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静态构造函数【转】

2013-09-25 08:08 260 查看
静态构造函数用于初始化任何
静态 数据,或用于执行仅需执行一次的特定操作。在创建第一个实例或引用任何静态成员之前,将自动调用静态构造函数。

class SimpleClass
{
// Static variable that must be initialized at run time.
static readonly long baseline;

// Static constructor is called at most one time, before any
// instance constructor is invoked or member is accessed.
static SimpleClass()
{
baseline = DateTime.Now.Ticks;
}
}


静态构造函数具有以下特点:

静态构造函数既没有访问修饰符,也没有参数。

在创建第一个实例或引用任何静态成员之前,将自动调用静态构造函数来初始化

无法直接调用静态构造函数。

在程序中,用户无法控制何时执行静态构造函数。

静态构造函数的典型用途是:当类使用日志文件时,将使用这种构造函数向日志文件中写入项。

静态构造函数在为非托管代码创建包装类时也很有用,此时该构造函数可以调用 LoadLibrary 方法。

如果静态构造函数引发异常,运行时将不会再次调用该构造函数,并且在程序运行所在的应用程序域的生存期内,类型将保持未初始化。

示例

在此示例中,类 Bus 有一个静态构造函数。创建
Bus 的第一个实例(bus1)时,将调用该静态构造函数来初始化该类。输出示例验证了即使创建
Bus 的两个实例,该静态构造函数也仅运行一次,并且在实例构造函数运行之前运行。

public class Bus
{
// Static variable used by all Bus instances.
// Represents the time the first bus of the day starts its route.
protected static readonly DateTime globalStartTime;

// Property for the number of each bus.
protected int RouteNumber { get; set; }

// Static constructor to initialize the static variable.
// It is invoked before the first instance constructor is run.
static Bus()
{
globalStartTime = DateTime.Now;

// The following statement produces the first line of output,
// and the line occurs only once.
Console.WriteLine("Static constructor sets global start time to {0}",
globalStartTime.ToLongTimeString());
}

// Instance constructor.
public Bus(int routeNum)
{
RouteNumber = routeNum;
Console.WriteLine("Bus #{0} is created.", RouteNumber);
}

// Instance method.
public void Drive()
{
TimeSpan elapsedTime = DateTime.Now - globalStartTime;

// For demonstration purposes we treat milliseconds as minutes to simulate
// actual bus times. Do not do this in your actual bus schedule program!
Console.WriteLine("{0} is starting its route {1:N2} minutes after global start time {2}.",
this.RouteNumber,
elapsedTime.TotalMilliseconds,
globalStartTime.ToShortTimeString());
}
}

class TestBus
{
static void Main()
{
// The creation of this instance activates the static constructor.
Bus bus1 = new Bus(71);

// Create a second bus.
Bus bus2 = new Bus(72);

// Send bus1 on its way.
bus1.Drive();

// Wait for bus2 to warm up.
System.Threading.Thread.Sleep(25);

// Send bus2 on its way.
bus2.Drive();

// Keep the console window open in debug mode.
System.Console.WriteLine("Press any key to exit.");
System.Console.ReadKey();
}
}
/* Sample output:
Static constructor sets global start time to 3:57:08 PM.
Bus #71 is created.
Bus #72 is created.
is starting its route 6.00 minutes after global start time 3:57 PM.
is starting its route 31.00 minutes after global start time 3:57 PM.
*/


原文地址:http://msdn.microsoft.com/zh-cn/library/k9x6w0hc
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