精进不休 .NET 4.0 (4) - C# 4.0 新特性之命名参数和可选参数, 动态绑定(dynamic), 泛型协变和逆变, CountdownEvent, Barrier

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精进不休 .NET 4.0 (4) - C# 4.0 新特性之命名参数和可选参数, 动态绑定(dynamic), 泛型协变和逆变, CountdownEvent, Barrier

作者： webabcd

介绍
C# 4.0 的新特性

Named And Optional Arguments - 命名参数和可选参数
Dynamic Binding - 动态绑定(dynamic 用于动态编程，其依赖于Dynamic Language Runtime)
Covariance - 泛型的协变
Contravariance - 泛型的逆变
CountdownEvent - 线程、任务同步类。线程或任务一直阻塞到 CountdownEvent 的计数为 0 为止
Barrier - 线程、任务同步类。其用来同步一个线程组或任务组中所有的线程或任务，先到达的线程或任务在此阻塞

示例
1、命名参数和可选参数的 Demo
NamedAndOptionalArguments.aspx.cs

代码

/*
* 命名参数和可选参数
* 命名参数：调用方法时，可以不按位置传递参数，而是指定参数的命名来传值
* 可选参数：声明方法中的参数时，可以为其设置默认值，那么在调用该方法时，这种可选参数是可以忽略的
*/

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace CSharp
{
 public partial class NamedAndOptionalArguments : System.Web.UI.Page
 {
 protected void Page_Load( object sender, EventArgs e)
 {
 Write( " hello " );
 Write( " hello " , " webabcd " );
 Write( " hello " , p3: false , p2: " webabcd " );
 }

 private void Write( string p1, string p2 = " p2 " , bool p3 = true )
 {
 Response.Write( string .Format( " p1:{0}; p2:{1}; p3:{2} " , p1, p2, p3.ToString()));
 Response.Write( " " );
 }
 }
}

/*
运行结果：
p1:hello; p2:p2; p3:True
p1:hello; p2:webabcd; p3:True
p1:hello; p2:webabcd; p3:False
*/

2、dynamic 的 Demo
DynamicBinding.aspx.cs

代码

/*
* dynamic - 用于动态编程，其依赖于Dynamic Language Runtime（DLR）
*
*/

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace CSharp
{
 public class DyanmicDemo
 {
 public string Hello( string name)
 {
 return " hello: " + name;
 }

 public string Name { get ; set ; }

 public string this [ string value]
 {
 get
 {
 return value;
 }
 }

 public dynamic GetNames()
 {
 List < string > names = new List < string > () { " web " , " webabc " , " webabcd " };

 return names;
 }
 }

 public partial class DynamicBinding : System.Web.UI.Page
 {
 protected void Page_Load( object sender, EventArgs e)
 {
 dynamic d = new DyanmicDemo();

 Response.Write(d.Hello( " method " ));
 Response.Write( " " );

 d.Name = " hello: property " ;
 Response.Write(d.Name);
 Response.Write( " " );

 Response.Write(d[ " hello: indexer " ]);
 Response.Write( " " );

 Response.Write(d.GetNames().Count.ToString());

 // 注意：下面这句会报错，因为不支持扩展方法
 // Response.Write(d.GetNames().Last());
 }
 }
}

/*
运行结果：
hello: method
hello: property
hello: indexer
3
*/

3、泛型的协变的 Demo
Covariance.aspx.cs

代码

/*
泛型协变规则：
泛型参数受 out 关键字约束，隐式转换目标的泛型参数类型必须是当前类型的“基类”
*/

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace CSharp
{
 public partial class Covariance : System.Web.UI.Page
 {
 protected void Page_Load( object sender, EventArgs e)
 {
 List < Human > human = new List < Human > ();
 human.Add( new Human { Name = " aaa " });
 human.Add( new Human { Name = " bbb " });
 human.Add( new Human { Name = " ccc " });

 List < Hero > hero = new List < Hero > ();
 hero.Add( new Hero { Name = " ddd " , Story = " 尿床 " });
 hero.Add( new Hero { Name = " eee " , Story = " 撒谎 " });
 hero.Add( new Hero { Name = " fff " , Story = " 打架 " });

 /*
 * List<T> 实现了如下接口 IEnumerable<out T> ，所以可以实现协变
 * public interface IEnumerable<out T> : IEnumerable
 * {
 * // Summary:
 * // Returns an enumerator that iterates through the collection.
 * //
 * // Returns:
 * // A System.Collections.Generic.IEnumerator<T> that can be used to iterate through
 * // the collection.
 * IEnumerator<T> GetEnumerator();
 * }
 */

 // Hero 的基类是 Human，所以 Hero 可以协变到 Human，所以下面的表达式成立
 List < Human > list = human.Union(hero).ToList();
 foreach (Human h in list)
 {
 Response.Write(h.Name);
 Response.Write( " " );
 }
 }

 class Human
 {
 public string Name { get ; set ; }
 }

 class Hero : Human
 {
 public string Story { get ; set ; }
 }
 }
}

/*
运行结果：
aaa
bbb
ccc
ddd
eee
fff
*/

4、泛型的逆变的 Demo
Contravariance.aspx.cs

代码

/*
泛型逆变规则：
泛型参数受 in 关键字约束，隐式转换目标的泛型参数类型必须是当前类型的“子类”
*/

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace CSharp
{
 public partial class Contravariance : System.Web.UI.Page
 {
 protected void Page_Load( object sender, EventArgs e)
 {
 IOutput < Human > human = new Output < Human > ();

