一个表达式表示一个整体，该表达式的值就是Lambda的结果
格式：( 显式的参数列表 )=>表达式
样例：
( string iReason, DateTime iDate ) => iDate == DateTime.Parse("2010-10-10")
( Person xiaozhang )=>xiaozhang.Height
C. 隐式类型的参数列表
格式：( 隐式参数列表 )=>表达式
样例：
( iReason, iDate ) => iDate == DateTime.Parse("2010-10-10")
( xiaozhang )=>xiaozhang.Height
D. 单一参数的快捷语法
格式：参数名 => 表达式
样例：xiaozhang=>xiaozhang.Height
在集合查询中应用 Lambda 表达式
Lambda 表达式应用注意事项简要说明
Lambda 表达式和匿名方法的比较
总体上说，匿名方法可以看做是 Lambda 表达式的功能子集，但是两者存在以下区别：
1. Lambda 表达式的参数允许不指明参数类型，而匿名方法的参数必须明确指明参数类型。
2. Lambda 表达式的方法体允许由单一表达式或者多条语句组成，而匿名方法不允许单一表达式形式
Lambda 表达式树
表达式树的构建使用例子
详细请看相关代码：
1 class Program
2 {
3 /// <summary>
4 /// 依据 IQueryable 集合，数据源构造一个查询，然后执行该查询。 代码将生成一个表达式树来表示以下查询：
5 /// companies.Where(company => (company.ToLower() == "coho winery" || company.Length > 16)).OrderBy(company => company)
6 /// </summary>
7 /// <param name="args"></param>
8 static void Main(string[] args)
9 {
10 string[] companies = { "Consolidated Messenger", "Alpine Ski House", "Southridge Video", "City Power & Light",
11 "Coho Winery", "Wide World Importers", "Graphic Design Institute", "Adventure Works",
12 "Humongous Insurance", "Woodgrove Bank", "Margie's Travel", "Northwind Traders",
13 "Blue Yonder Airlines", "Trey Research", "The Phone Company",
14 "Wingtip Toys", "Lucerne Publishing", "Fourth Coffee" };
15
16 // 转型为 IQueryable<String>
17 IQueryable<String> queryableData = companies.AsQueryable<string>();
18
19 // 组合表达式树作为参数推断的依据
20 ParameterExpression pe = Expression.Parameter(typeof(string), "company");
21
22 #region 构建推断条件：Where(company => (company.ToLower() == "coho winery" || company.Length > 16))
23 // 创建表达式树代表条件 'company.ToLower() == "coho winery"'.
24 Expression left = Expression.Call(pe, typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));
25 Expression right = Expression.Constant("coho winery");
26 Expression e1 = Expression.Equal(left, right);
27
28 // 创建表达式树代表条件 'company.Length > 16'.
29 left = Expression.Property(pe, typeof(string).GetProperty("Length"));
30 right = Expression.Constant(16, typeof(int));
31 Expression e2 = Expression.GreaterThan(left, right);
32
33 // 组合表达式树代表： '(company.ToLower() == "coho winery" || company.Length > 16)'.
