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
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
所以我使用.Net 4.5的lazy源码
一、最常使用的属性Value
[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
/// <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
/// <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;
}
原文地址：https://www.cnblogs.com/qixinbo/p/11534379.html
时间： 2024-10-03 22:29:04