靜態建構函式 (C# 程式設計手冊)
靜態建構函式可以用來初始化任何靜態資料，或執行只需執行一次的特定動作。 在建立第一個執行個體或參考任何靜態成員之前，會自動呼叫靜態建構函式。
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;
}
}
靜態建構函式有下列屬性：
靜態建構函式並不使用存取修飾詞，也沒有參數。
在建立第一個執行個體或參考任何靜態成員之前，就會自動呼叫靜態建構函式以初始化類別。
不能直接呼叫靜態建構函式。
使用者無法控制程式中靜態建構函式執行的時間。
靜態建構函式通常用在當類別使用記錄檔，而建構函式被用來將項目寫入該檔案。
當靜態建構函式可呼叫 LoadLibrary 方法時，也可以使用這種建構函式為 Unmanaged 程式碼建立包裝函式類別。
如果靜態建構函式擲回例外狀況，執行階段將不會再一次叫用它，且在您的程式執行的應用程式定義域存留期中，型別都將保持未初始化狀態。
範例
在這個範例中，類別 Bus 有一個靜態建構函式。 建立 Bus 的第一個執行個體 (bus1) 時，會呼叫此建構函式來初始化該類別。 這個範例輸出確認靜態建構函式只會執行一次，即使建立了兩個 Bus 的執行個體也是如此，並且確認該建構函式會在執行個體建構函式執行之前執行。
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.
*/