34 Expression predicateBody = Expression.OrElse(e1, e2);
35
36 // 创建表达式树代表条件 'queryableData.Where(company => (company.ToLower() == "coho winery" || company.Length > 16))'
37 MethodCallExpression whereCallExpression = Expression.Call(
38 typeof(Queryable),
39 "Where",
40 new Type[] { queryableData.ElementType },
41 queryableData.Expression,
42 Expression.Lambda<Func<string, bool>>(predicateBody, new ParameterExpression[] { pe }));
43 #endregion
44
45 #region 构建排序条件：OrderBy(company => company)
46 // 创建表达式树代表条件 'whereCallExpression.OrderBy(company => company)'
47 MethodCallExpression orderByCallExpression = Expression.Call(
48 typeof(Queryable),
49 "OrderBy",
50 new Type[] { queryableData.ElementType, queryableData.ElementType },
51 whereCallExpression,
52 Expression.Lambda<Func<string, string>>(pe, new ParameterExpression[] { pe }));
53 #endregion
54
55 // 执行处理结果
56 IQueryable<string> results = queryableData.Provider.CreateQuery<string>(orderByCallExpression);
57
58 // 枚举结果
59 foreach (string company in results)
60 Console.WriteLine(company);
61
62 Console.ReadKey();
63 }
64 }View Code
常见的使用委托和 Lambda 表达式的场景与实现
1 public class Person
2 {
3 public Guid ID { get; set; }
4 public string Name { get; set; }
5 public string Description { get; set; }
6 public string SortCode { get; set; }
7 public string FirstName { get; set; }
8 public string LastName { get; set; }
9 public string Mobile { get; set; }
10 public string Email { get; set; }
11
12 public virtual Department Department { get; set; }
13
14 public Person()
15 {
16 this.ID = Guid.NewGuid();
17 this.SortCode = DateTime.Now.ToString("yyyyMMddhhmmssfff");
18 }
19 }
20
21 public class Department
22 {
23 public Guid ID { get; set; }
24 public string Name { get; set; }
25 public string Description { get; set; }
26 public string SortCode { get; set; }
27
28 public virtual Department ParentDepartment { get; set; }
29
30 public Department()
31 {
32 this.ID = Guid.NewGuid();
33 }
34 }基本业务代码
1 public static class PersonRepository
2 {
3 public static List<Department> Departments { get; set; }
4 public static List<Person> People { get; set; }
5
6 static PersonRepository()
7 {
8 _Initializer();
9 }
10
11 public static void _Initializer()
12 {
13
14 #region 初始化部门数据
15
16 Departments = new List<Department>();
17 var dept01 = new Department
18 {
19 Name = "南宁市信息技术有限责任公司",
20 Description = "",
21 SortCode = "01"
22 };
23 dept01.ParentDepartment = dept01;
24 Departments.Add(dept01);
25
26 var dept0101 = new Department
27 {
28 Name = "综合管理部",
29 Description = "负责公司办公、人事、财务日常管理工作。",
30 SortCode = "0101",
31 ParentDepartment = dept01
32 };
33 Departments.Add(dept0101);
34
35 var dept0102 = new Department
36 {
37 Name = "营销部",
38 Description = "负责公司商业项目售前与售后支持工作。",
39 SortCode = "0102",
40 ParentDepartment = dept01
41 };
42 Departments.Add(dept0102);
43
44 var dept0103 = new Department
45 {
46 Name = "技术部",
47 Description = "负责公司商业项目具体实施开发工作",
48 SortCode = "0102",
49 ParentDepartment = dept01
50 };
51 Departments.Add(dept0103);
52
53 #endregion
54
55 #region 初始化人员数据
56
57 People = new List<Person>
58 {
59 new Person { Name = "张小祎", Description = "北京", Department = dept01, Email = "zhangxw@hotmail.com" },
60 new Person { Name = "李珊柳", Description = "北京", Department = dept01, Email = "lishsh@hotmail.com" },
61 new Person { Name = "李华语", Description = "河北", Department=dept0101, Email = "lihy@hotmail.com" },
62 new Person { Name = "黄慧琳", Description = "河北", Department=dept0101,Email = "huanghl@hotmail.com" },
63 new Person { Name = "潘加伟", Description = "河北", Department=dept0101, Email = "panjw@hotmail.com" },
64 new Person { Name = "秦小梨", Description = "河北", Department=dept0101, Email = "qinxl@hotmail.com" },
