C#的单例模式实现
只能生成一个实例的类是实现了Singleton(单例)模式的类。以下为C#实现单例模式的方式。 方式一只使用于单线程环境 // 把构造函数设为私有函数以禁止他人创建实例 // 定义一个静态的实例在需要的时候创建该实例 // 在Singlrton的静态属性Instance中只有在instance为null的时候才创建一个实例以避免 // 重复创建 // 把构造函数定义为私有函数 public sealed class Singleton1 { public int a = 2; private Singleton1() { } private static Singleton1 instance = null; public static Singleton1 Instance { get { if (instance == null) instance = new Singleton1(); return instance; } } } 方式二虽然在多线程环境中能工作但效率不高 // 每次通过属性Instance得到Singleton2的实例都会试图加上一个同步锁 // 而加锁是一个非常耗时的操作在没有必要的时候应该尽量避免 public sealed class Singleton2 { public int a = 2; private Singleton2(){} private static readonly object syncObj = new object(); private static Singleton2 instance = null; public static Singleton2 Instance { get { lock (syncObj) { if (instance == null) instance = new Singleton2(); } return instance; } } } 可行的解法 加同步锁前后两次判断实例是否已存在 // 只有instance为null即没有创建时需要加锁操作。 public sealed class Singleton3 { private Singleton3() { } private static readonly Object syncObj = new Object(); private static Singleton3 instance = null; public static Singleton3 Instance { get { if(instance == null) { lock(syncObj) { if(instance == null) instance = new Singleton3(); } } return instance; } } } 推荐的解法一利用静态构造函数 // 在初始化静态变量instance的时候创建一个实例 // 由于C#是在调用静态构造函数时初始化静态变量.NET运行时能够确保只调用一次静态构造 // 函数保证只初始化一次instance public sealed class Singleton4 { private Singleton4() { } private static Singleton4 instance = new Singleton4(); public static Singleton4 Instance { get { return instance; } } } 推荐的解法二 实现按需创建实例 // 在内部定义了一个私有类型Nested。 // 当第一次用到这个嵌套类的时候会调用静态构造函数创建Singleton5的实例instance public sealed class Singleton5 { private Singleton5() { } public static Singleton5 Instance { get { return Nested.instance; } } class Nested { static Nested() { } internal static readonly Singleton5 instance = new Singleton5(); } } 扩展 定义一个表示总统的类型President可以从该类型继承出FrenchPresident 和AmericanPresident等类型。这些派生类型都只能产生一个实例 public class President { private string name = ""; private string country = ""; public President() { } public string Name { get { return name; } set { name = value; } } public string Country { get { return country; } set { country = value; } } } public sealed class FrenchPresident: President { private FrenchPresident():base() { } private static FrenchPresident instance = new FrenchPresident(); public static FrenchPresident Instance { get { return (FrenchPresident)(Nested.instance); } } private class Nested { static Nested() { } internal static readonly FrenchPresident instance = new FrenchPresident(); } } public sealed class AmericanPresident : President { private AmericanPresident() : base() { } private static AmericanPresident instance = new AmericanPresident(); public static AmericanPresident Instance { get { return Nested.instance; } } private class Nested { static Nested() { } internal static readonly AmericanPresident instance = new AmericanPresident(); } } 实现泛型单例模式 public class SingletonExamplewhere T : class, new() { public static T Instance { get { return Nested.instance; } } private class Nested { static Nested() { } internal static readonly T instance = new T(); } } public class Two: SingletonExample { public int a = 2; public void Show() { Console.WriteLine(a); } }
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