靜態構造函數（C# 編程指南）

靜態構造函數用於初始化任何 靜態 數據，或用於執行僅需執行一次的特定操做。在建立第一個實例或引用任何靜態成員以前，將自動調用靜態構造函數。

C#

 

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;
    }
}

靜態構造函數具備如下特色：

• 靜態構造函數既沒有訪問修飾符，也沒有參數。

• 在建立第一個實例或引用任何靜態成員以前，將自動調用靜態構造函數來初始化類。

• 沒法直接調用靜態構造函數。

• 在程序中，用戶沒法控制什麼時候執行靜態構造函數。

• 靜態構造函數的典型用途是：當類使用日誌文件時，將使用這種構造函數向日志文件中寫入項。

• 靜態構造函數在爲非託管代碼建立包裝類時也頗有用，此時該構造函數能夠調用 LoadLibrary 方法。

• 若是靜態構造函數引起異常，運行時將不會再次調用該構造函數，而且在程序運行所在的應用程序域的生存期內，類型將保持未初始化。

在此示例中，類 Bus 有一個靜態構造函數。建立 Bus 的第一個實例（bus1）時，將調用該靜態構造函數來初始化該類。輸出示例驗證了即便建立 Bus 的兩個實例，該靜態構造函數也僅運行一次，而且在實例構造函數運行以前運行。

C#

 

 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.      
*/