65 new Person { Name = "覃晓琳", Description = "河北", Department=dept0101,Email = "qinxlin@hotmail.com" },
66 new Person { Name = "韦长英", Description = "广西", Department=dept0101, Email = "weichy@hotmail.com" },
67 new Person { Name = "韦大东", Description = "广西", Department=dept0102, Email = "1918625568@qq.com" },
68 new Person { Name = "韦家文", Description = "广西", Department=dept0101, Email = "2918625578@qq.com" },
69 new Person { Name = "黎文新", Description = "广西", Department=dept0101,Email = "1918625592@qq.com" },
70 new Person { Name = "黎子流", Description = "广东", Department=dept0102, Email = "1918625593@qq.com" },
71 new Person { Name = "余卫东", Description = "广东", Department=dept0102, Email = "1918625591@qq.com" },
72 new Person { Name = "何家宝", Description = "广东", Department=dept0102, Email = "1918625566@qq.com" },
73 new Person { Name = "何欣俊", Description = "广东", Department=dept0102,Email = "1918625555@qq.com" },
74 new Person { Name = "余华亮", Description = "广东", Department=dept0102, Email = "1918625533@qq.com" },
75 new Person { Name = "汤富贵", Description = "广东", Department=dept0102,Email = "1918625532@qq.com" },
76 new Person { Name = "唐富贵", Description = "广东", Department=dept0102, Email = "1918625536@qq.com" },
77 new Person { Name = "唐蔚佳", Description = "广东", Department=dept0103, Email = "tangwj@hotmail.com" },
78 new Person { Name = "谢显才", Description = "广东", Department=dept0103,Email = "xiexc@hotmail.com" },
79 new Person { Name = "解晓东", Description = "广东", Department=dept0103, Email = "xiexd@hotmail.com" },
80 new Person { Name = "谢家麟", Description = "广东", Department=dept0103,Email = "xiejl@hotmail.com" },
81 new Person { Name = "谢子怡", Description = "广东", Department=dept0103, Email = "xieziy@hotmail.com" },
82 new Person { Name = "张建光", Description = "广东", Department=dept0103, Email = "zhangjg@163.com" },
83 new Person { Name = "李振书", Description = "广东", Department=dept0103, Email = "lizhsh@163.com" },
84 new Person { Name = "陈丰州", Description = "广东", Department=dept0103, Email = "chenfzh@163.com" },
85 new Person { Name = "陈卫东", Description = "广东", Department=dept0103, Email = "chenwd@163.com" },
86 new Person { Name = "陈峰受", Description = "广东", Department=dept0103, Email = "chenfsh@163.com" },
87 new Person { Name = "陈金健", Description = "广东", Department=dept0103,Email = "chenjj@163.com" },
88 new Person { Name = "韦海波", Description = "广东", Department=dept0103, Email = "weihb@163.com" },
89 new Person { Name = "祁宣明", Description = "广东", Department=dept0103, Email = "qixm@163.com" },
90 new Person { Name = "戚计生", Description = "上海", Department=dept01, Email = "qijsh@163.com" },
91 new Person { Name = "石智生", Description = "广西", Department=dept0101,Email = "shizhsh@163.com" },
92 new Person { Name = "苏晓琳", Description = "广西", Department=dept0101, Email = "suxl@hotmail.com" },
93 new Person { Name = "苏振彪", Description = "广西", Department=dept0101, Email = "suzhb@sina.com.cn" },
94 new Person { Name = "谭家伟", Description = "广西", Department=dept0101, Email = "tanjw@sina.com.cn" },
95 new Person { Name = "谭俊杰", Description = "广西", Department=dept01,Email = "tanjj@sina.com.cn" },
96 new Person { Name = "王定祠", Description = "广西", Department=dept01, Email = "wangdc@sina.com.cn" },
97 new Person { Name = "王金生", Description = "广西", Department=dept01, Email = "wangjsh@sina.com.cn" },
98 new Person { Name = "王宝军", Description = "广西", Department=dept0103, Email = "wangbj@sina.com.cn" },
99 new Person { Name = "吴克标", Description = "广西", Department=dept0103, Email = "wukb@hotmail.com" },
100 new Person { Name = "吴斌", Description = "江西", Department=dept0103, Email = "wubing@hotmail.com" },
