静态构造函数【转】

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类。

• 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.
        71 is starting its route 6.00 minutes after global start time 3:57 PM.
        72 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