靜態建構函式（C# 編程指南）

Visual Studio 2010  其他版本 此主題尚未評級 評價此主題 

 

靜態建構函式用於初始化任何 靜態 資料，或用於執行僅需執行一次的特定操作。 在建立第一個執行個體或引用任何靜態成員之前，將自動調用靜態建構函式。

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;    }}

靜態建構函式具有以下特點：

• 靜態建構函式既沒有存取修飾詞，也沒有參數。

• 在建立第一個執行個體或引用任何靜態成員之前，將自動調用靜態建構函式來初始化類。

• 無法直接調用靜態建構函式。

• 在程式中，使用者無法控制何時執行靜態建構函式。

• 靜態建構函式的典型用途是：當類使用記錄檔時，將使用這種建構函式向記錄檔中寫入項。

• 靜態建構函式在為Unmanaged 程式碼建立封裝類時也很有用，此時該建構函式可以調用 LoadLibrary 方法。

• 如果靜態建構函式引發異常，運行時將不會再次調用該建構函式，並且在程式運行所在的應用程式定義域的生存期內，類型將保持未初始化。

樣本

在此樣本中，類 Bus 有一個靜態建構函式。 建立 Bus 的第一個執行個體（bus1）時，將調用該靜態建構函式來初始化該類。 輸出樣本驗證了即使建立 Bus 的兩個執行個體，該靜態建構函式也僅運行一次，並且在執行個體建構函式運行之前運行。

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.         */

請參見

參考類和結構（C# 編程指南）建構函式（C# 編程指南）解構函式（C# 編程指南）概念C# 編程指南