101 new Person { Name = "游毅峰", Description = "湖南", Department=dept0103, Email = "youyf@hotmail.com" },
102 new Person { Name = "柳君华", Description = "湖南", Department=dept0103, Email = "liujh@hotmail.com" },
103 new Person { Name = "刘小薇", Description = "湖南", Department=dept0103, Email = "liuxw@hotmail.com" },
104 new Person { Name = "陈东风", Description = "湖南", Department=dept0103, Email = "chendf@hotmail.com" },
105 new Person { Name = "欧阳海", Description = "湖南", Department=dept0103, Email = "ouyangpj@hotmail.com" },
106 new Person { Name = "柳三变", Description = "湖南", Department=dept0103, Email = "liusb@hotmail.com" },
107 new Person { Name = "郭骁", Description = "北京", Department=dept0103,Email = "zhanguoxiao@hotmail.com" },
108 new Person { Name = "郭家铭", Description = "香港", Department=dept0103, Email = "guojm@hotmail.com" },
109 new Person { Name = "陈兆年", Description = "四川", Department=dept0103, Email = "chanzhn@hotmail.com" },
110 new Person { Name = "农卫红", Description = "四川", Department=dept0103, Email = "nongwh@hotmail.com" },
111 new Person { Name = "农志升", Description = "四川", Department=dept0103, Email = "nongzhsh@hotmail.com" },
112 new Person { Name = "农小琳", Description = "四川", Department=dept0103, Email = "nongxl@hotmail.com" },
113 new Person { Name = "徐荣国", Description = "四川", Department=dept0103, Email = "xurg@live.com" },
114 new Person { Name = "聂小威", Description = "四川", Department=dept0103, Email = "niexw@live.com" },
115 new Person { Name = "邱福林", Description = "四川", Department=dept0103, Email = "qiufl@live.com" },
116 new Person { Name = "范思坦", Description = "四川", Department=dept0103, Email = "fanst@live.com" },
117 new Person { Name = "邓唯佳", Description = "四川", Department=dept0103, Email = "dengwj@hotmail.com" },
118 new Person { Name = "马晓东", Description = "云南", Department=dept0103, Email = "maxd@hotmail.com" },
119 new Person { Name = "魏明翠", Description = "云南", Department=dept0103, Email = "weimc@hotmail.com" },
120 new Person { Name = "尹相杰", Description = "贵州", Department=dept0103, Email = "yinxj@hotmail.com" },
121 new Person { Name = "张小祎", Description = "贵州", Department=dept0103, Email = "zhangxw@163.com" }
122 };
123
124 #endregion
125 }
126 }初始化数据代码
1.基本查询操作符-获取数据：
Select() 方法：
public static IEnumerable<TResult> Select<TSource, TResult> (this IEnumerable<TSource> source, Func<TSource, TResult> selector )
说明：
。Select 方法本身是一个泛型扩展方法
。它作用于 IEnumerable<TSource> 类型
。它只接受一个 Func<TSoursce,TResult> 类型参数
。Func<TSource,TResult>是一个泛型委托，位于 System 名字空间下，System.Core.dll 中
。在这里 selector 是一个提取器
2.基本查询操作符-过滤数据
Where()方法：
public static IEnumerable<TSource> Where<TSource>( this IEnumerable<TSource> source, Func<TSource, bool> predicate )
说明：
。Where方法也是一个泛型扩展方法
。它和 Select() 一样作用于IEnumerable<TSource>类型
。它只接受一个 Func<TSource, bool> 泛型委托参数
。在这里 predicate 是一个判断条件
3.基本查询操作符-排序数据：
OrderBy()方法：
public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>( this IEnumerable<TSource> source, Func<TSource, TKey> keySelector )
说明
。OrderBy方法也是一个泛型扩展方法
。它和 Select() 一样作用于IEnumerable<TSource>类型
。它只接受一个 Func<TSource, TKey > 类型参数
。在这里 keySelector 指定要排序的字段
。如果想降序排列可以使用OrderByDescending方法
4.基本查询操作符-分组数据：
GroupBy()方法
public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector )
说明
。GroupBy方法和OrderBy方法非常类似，它也是一个泛型扩展方法
。它和 OrderBy() 一样作用于IEnumerable<TSource>类型
。它只接受一个 Func<TSource, TKey > 类型参数
。在这里 keySelector 指定要分组的字段
参考网址：https://docs.microsoft.com/zh-cn/dotnet/csharp/programming-guide/statements-expressions-operators/anonymous-functions
转载于:https://www.cnblogs.com/huangzewei/p/7286255.html
part01.03 委托与 Lambda 表达式（三）：Lambda 表达式相关推荐
- C# 从CIL代码了解委托，匿名方法，Lambda 表达式和闭包本质
前言 C# 3.0 引入了 Lambda 表达式,程序员们很快就开始习惯并爱上这种简洁并极具表达力的函数式编程特性. 本着知其然,还要知其所以然的学习态度,笔者不禁想到了几个问题. (1)匿名函数(匿 ...
