C#快速学习笔记（译）续一

6.虚拟和非虚拟函数

下面是一个非虚拟函数

using System;
namespace Test2 {
class Plane { public double TopSpeed() {return 300.0D; }}
class Jet : Plane { public double TopSpeed() { return 900.0D;}}
class Airport {
static void Main(string[] args) {
   Plane plane = new Jet();
   Console.WriteLine("planes top speed: {0}",plane.TopSpeed()); //300
   Console.ReadLine();
}
}
}

因为TopSpeed（）是非虚拟的函数，上面的代码将打印出300。为了修正这个问题，我们需要用virtual声明这个方法，并且在子类中用override重写它，如果不设置Jet（）为override，结果依然是300，注意观察下面的例子。

class Plane { public virtual double TopSpeed() { return 300.0D;}}
class Jet : Plane { public override double TopSpeed() { return 900.0D;}}
class Airport {
static void Main(string[] args) {
   Plane plane = new Jet();
   Console.WriteLine("planes top speed: {0}",plane.TopSpeed()); //900
   Console.ReadLine();
}

7.隐藏继承成员

在子类中，你可以重复使用变量名，但编译器会提醒你，子类的成员是“隐藏”父类的变量。

public class Elephant { public int Weight = 15000; }
public class AsianElephant : Elephant { public int Weight = 13000; }
void Main()
{
Console.WriteLine(new AsianElephant().Weight); //13000
}

如果你真想得到一个新的变量，那么告诉编译器将停止抱怨它，通过使用“new”修饰符（不被与“new”运算符混淆）。

public class AsianElephant : Elephant { public new int Weight = 1300; }

8.重载运算符示例。注意，血多运算符必须重载为一对，如〉、〈

public class Plane {
      public virtual double TopSpeed() { return 300.0D;}
      public static bool operator>(Plane one, Plane two) {   
         return one.TopSpeed() > two.TopSpeed();
      }
      public static bool operator<(Plane one, Plane two) {   
         return one.TopSpeed() < two.TopSpeed();
      }
   }
   class Jet : Plane {
      public override double TopSpeed() { return 900.0D; }
      public override string ToString() { return "I'm a Jet"; }
   }
      class Airport {
      static void Main(string[] args) {
         Plane plane = new Jet();
         Console.WriteLine("plane's top speed: {0}",plane.TopSpeed());
         Jet jet = new Jet();
         Console.WriteLine("jet's top speed: {0}",jet.TopSpeed());
         Console.WriteLine("Plane > Jet = {0}", plane > jet);
         Console.ReadLine();
      }
   }

9.重载参数

通常在编译时根据声明的类型参数来决定调用重载方法。即使“mamal”的对象是真正的Tiger型，编译器将调用Mammal重载 - 除非它转换为“dynamic”的类型，在这种情况下，它会调用此基础上该方法真正的对象类型。

using System;
namespace ScratchPad
{
public class Linnaeus 
{
    public class Mammal {}
    public class Tiger : Mammal{}
    public static void Zoo(Mammal mammal)
    {
        Console.Out.WriteLine("mammal");
    }
    public static void Zoo(Tiger tiger)
    {
        Console.Out.WriteLine("tiger");
    }
    public static void Main(string[] args)
    {
        Mammal mammal = new Tiger();
        Zoo(mammal); //writes mammal because variable is that type
        Zoo((dynamic) mammal); //writes tiger because the object is a Tiger
        Console.ReadKey();
    }
}
}

10.使用属性访问器方法的例子。注意set和value变量的特殊用途。

public class Plane {
 protected double mySpeed = 300.0D;
 public double TopSpeed { 
                 get {return mySpeed;} 
                 set { mySpeed = value; } 
                         }
}
class Jet : Plane {
   public Jet() { TopSpeed = 900.0D; }
}
   class Airport {
   static void Main(string[] args) {
      Plane plane = new Plane();
      Console.WriteLine("plane's top speed: {0}",plane.TopSpeed);
      Jet jet = new Jet();
      Console.WriteLine("jet's top speed: {0}",jet.TopSpeed);
      Console.ReadLine();
   }
}

11.输出当前时间

DateTime dt = DateTime.Now;
Console.WriteLine("Current Time is {0} ",dt.ToString());

要指定一个格式: dt.ToString("yyyy/MM/dd")

当前独立的通用格式: dt.ToString("u") 
将输出 "yyyy'-'MM'-'dd HH':'mm':'ss'Z'"

12.写入几行文字到文件中

using System.IO;
...
try {
 StreamWriter streamWriter = new StreamWriter("tmp.txt");

 streamWriter.WriteLine("This is the first line.");
 streamWriter.WriteLine("This is the second line.");
} finally {
 if(streamWriter != null) {
     streamWriter.Close();
     }
}

未完待续。