Core源码（三） Lazy<T>

悠悠 2023-08-17 15:42 90阅读 0赞

### Lazy<T>解决什么问题？ ###

1、大对象加载

考虑下面的需求，有个对象很大，创建耗时，并且要在托管堆上分配一大块空间。我们当然希望，用到它的时候再去创建。也就是延迟加载，等到真正需要它的时候，才去加载。

显然，这里需要加一个中间层，将大对象封装起来，暴露接口，开始并不创建大对象，等到用户真正访问对象的时候，再去创建。另外，这个中间层应该可以封装不同类型的大对象，因此需要类模版。Lazy<T>就是为了解决这个问题。

典型的使用

public Lazy<AccountService> AccountServ = new Lazy<AccountService>();
    public Lazy<ProductService> ProductService = new Lazy<ProductService>();

2、将委托或者方法对象保存，并在需要的时候调用。

private readonly Lazy<IDbConnection> _connectionLazy; public CallHistoryRepository(ConnectionFactory connectionFactory) { _connectionLazy = new Lazy<IDbConnection>(()=>connectionFactory.Connection); }

一旦使用.Vale，那么对应的变量就会被实例化，IsValueCreated属性也就变成了true。

### 实现自己的Lazy<T> ###

在.NET Framework 4.0之前，大对象就是存在的，那么对于一个大型系统而言，怎么样对付一个大对象呢。主要有两点：**延迟加载**和**即时清理**。前者解决创建问题，后者解决回收问题。

那么在来看Lazy<T>的.NET Framework实现之前，我们先来自己实现一个简单的Lazy<T>吧。

class MyLazy<T> where T : new()
    {
        private T value;
        private bool isLoaded;
        public MyLazy()
        {
            isLoaded = false;
        }
        public T Value
        {
            get
            {
                if (!isLoaded)
                {
                    value = new T();
                    isLoaded = true;
                }
                return value;
            }
        }
    }

这应该是最简单版本的Lazy<T>了，**没有线程安全检测**，只有着访问时创建真实对象，可是对于我们一般的应用来说也许就已经足够了。

### .NET Lazy<T> 实现 ###

.NET Core和我们的实现，有两点主要的不同：

#### 1、 引入了Boxed内部类： ####

[Serializable]
    private class Boxed
    {
        // Fields
        internal T m_value;
    
        // Methods
        [TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
        internal Boxed(T value)
        {
            this.m_value = value;
        }
    }

该内部类取代了我在上面实现中的泛型约束，使之更通用。

但是我们也应该注意到，如果T为结构体，那么由于T很大，所以装箱拆箱反而也许是个更耗费效率的事情，因此，个人建议，**对值类型慎用Lazy<T>**。

#### 2、 线程安全的控制 ####

在线程安全的控制选项中，.NET Framework为我们提供了这样的枚举选项：

public enum LazyThreadSafetyMode
    {
        None,
        PublicationOnly,
        ExecutionAndPublication
    }

默认值为ExecutionAndPublication

枚举选项MSDN介绍如下

[http://msdn.microsoft.com/en-us/library/system.threading.lazythreadsafetymode%28VS.100%29.aspx][http_msdn.microsoft.com_en-us_library_system.threading.lazythreadsafetymode_28VS.100_29.aspx]

isThreadSafe则应用于多线程环境下，**如果isThreadSafe为false，那么延迟加载对象则一次只能创建于一个线程。**

### Lazy<T>源码 ###

System.Runtime命名空间下

由于core中的lazy源码我只找到了下图这个，再往下lazy的实现并没找

![1139419-20190917154831692-1051932416.png][]

所以我使用.Net 4.5的lazy源码

![1139419-20190917154308306-1584512224.png][]

### 一、最常使用的属性Value ###

![ContractedBlock.gif][] ![ExpandedBlockStart.gif][]

[DebuggerBrowsable(DebuggerBrowsableState.Never)]
    public T Value
    {
        get
        {
            Boxed boxed = null;
            if (m_boxed != null )
            {
                // Do a quick check up front for the fast path.
                boxed = m_boxed as Boxed;
                if (boxed != null)
                {
                    return boxed.m_value;
                }
                LazyInternalExceptionHolder exc = m_boxed as LazyInternalExceptionHolder;
                Contract.Assert(m_boxed != null);
                exc.m_edi.Throw();
            }
            return LazyInitValue();
        }
    }
    //null --> value is not created
    //m_value is Boxed --> the value is created, and m_value holds the value
    //m_value is LazyExceptionHolder --> it holds an exception
    private object m_boxed;

如果m\_boxed有值，就直接装箱返回对应值（这里就要注意值类型装箱的性能损失了）。这个装箱是为了检验下m\_boxed的值，因为其中有可能是异常。

### 二、LazyInitValue方法 ###

如果m\_boxed为空，就调用LazyInitValue方法，这里有针对线程安全模式的判断

![ContractedBlock.gif][] ![ExpandedBlockStart.gif][]

