Lambda表达式

一、什么时候使用Lambda表达式

总的来说，Lambda 表达式可以用在任何需要使用匿名方法，或是代理的地方。编译器会将Lambda表达式编译为标准的匿名方法（可以使用ildasm.exe or reflector.exe得到确认）。

比如：

List<int> evenNumbers = list.FindAll(i => (i%2) == 0);

被编译为

List<int> evenNumbers = list.FindAll(delegate (int i)
{
return (i % 2) ==0;
});

二、Lambda表达式的解读

Lambda表达式的写法

ArgumentsToProcess （参数）=> StatementsToProcessThem（报表）

比如

// "i" is our parameter list.
// "(i % 2) == 0" is our statement set to process "i".
List<int> evenNumbers = list.FindAll(i => (i%2) == 0);

应该这样来理解

// My list of parameters (in this case a singleinteger named i)
// will be processed by the expression (i % 2) == 0.
List<int> evenNumbers = list.FindAll((i) => ((i%2) == 0));

可以显式指定输入参数的类型

List<int> evenNumbers = list.FindAll((int i) => (i %2) == 0);

可以使用括号把输入参数和表达式括起来，如果参数或处理表达式只有一个，可以省略括号

List<int> evenNumbers = list.FindAll((i) => ((i%2) == 0));

有多行处理表达式时，需要使用花括号包起来

List<int> evenNumbers = list.FindAll((i) =>
{
Console.WriteLine("valueof i is currently: {0}", i);
bool isEven = ((i % 2) ==0);
return isEven;
});

当输入参数有多个时

SimpleMath m = new SimpleMath();
m.SetMathHandler((msg, result) =>
{Console.WriteLine("Message:{0}, Result: {1}", msg, result);});

或者显式指定输入参数类型

m.SetMathHandler((string msg,int result) =>
{Console.WriteLine("Message:{0}, Result: {1}", msg, result);});

 
当没有输入参数时

VerySimpleDelegate d = new VerySimpleDelegate( () => {return "Enjoy your string!";} );

 

Lambda表达式可以在使用代理和匿名代理的地方

1、命名函数

publicclass Common
{
public delegate bool IntFilter(int i);
public static int[] FilterArrayOfInts(int[] ints, IntFilter filter)
{
ArrayList aList = new ArrayList();
foreach (int i in ints)
{
if (filter(i))
{
aList.Add(i);
}
}
return ((int[])aList.ToArray(typeof(int)));
}
}

 

publicclass Application
{
public static bool IsOdd(int i)
{
return ((i & 1) ==1);
}
}

 

using System.Collections;
int[] nums = { 1,2, 3, 4, 5, 6,7, 8, 9, 10 };
int[] oddNums = Common.FilterArrayOfInts(nums,Application.IsOdd);
foreach (int i in oddNums)
Console.WriteLine(i);
2、匿名函数

int[] nums = {1, 2, 3, 4, 5,6, 7, 8, 9, 10 };
int[] oddNums =
Common.FilterArrayOfInts(nums, delegate(int i) {return ((i & 1) ==1); });
foreach (int i in oddNums)
Console.WriteLine(i);
3、Lambda表达式

int[] nums = {1, 2, 3, 4, 5,6, 7, 8, 9, 10 };
int[] oddNums = Common.FilterArrayOfInts(nums,i => ((i &1) == 1));
foreach (int i in oddNums)
Console.WriteLine(i);
三种方式的比较

int[] oddNums =// using named method
Common.FilterArrayOfInts(nums,Application.IsOdd);
int[] oddNums = // using anonymous method
Common.FilterArrayOfInts(nums,delegate(int i){return((i &1) == 1);});
int[] oddNums = // using lambda expression
Common.FilterArrayOfInts(nums,i => ((i & 1) ==1));
命名函数虽然简短，但是而外还要定义处理函数，优点是可以重用

 

---

匿名方法

---

匿名方法就是没名儿的委托。虽然没名，但是必须加”delegate“来表示我没名。

// Create a delegateinstance
delegate void Del(int x);

// Instantiate the delegate using an anonymous method
Del d = delegate(int k) { /* ... */ };
Del d = delegate() { System.Console.WriteLine("Copy #:{0}", ++n);};
delegate void Printer(string s);
Printer p = delegate(string j)
{
System.Console.WriteLine(j);
};
p("The delegate using the anonymous method is called.");

delegate string Printer(string s);
private void button1_Click(object sender, EventArgs e)
{
 Printer p =delegate(string j)
{
   return (j)+"烦死了";
};
Console.WriteLine(p("The delegate using the anonymous method is called."));
}

