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

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

/* 
* 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 
        { 
void Page_Load() void Page_Load(object sender, EventArgs e) 
                { 
                        Write("hello"); 
                        Write("hello", "webabcd"); 
                        Write("hello", p3: false, p2: "webabcd"); 
                }

void Write() 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("<br />"); 
                } 
        } 
}

/* 
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

/* 
* 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 
        { 
string Hello() string Hello(string name) 
                { 
                        return "hello: " + name; 
                }

public string Name { get; set; }

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

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

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

Response.Write(d.Hello("method")); 
                        Response.Write("<br />");

d.Name = "hello: property"; 
                        Response.Write(d.Name); 
                        Response.Write("<br />");

Response.Write(d["hello: indexer"]); 
                        Response.Write("<br />");

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

/* 
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 
        { 
void Page_Load() 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("<br />"); 
                        } 
                }

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

/* 
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 
        { 
void Page_Load() 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 
                { 
void Write() 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

/* 
* 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 = "";

static readonly object objLock = new object() static readonly object objLock = new object();

void Page_Load() 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); 
                }

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

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

// 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

/* 
* 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 = "";

static readonly object objLock = new object() static readonly object objLock = new object();

void Page_Load() 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); 
                }

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

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 += "<br />"; 
                        } 
                } 
        } 
}

/* 
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 
[源码下载] 

     本文转自webabcd 51CTO博客，原文链接：http://blog.51cto.com/webabcd/341285，如需转载请自行联系原作者