私有構造函數(shù)
私有構造函數(shù)是一種特殊的實例構造函數(shù)。它通常用在只包含靜態(tài)成員的類中。如果類具有一個或多個私有構造函數(shù)而沒有公共構造函數(shù)，則其他類（除嵌套類外）無法創(chuàng)建該類的實例。例如：
class NLog{ // Private Constructor: private NLog() { } public static double e = Math.E; //2.71828...}聲明空構造函數(shù)可阻止自動生成默認構造函數(shù)。注意，如果您不對構造函數(shù)使用訪問修飾符，則在默認情況下它仍為私有構造函數(shù)。但是，通常顯式地使用 private 修飾符來清楚地表明該類不能被實例化。
當沒有實例字段或?qū)嵗椒ǎㄈ?Math 類）時或者當調(diào)用方法以獲得類的實例時，私有構造函數(shù)可用于阻止創(chuàng)建類的實例。如果類中的所有方法都是靜態(tài)的，可考慮使整個類成為靜態(tài)的。
下面是使用私有構造函數(shù)的類的示例。
public class Counter{ private Counter() { } public static int currentCount; public static int IncrementCount() { return ++currentCount; }}class TestCounter{ static void Main() { // If you uncomment the following statement, it will generate // an error because the constructor is inaccessible: // Counter aCounter = new Counter(); // Error Counter.currentCount = 100; Counter.IncrementCount(); Console.WriteLine("New count: {0}", Counter.currentCount); // Keep the console window open in debug mode. Console.WriteLine("Press any key to exit."); Console.ReadKey(); }}輸出：
New count: 101
注意，如果您取消注釋該示例中的以下語句，它將生成一個錯誤，因為該構造函數(shù)受其保護級別的限制而不可訪問：
// Counter aCounter = new Counter(); // Error
靜態(tài)構造函數(shù)
靜態(tài)構造函數(shù)用于初始化任何靜態(tài)數(shù)據(jù)，或用于執(zhí)行僅需執(zhí)行一次的特定操作。在創(chuàng)建第一個實例或引用任何靜態(tài)成員之前，將自動調(diào)用靜態(tài)構造函數(shù)。
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(tài)構造函數(shù)具有以下特點：
靜態(tài)構造函數(shù)的典型用途是：當類使用日志文件時，將使用這種構造函數(shù)向日志文件中寫入項。
靜態(tài)構造函數(shù)在為非托管代碼創(chuàng)建包裝類時也很有用，此時該構造函數(shù)可以調(diào)用 LoadLibrary 方法。
如果靜態(tài)構造函數(shù)引發(fā)異常，運行時將不會再次調(diào)用該構造函數(shù)，并且在程序運行所在的應用程序域的生存期內(nèi)，類型將保持未初始化。
在此示例中，類 Bus 有一個靜態(tài)構造函數(shù)。創(chuàng)建 Bus 的第一個實例（bus1）時，將調(diào)用該靜態(tài)構造函數(shù)來初始化該類。輸出示例驗證了即使創(chuàng)建 Bus 的兩個實例，該靜態(tài)構造函數(shù)也僅運行一次，并且在實例構造函數(shù)運行之前運行。
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(); } }輸出：
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.
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