Chapter162:GarbageCollectorin.Net

Section162.1:WeakReferences

In .NET, the GC allocates objects when there are no references left to them. Therefore, while an object can still be reachedfromcode(thereisastrongreferencetoit),theGCwillnotallocatethisobject.Thiscanbecomeaproblem if there are a lot of large objects.

Aweakreferenceisareference,thatallowstheGCtocollecttheobjectwhilestillallowingtoaccesstheobject.A weakreferenceisvalidonlyduringtheindeterminateamountoftimeuntiltheobjectiscollectedwhennostrong referencesexist.Whenyouuseaweakreference,theapplicationcanstillobtainastrongreferencetotheobject, whichpreventsitfrombeingcollected.Soweakreferencescanbeusefulforholdingontolargeobjectsthatare expensivetoinitialize,butshouldbeavailableforgarbagecollectioniftheyarenotactivelyinuse.

Simpleusage:

WeakReferencereference=newWeakReference(newobject(),false); GC.Collect();

objecttarget=reference.Target;

if(target!=null)

DoSomething(target);

So weak references could be used to maintain, for example, a cache of objects. However, it is important to remember that there is always the risk that the garbage collector will get to the object before a strong reference is reestablished.

Weakreferencesarealsohandyforavoidingmemoryleaks.Atypicalusecaseiswithevents.

Supposewehavesomehandlertoaneventonasource:

Source.Event+=newEventHandler(Handler)

This code registers an event handler and creates a strong reference from the event source to the listening object. If the source object has a longer lifetime than the listener, and the listener doesn’t need the event anymore when there are no other references to it, using normal .NET events causes a memory leak: the source object holds listener objects in memory that should be garbage collected.

Inthiscase,itmaybeagoodideaistousetheWeakEventPattern.

Somethinglike:

publicstaticclassWeakEventManager

{

publicstaticvoidSetHandler<S,TArgs>(

Action<EventHandler<TArgs>>add,

Action<EventHandler<TArgs>>remove,

Ssubscriber,

Action<S,TArgs>action)

whereTArgs:EventArgs

whereS:class

{

varsubscrWeakRef=newWeakReference(subscriber);

EventHandler<TArgs>handler=null;

handler=(s,e)=>

{

varsubscrStrongRef=subscrWeakRef.TargetasS; if

(subscrStrongRef!=null)

{

action(subscrStrongRef,e);

}

else

{

remove(handler);

handler=null;

}

};

add(handler);

}

}

andusedlikethis:

EventSources=newEventSource();

Subscribersubscriber=newSubscriber();

WeakEventManager.SetHandler<Subscriber,SomeEventArgs>(a=>s.Event+=a,r=>s.Event-=r,

subscriber,(s,e)=>{s.HandleEvent(e); });

Inthiscaseofcoursewehavesomerestrictions-theeventmustbea

publiceventEventHandler<SomeEventArgs>Event;

AsMSDNsuggests:

• Uselongweakreferencesonlywhennecessaryasthestateoftheobjectisunpredictableafterfinalization. Avoid using weak references to small objects because the pointer itself may be as large or larger.
• Avoid using weak references as an automatic solution to memory management problems. Instead, develop an effective caching policy for handling your application’s objects.

Section162.2:LargeObjectHeapcompaction

BydefaulttheLargeObjectHeapisnotcompactedunliketheclassicObjectHeapwhichcanleadtomemoryfragmentationand further, can lead to OutOfMemoryExceptions

Startingwith.NET4.5.1thereisanoptiontoexplicitlycompacttheLargeObjectHeap(alongwithagarbage collection):

GCSettings.LargeObjectHeapCompactionMode=GCLargeObjectHeapCompactionMode.CompactOnce; GC.Collect();

Justasanyexplicitgarbagecollectionrequest(it’scalledrequestbecausetheCLRisnotforcedtoconductit)use with care and by default avoid it if you can since it can de-calibrate GCs statistics, decreasing its performance.