或者有名。。。。的匿名委托。。。。哈哈，我在抽风还是微软在抽风。让我死吧。

delegate void Printer(string s);
private void button1_Click(object sender, EventArgs e)
{
Printer p = new Printer(Form1.DoWork);
p("http://www.dc9.cn/");
}
static void DoWork(string k)
{
System.Console.WriteLine(k);
}

匿名方法就完了。

---

Lamdba就是 (int x)=>{x+1}就是这样的。。。例子呢就是上面写了的一段用lambda就这样写

delegate string Printer(string s);
private void button1_Click(object sender, EventArgs e)
{
Printer p = j => j+"烦死了！！！";
Console.WriteLine(p("The delegate using the anonymous method is called."));
}

还能这么用
和上面那个一样，就是简化了。

public Form1()
{
InitializeComponent();
 this.Click += (s, e) => { MessageBox.Show(((MouseEventArgs)e).Location.ToString());};
}

---

超难+===>?其实不难。难的是转换成一般的函数怎么写呢？？？？

---

Where是Enumerable的一个方法。3.5才有的。里面的参数是Func<(Of <(T, TResult>)>)泛型委托

---

Func<(Of<(T, TResult>)>) 泛型委托

---

using System;
delegate string ConvertMethod(string inString);

public class DelegateExample
{
   public static void Main()
   {
  // Instantiate delegate to referenceUppercaseString method
  ConvertMethod convertMeth = UppercaseString;
  string name = "Dakota";
  // Use delegate instance to call UppercaseStringmethod
  Console.WriteLine(convertMeth(name));
   }

   private static string UppercaseString(stringinputString)
   {
  return inputString.ToUpper();
   }
}

写成泛型委托是

using System;
public class GenericFunc
{
   public static void Main()
   {
  // Instantiate delegate to referenceUppercaseString method
  Func<string, string> convertMethod= UppercaseString;
  string name = "Dakota";
  // Use delegate instance to call UppercaseStringmethod
  Console.WriteLine(convertMethod(name));
   }
   private static string UppercaseString(stringinputString)
   {
  return inputString.ToUpper();
   }
}

---

于是应用到Linq，再变换到lambda

delegate bool TestFunc(string fruit);
private void button1_Click(object sender, EventArgs e)
{
List<string> fruits =new List<string> { "apple", "http://www.dc9.cn","banana", "mango",
"orange", "blueberry", "grape", "strawberry"};
TestFunc f = new TestFunc(DoWork);
Func<string, bool> f2 = DoWork;
IEnumerable<string> query = fruits.Where(f2);
foreach (string fruit in query)
Console.WriteLine(fruit);
   }

private static bool DoWork(string k)
{
  return   k.Length < 6;
   }

能用。

==========================================================

 

delegate bool TestFunc(string fruit);
private void button1_Click(object sender, EventArgs e)
{
List<string> fruits =new List<string> { "apple", "passionfruit","banana", "mango",
"orange", "blueberry", "grape", "http://www.dc9.cn"};
TestFunc f = DoWork;
Func<string, bool> f2 =k=> k.Length < 6;
IEnumerable<string> query = fruits.Where(f2);
foreach (string fruit in query)
Console.WriteLine(fruit);
   }

private static bool DoWork(string k)
{
  return   k.Length < 6;
   }

也能用

 

========================================================

漫画

private void button1_Click(object sender, EventArgse)
{
List<string> fruits =new List<string> { "apple", "passionfruit","banana", "mango",
"orange", "blueberry", "grape", "http://www.dc9.cn"};
IEnumerable<string> query = fruits.Where(
delegate(string k){
return k.Length < 6;
}
);
foreach (string fruit in query)
Console.WriteLine(fruit);
   }

能用～

private void button1_Click(object sender, EventArgse)
{
List<string> fruits =new List<string> { "apple", "passionfruit","banana", "mango",
"orange", "blueberry", "grape", "http://www.dc9.cn"};
IEnumerable<string> query = fruits.Where(k=>k.Length<6);
foreach (string fruit in query)
Console.WriteLine(fruit);
   }

最后，lambda,能用～～～～就酱紫了～～～～

  publicdelegate int mydg(int a, int b);
  public static class LambdaTest
  {
  public static int oper(this int a,int b, mydg dg)
  {
  return dg(a, b);
  }
  }
Console.WriteLine(1.oper(2, (a, b) => a + b));
Console.WriteLine(2.oper(1, (a, b) => a - b));