.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 string
foreach(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 name
foreach(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);
}
}