- java三目表达式_Java8新特性Lambda表达式
1 Lambda表达式 对于很多计算机语言来说,Lambda表达式并不是陌生的语法格式,而对于Java而言,它的到来比较晚,直到Java8更新之后,Lambda表达式才正式出现在Java语法中.所以如 ...
- Linq表达式、Lambda表达式你更喜欢哪个？
什么是Linq表达式?什么是Lambda表达式? 如图: 由此可见Linq表达式和Lambda表达式并没有什么可比性. 那与Lambda表达式相关的整条语句称作什么呢?在微软并没有给出官方的命名,在& ...
- C#3.0之神奇的Lambda表达式和Lambda语句
"Lambda 表达式"是一个匿名函数,它可以包含表达式和语句,并且可用于创建委托或表达式目录树类型.所有 Lambda 表达式都使用 Lambda 运算符 =>,该运算符读 ...
- c++ lambda函数_C++ Lambda表达式
利用Lambda表达式,可以方便的定义和创建匿名函数.Lambda 表达式把函数看作对象.Lambda 表达式可以像对象一样使用,比如可以将它们赋给变量和作为参数传递,还可以像函数一样对其求值. 当一 ...
- Java代码简化之朗母达表达式（Lambda Express）
Java代码简化之朗母达表达式(Lambda Express) 本文目录: Lambda Express基本概念 Lambda Express的解析 1. Lambda Express基本概念 λ表达 ...
- Linq表达式和Lambda表达式用法对比
什么是Linq表达式?什么是Lambda表达式? 前一段时间用到这个只是,在网上也没找到比较简单明了的方法,今天就整理了一下相关知识,有空了再仔细研究研究 public Program() { Lis ...
- python基础学习1-三元表达式和lambda表达式
#!/usr/bin/env python # -*- coding:utf-8 -*- 三元运算 if else 的简写name ="alex" if 1==1 else &qu ...
- c++11标准：匿名函数（匿名表达式）lambda
lambda: C++11提供了对匿名函数的支持,称为Lambda函数(也叫Lambda表达式). Lambda表达式具体形式如下: 匿名函数定义/匿名表达式声明:[capture](paramete ...
最新文章
- Web前端JQuery面试题（三）
- CSS实现英文或拼音单词首字母大写
- 有关UNICODE、ANSI字符集和相关字符串操作的总结[转]
- 线程的状态、进程的状态
- Java黑皮书课后题第3章：*3.13（金融应用：计算税款）程序清单3-5给出了计算单身登记人税款的源代码。将程序清单3-5补充完整，从而计算所有登记的婚姻状态的税款
- 【Asp.net】Cookie对象
- python 支持向量机预测结果相同_Python机器学习算法 — 支持向量机（SVM）
- 唏嘘！2019榜单出炉：铁打的Python连续3年第一，它居然跌出前十？
- 关于grep,egrep正则表达式的常用用法
- [译]LightSwitch 如何实现：在查询中创建和使用全局值(Eric Erhardt)
- java 泛型 比较_java 泛型和object比较
- codeforces 762E（cdq分治）
- 把文本框的值转换成Image
- c语言函数二维数组调用,C语言之调用函数传递二维数组
- IPC 38模组板图分析-雄迈IPG-50H10PE-SL-Hi3518E+OV9712
- 网页游戏《Drakensang》评测——改进道具收费模式可行性分析
- go 并发goroutines，channal
- 前端面试基础题总结 (必会)
- 元学习 迁移学习_元学习就是您所需要的
- 2018版ISTQB FL基础级大纲全解析