Lambda 表达式

.NET 3.x新特性之Lambda表达式

lambda 表达式格式为：
(参数列表) => 表达式或者语句块
分为三个步骤
1、(参数列表)：
2、=>符号可以理解为从
3、表达式或者语句块

可以有多个参数，一个参数，或者无参数。参数类型可以隐失或者显示。例如：
(x,y) => x * y    //多参数，隐式类型=>表达式
x => x * 10    //单参数，隐式类型=>表达式
x => {return x * 10;}   //单参数，隐失类型=>语句块
(int x) => x * 10   //单参数，显示类型=>表达式
(int x) => {return x * 10} //单参数，显示类型=>语句块
() => Console.WriteLing() //无参数

 

Lambda表达式其实是一个委托类型实例,编译器会根据参数的类型建立一个同类型的同参数的委托：
比如
x => x * 10
相当于是下面一个委托类型的实例
delegate int MyDeg(int i);//这个是由编译器根据推断出的类型而建立的
只不过这些工作由编译器帮我们做了

 

Lambda 表达式格式要点
- Lambda表达式的参数类型可以忽略，因为可以根据使用的上下文进行推断。
- Lambda表达式的主体(body)可以是表达式，也可以是语句块。
- Lambda表达式传入的实参将参与类型推断，以及方法重载辨析。
- Lambda表达式和表达式体可以被转换为表达式树。

 

Lambda 表达式与委托类型
Lambda 表达式L可以被转换为委托类型D，需要满足以下条件：
- L和D拥有相同的参数个数。
- L的参数类型要与D的的参数类型相同。注意隐式类型要参与类型辨析。
- D的返回类型与L相同，无论L是表达式，还是语句块。

下面我们来看一下编译器到底都做了些什么？
比如在程序中有下面Lambda表达式:

delegate bool Mydelegate(string s);class jh{    public static Process(Mydelegate myd)        {             myd("wo shi jhxz");        }}jh.Process(s => s.Indexof("jhxz") > 0) //Process方法接受一个Mydelegate的委托

 

编译其实有Lambda表达式那条语句编译为以下这样的代码：

Mydelegate md=new Mydelegate(IsValidXXXX);//编译器生成jh.Process(md1)public static bool IsValidXXXX(string s)//编译器生成{　　return s.Indexof("jhxz") > 0;} 

体验:
1.准备测试数据

static int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };static string[] strings = new string[] { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"};class Person {public string Name;public int Level;}static Person[] persons = new Person[] {new Person {Name="Matt", Level=3},new Person {Name="Luca", Level=3},new Person {Name="Jomo", Level=5},new Person {Name="Dinesh", Level=3},new Person {Name="Charlie", Level=3},new Person {Name="Mads", Level=3},new Person {Name="Anders", Level=9}};

 

2.过滤数据

 

public static void Sample1() {// use Where() to filter out elements matching a particular condition               IEnumerable<int> fnums = numbers.Where(n => n < 5);Console.WriteLine("Numbers < 5");foreach(int x in fnums) {Console.WriteLine(x);}}

 

 

3.匹配首个字母

 

public static void Sample2() {// use First() to find the one element matching a particular condition         string v = strings.First(s => s[0] == 'o');Console.WriteLine("string starting with 'o': {0}", v);}4.根据numbers排序public static void Sample3() {// use Select() to convert each element into a new value        IEnumerable<string> snums = numbers.Select(n => strings[n]);Console.WriteLine("Numbers");foreach(string s in snums) {Console.WriteLine(s);}}

5

.匿名类型,注意var关键字

 

public static void Sample4(){// use Anonymous Type constructors to construct multi-valued results on the fly        var q = strings.Select(s => new {Head = s.Substring(0,1), Tail = s.Substring(1)});foreach(var p in q) {Console.WriteLine("Head = {0}, Tail = {1}", p.Head, p.Tail);}}

6

.联合查询(即使用两个以上的查询条件)

 

public static void Sample5() {// Combine Select() and Where() to make a complete query        var q = numbers.Where(n => n < 5).Select(n => strings[n]);Console.WriteLine("Numbers < 5");foreach(var x in q) {Console.WriteLine(x);}}

7

.使用ToList方法

 

public static void Sample6() {// Sequence operators form first-class queries are not executed until you enumerate them.        int i = 0;var q = numbers.Select(n => ++i);// Note, the local variable 'i' is not incremented until each element is evaluated (as a side-effect).        foreach(var v in q) {Console.WriteLine("v = {0}, i = {1}", v, i);}Console.WriteLine();// Methods like ToList() cause the query to be executed immediately, caching the results        int i2 = 0;var q2 = numbers.Select(n => ++i2).ToList();// The local variable i2 has already been fully incremented before we iterate the results        foreach(var v in q2) {Console.WriteLine("v = {0}, i2 = {1}", v, i2);}}

8

.分组查询

 

public static void Sample7() {// use GroupBy() to construct group partitions out of similar elements        var q = strings.GroupBy(s => s[0]); // <- group by first character of each stringforeach(var g in q) {Console.WriteLine("Group: {0}", g.Key);foreach(string v in g) {Console.WriteLine("\tValue: {0}", v);}}}

9

.统计聚合

 

public static void Sample8() {// use GroupBy() and aggregates such as Count(), Min(), Max(), Sum(), Average() to compute values over a partition        var q = strings.GroupBy(s => s[0]).Select(g => new {FirstChar = g.Key, Count = g.Count()});foreach(var v in q) {Console.WriteLine("There are {0} string(s) starting with the letter {1}", v.Count, v.FirstChar);}}

10

.排序

 

// use OrderBy()/OrderByDescending() to give order to your resulting sequence        var q = strings.OrderBy(s => s); // order the strings by their nameforeach(string s in q) {Console.WriteLine(s);}}

11

二次排序

public static void Sample9a() {// use ThenBy()/ThenByDescending() to provide additional ordering detail        var q = persons.OrderBy(p => p.Level).ThenBy(p => p.Name);foreach(var p in q) {Console.WriteLine("{0} {1}", p.Level, p.Name);}}

 

 

 

 

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