 // 因为 IOutput<in T> ，并且 Human 的子类是 Hero ，所以 IOutput<Human> 可以逆变到 IOutput<Hero>
 IOutput < Hero > hero = human;
 hero.Write( new Hero { Name = " webabcd " });
 }

 interface IOutput < in T >
 {
 void Write(T o);
 }

 class Output < T > : IOutput < T >
 where T : Human
 {
 public void Write(T o)
 {
 HttpContext.Current.Response.Write(o.Name);
 }
 }

 class Human
 {
 public string Name { get ; set ; }
 }

 class Hero : Human
 {
 public string Story { get ; set ; }
 }
 }
}

/*
运行结果：
webabcd
*/

5、CountdownEvent 的 Demo
CountdownEventDemo.aspx.cs

代码

/*
* CountdownEvent - 线程、任务同步类。线程或任务一直阻塞到 CountdownEvent 的计数为 0 为止
* 1、当有新的需要同步的线程或任务产生时，就调用 AddCount 增加 CountdownEvent 的计数
* 2、当有线程或任务到达同步点时，就调用 Signal 函数减小 CountdownEvent 的计数
* 3、当 CountdownEvent 的计数为 0 时，就表示所有需要同步的任务已经完成。通过 Wait 来阻塞线程
*/

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

using System.Threading;

namespace CSharp
{
 public partial class CountdownEventDemo : System.Web.UI.Page
 {
 private string _result = "" ;

 private static readonly object objLock = new object ();

 protected void Page_Load( object sender, EventArgs e)
 {
 // CountdownEvent(int initialCount) - 实例化一个 CountdownEvent
 // int initialCount - 初始计数
 using (var countdown = new CountdownEvent( 1 ))
 {
 Thread t1 = new Thread(() => ThreadWork( " aaa " , TimeSpan.FromSeconds( 1 ), countdown));
 // 增加 1 个计数
 countdown.AddCount();
 t1.Start();

 Thread t2 = new Thread(() => ThreadWork( " bbb " , TimeSpan.FromSeconds( 2 ), countdown));
 countdown.AddCount();
 t2.Start();

 Thread t3 = new Thread(() => ThreadWork( " ccc " , TimeSpan.FromSeconds( 3 ), countdown));
 countdown.AddCount();
 t3.Start();

 // 减少 1 个计数
 countdown.Signal();
 // 阻塞当前线程，直到 CountdownEvent 的计数为零
 countdown.Wait();
 }

 Response.Write(_result);
 }

 private void ThreadWork( string name, TimeSpan sleepTime, CountdownEvent countdown)
 {
 Thread.Sleep(sleepTime);

 _result += " hello: " + name + " " + DateTime.Now.ToString( " HH:mm:ss " );
 _result += " " ;

 // 减少 1 个计数
 countdown.Signal();
 }
 }
}

/*
运行结果：
hello: aaa 15:18:55
hello: bbb 15:18:56
hello: ccc 15:18:57
*/

6、Barrier 的 Demo
BarrierDemo.aspx.cs

代码

/*
* Barrier - 线程、任务同步类。其用来同步一个线程组或任务组中所有的线程或任务，先到达的线程或任务在此阻塞
* 1、实例化 Barrier 指定其需要阻塞的线程或任务数
* 2、通过 SignalAndWait ，可以实现当指定的线程或任务数完成的时候取消阻塞
*/

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

using System.Threading;

namespace CSharp
{
 public partial class BarrierDemo : System.Web.UI.Page
 {
 private Barrier _barrier;
 private string _result = "" ;

 private static readonly object objLock = new object ();

 protected void Page_Load( object sender, EventArgs e)
 {
 // Barrier(int participantCount) - 实例化一个 Barrier
 // int participantCount - 需要阻塞的相关线程或任务数
 _barrier = new Barrier( 2 );

 Thread t1 = new Thread(() => ThreadWork( " aaa " , TimeSpan.FromSeconds( 1 )));
 t1.Start();
 Thread t2 = new Thread(() => ThreadWork( " bbb " , TimeSpan.FromSeconds( 2 )));
 t2.Start();
 Thread t3 = new Thread(() => ThreadWork( " ccc " , TimeSpan.FromSeconds( 3 )));
 t3.Start();

 Thread.Sleep( 5 * 1000 );
 Response.Write(_result);
 }

 private void ThreadWork( string name, TimeSpan sleepTime)
 {
 lock (objLock)
 {
 _result += " Barrier之前： " + name + " " + DateTime.Now.ToString( " HH:mm:ss " );
 _result += " " ;
 }

 Thread.Sleep(sleepTime);

 // 当指定数量的线程或任务完成后，同步这些线程或任务
 _barrier.SignalAndWait();

 lock (objLock)
 {
 _result += " Barrier之后： " + name + " " + DateTime.Now.ToString( " HH:mm:ss " );
 _result += " " ;
 }
 }
 }
}

/*
运行结果：
Barrier之前：aaa 17:38:01
Barrier之前：ccc 17:38:01
Barrier之前：bbb 17:38:01
Barrier之后：bbb 17:38:03
Barrier之后：aaa 17:38:03
*/

OK
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