public class LazySinleton
{
    private LazySingleton()
    {}
 
    public static LazySingleton Instance
    {
        get
        {
            return Lazy.data;
        }
    }
 
    private class Lazy
    {
        static Lazy()
        {}
 
        internal static readonly LazySingleton data = new LazySingleton();
    }
}

以上

通過私有構(gòu)造函數(shù)屏蔽了LazySingleton類通過構(gòu)造函數(shù)創(chuàng)建的方式
私有嵌套類Lazy的data字段負(fù)責(zé)提供一個LazySingleton的實例
只能通過LazySingleton的屬性Instance，才能拿到內(nèi)部類Lazy.data所代表的實例

Lazy<T>實例

先看Lazy<T>的定義：

public class Lazy<T>
{
    public Lazy();
    public Lazy(bool isThreadSafe);
    public Lazy(Func<T> valueFactory);
    public Lazy(LazyThreadSafeMode mode);
    public Lazy(Func<T> valueFactory, bool isThreadSafe);
    public Lazy(Funct<T> valueFactory, LazyThreadSafetyMode mode);

    public bool IsValueCreated{get;}
    public T Value {get;}
    public override string ToStirng();
}

通過Lazy<T>的構(gòu)造函數(shù)重載創(chuàng)建對象，再通過體現(xiàn)延遲加載的Value屬性來實現(xiàn)對象的創(chuàng)建，并獲取對象實例。

public class SomeClass
{
    public int ID{get;set;}
}

Lazy<SomeClass> temp = new Lazy<SomeClass>();
Console.WriteLine(temp.Value.ID);

以上，只適用于沒有構(gòu)造函數(shù)的情況，如果有構(gòu)造函數(shù)如何處理呢？
--使用public Lazy(Func<T> valueFactory)，通過委托創(chuàng)建對象

pubic class SomeClass
{
    public int ID{get;set;}
    public SomeClass(int id)
    {
        this.ID = id;
    }
}

Lazy<SomeClass> temp = new Lazy<SomeClass>(() => new Big(100));
Console.WriteLine(temp.Value.ID);

延遲加載的本質(zhì)

創(chuàng)建自定義延遲加載類。

    public class MyLazy<T>
    {
        private volatile object boxed; //volatile說明在多線程狀況下，也可以修改該字段
        private Func<T> valueFactory; //委托，用來生產(chǎn)T對象實例

        static MyLazy(){}
        public MyLazy(){}

        public MyLazy(Func<T> valueFactory)
        {
            this.valueFactory = valueFactory;
        }

        public T Value
        {
            get
            {
                Boxed boxed = null;
                if (this.boxed != null)
                {
                    boxed = this.boxed as Boxed;
                    if (boxed != null)
                    {
                        return boxed.value;
                    }
                }
                return this.Init();
            }
        }

        //初始化對象實例
        private T Init()
        {
            Boxed boxed = null;
            if (this.boxed == null)
            {
                boxed = this.CreateValue();
                this.boxed = boxed;
            }
            return boxed.value;
        }

        //創(chuàng)建內(nèi)部類實例
        private Boxed CreateValue()
        {
            //如果創(chuàng)建對象實例的委托valueFactory存在
            if (this.valueFactory != null)
            {
                //就通過委托生成對象實例
                return new Boxed(this.valueFactory());
            }
            else
            {
                //否則，通過反射生成對象實例
                return new Boxed((T)Activator.CreateInstance(typeof(T)));
            }
        }

        //內(nèi)部嵌套類，通過構(gòu)造函數(shù)對其字段賦值
        private class Boxed
        {
            internal T value;
            internal Boxed(T value)
            {
                this.value = value;
            }
        }
    }

自定義帶構(gòu)造函數(shù)的類。

    public class Big
    {
        public int ID { get; set; }
        public Big(int id)
        {
            this.ID = id;
        }
    }

自定義創(chuàng)建對象實例的工廠類。

    public class BigFactory
    {
        public static Big Build()
        {
            return new Big(10);
        }
    }

客戶端調(diào)用。

    class Program
    {
        static void Main(string[] args)
        {
            MyLazy<Big> temp = new MyLazy<Big>(() => BigFactory.Build());
            Console.WriteLine(temp.Value.ID);
            Console.ReadKey();
        }
    }