只能生成一個(gè)實(shí)例的類是實(shí)現(xiàn)了Singleton(單例)模式的類。以下為C#實(shí)現(xiàn)單例模式的方式。
    
方式一只使用于單線程環(huán)境
    // 把構(gòu)造函數(shù)設(shè)為私有函數(shù)以禁止他人創(chuàng)建實(shí)例
    // 定義一個(gè)靜態(tài)的實(shí)例在需要的時(shí)候創(chuàng)建該實(shí)例
    // 在Singlrton的靜態(tài)屬性Instance中只有在instance為null的時(shí)候才創(chuàng)建一個(gè)實(shí)例以避免
    // 重復(fù)創(chuàng)建
    // 把構(gòu)造函數(shù)定義為私有函數(shù)
    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;            }        }    }     方式二雖然在多線程環(huán)境中能工作但效率不高     
// 每次通過屬性Instance得到Singleton2的實(shí)例都會試圖加上一個(gè)同步鎖
    // 而加鎖是一個(gè)非常耗時(shí)的操作在沒有必要的時(shí)候應(yīng)該盡量避免
    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 加同步鎖前后兩次判斷實(shí)例是否已存在        
// 只有instance為null即沒有創(chuàng)建時(shí)需要加鎖操作。
    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;
            }
        }
    }
        推薦的解法一利用靜態(tài)構(gòu)造函數(shù)     
   // 在初始化靜態(tài)變量instance的時(shí)候創(chuàng)建一個(gè)實(shí)例
    // 由于C#是在調(diào)用靜態(tài)構(gòu)造函數(shù)時(shí)初始化靜態(tài)變量.NET運(yùn)行時(shí)能夠確保只調(diào)用一次靜態(tài)構(gòu)造
    // 函數(shù)保證只初始化一次instance
    public sealed class Singleton4
    {
        private Singleton4() { }
        private static Singleton4 instance = new Singleton4();
        public static Singleton4 Instance
        {
            get
            {
                return instance;
            }
        }
    }
        推薦的解法二 實(shí)現(xiàn)按需創(chuàng)建實(shí)例        
// 在內(nèi)部定義了一個(gè)私有類型Nested。
    // 當(dāng)?shù)谝淮斡玫竭@個(gè)嵌套類的時(shí)候會調(diào)用靜態(tài)構(gòu)造函數(shù)創(chuàng)建Singleton5的實(shí)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();
        }
    }
        擴(kuò)展 定義一個(gè)表示總統(tǒng)的類型President可以從該類型繼承出FrenchPresident    和AmericanPresident等類型。這些派生類型都只能產(chǎn)生一個(gè)實(shí)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();
        }
    }
        實(shí)現(xiàn)泛型單例模式        
public class SingletonExample where 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);
        }
    }