/// <summary>
    /// local helper method to initialize the value 
    /// </summary>
    /// <returns>The inititialized T value</returns>
    private T LazyInitValue()
    {
        Boxed boxed = null;
        LazyThreadSafetyMode mode = Mode;
        if (mode == LazyThreadSafetyMode.None)
        {
            boxed = CreateValue();
            m_boxed = boxed;
        }
        else if (mode == LazyThreadSafetyMode.PublicationOnly)
        {
            boxed = CreateValue();
            if (boxed == null ||
                Interlocked.CompareExchange(ref m_boxed, boxed, null) != null)
            {
                // If CreateValue returns null, it means another thread successfully invoked the value factory
                // and stored the result, so we should just take what was stored.  If CreateValue returns non-null
                // but we lose the ---- to store the single value, again we should just take what was stored.
                boxed = (Boxed)m_boxed;
            }
            else
            {
                // We successfully created and stored the value.  At this point, the value factory delegate is
                // no longer needed, and we don't want to hold onto its resources.
                m_valueFactory = ALREADY_INVOKED_SENTINEL;
            }
        }
        else
        {
            object threadSafeObj = Volatile.Read(ref m_threadSafeObj);
            bool lockTaken = false;
            try
            {
                if (threadSafeObj != (object)ALREADY_INVOKED_SENTINEL)
                    Monitor.Enter(threadSafeObj, ref lockTaken);
                else
                    Contract.Assert(m_boxed != null);
    
                if (m_boxed == null)
                {
                    boxed = CreateValue();
                    m_boxed = boxed;
                    Volatile.Write(ref m_threadSafeObj, ALREADY_INVOKED_SENTINEL);
                }
                else // got the lock but the value is not null anymore, check if it is created by another thread or faulted and throw if so
                {
                    boxed = m_boxed as Boxed;
                    if (boxed == null) // it is not Boxed, so it is a LazyInternalExceptionHolder
                    {
                        LazyInternalExceptionHolder exHolder = m_boxed as LazyInternalExceptionHolder;
                        Contract.Assert(exHolder != null);
                        exHolder.m_edi.Throw();
                    }
                }
            }
            finally
            {
                if (lockTaken)
                    Monitor.Exit(threadSafeObj);
            }
        }
        Contract.Assert(boxed != null);
        return boxed.m_value;
    }

### 三、CreateValue方法 ###

返回一个实例对象T，采用传入的func方法，这里会对是否已经返回做出判断，如果已经返回，就返回null

![ContractedBlock.gif][] ![ExpandedBlockStart.gif][]

/// <summary>Creates an instance of T using m_valueFactory in case its not null or use reflection to create a new T()</summary>
    /// <returns>An instance of Boxed.</returns>
    private Boxed CreateValue()
    {
        Boxed boxed = null;
        LazyThreadSafetyMode mode = Mode;
        if (m_valueFactory != null)
        {
            try
            {
                // check for recursion
                if (mode != LazyThreadSafetyMode.PublicationOnly && m_valueFactory == ALREADY_INVOKED_SENTINEL)
                    throw new InvalidOperationException(Environment.GetResourceString("Lazy_Value_RecursiveCallsToValue"));
                Func<T> factory = m_valueFactory;
                if (mode != LazyThreadSafetyMode.PublicationOnly) // only detect recursion on None and ExecutionAndPublication modes
                {
                    m_valueFactory = ALREADY_INVOKED_SENTINEL;
                }
                else if (factory == ALREADY_INVOKED_SENTINEL)
                {
                    // Another thread ----d with us and beat us to successfully invoke the factory.
                    return null;
                }
                boxed = new Boxed(factory());
            }
            catch (Exception ex)
            {
                if (mode != LazyThreadSafetyMode.PublicationOnly) // don't cache the exception for PublicationOnly mode
                    m_boxed = new LazyInternalExceptionHolder(ex);
                throw;
            }
        }
        else
        {
            try
            {
                boxed = new Boxed((T)Activator.CreateInstance(typeof(T)));
    
            }
            catch (System.MissingMethodException)
            {
                Exception ex = new System.MissingMemberException(Environment.GetResourceString("Lazy_CreateValue_NoParameterlessCtorForT"));
                if (mode != LazyThreadSafetyMode.PublicationOnly) // don't cache the exception for PublicationOnly mode
                    m_boxed = new LazyInternalExceptionHolder(ex);
                throw ex;
            }
        }
        return boxed;
    }

转载于:https://www.cnblogs.com/qixinbo/p/11534379.html

[http_msdn.microsoft.com_en-us_library_system.threading.lazythreadsafetymode_28VS.100_29.aspx]: http://msdn.microsoft.com/en-us/library/system.threading.lazythreadsafetymode%28VS.100%29.aspx
[1139419-20190917154831692-1051932416.png]: /images/20230810/70b34f7781484dd885e889e642c45209.png
[1139419-20190917154308306-1584512224.png]: /images/20230810/1ee9cf3720ff4159ba64063a2bb0241d.png
[ContractedBlock.gif]: https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif
[ExpandedBlockStart.gif]: /images/20230810/739b1272e2354ebe92182995b866ea20.png