I have a class shown below. Server instance holds reference to listener instance. Listener holds reference to server instance through event delegate. Will this prevent GC from collecting server instance? If so, how to break this cycle? Should I implement IDisposable or override Finalize method or do something else?
public class Server
{
public Listener Listener { get; private set; }
public Server(Listener listener)
{
Listener = listener;
Listener.ClientChannelConnected += new EventHandler<ClientChannelConnectedArgs>(listener_ClientChannelConnected);
}
void listener_ClientChannelConnected(object sender, ClientChannelConnectedArgs e)
{
...
}
}
No. The .NET garbage collector is smart enough to resolve circular references.
Strangely, I found that this type of code consistently leaks in .net, and had to be careful to -= the event handler. It is true that the memory is released eventually, but in practice you want to clean up after yourself sooner than that.
Also, never ever use a lambda expression to handle an event, as you will never have a reference back to that object.
Read this article about memory leaks. Sometimes you should use -=.
Related
The Problem:
I am using asynchronous event handling in a class that invokes an event about 60 times per second. If I run the software for several hours I find that it eventually gets to a state where the event stops invoking the assigned delegate without seeing any exceptions or user input.
Here is the distilled version of the event code I am using:
class DataSource
{
public event EventHandler<DataReadyEventArgs> DataReady;
private void OnDataReady()
{
EventHandler<DataReadyEventArgs> evt = DataReady;
if (evt != null)
{
Trace.WriteLine("Invoking DataReady...");
DataReadyEventArgs args = new DataReadyEventArgs();
evt.BeginInvoke(this, args, new AsyncCallback(DataReadyHandled), null);
}
}
private void DataReadyHandled(IAsyncResult result)
{
AsyncResult aresult = result as AsyncResult;
EventHandler<DataReadyEventArgs> evt = aresult.AsyncDelegate as EventHandler<DataReadyEventArgs>;
if (evt != null)
{
try
{
evt.EndInvoke(result);
Trace.WriteLine("DataReady was handled.");
}
catch
{ }
}
}
}
class DataConsumer
{
public DataConsumer(DataSource src)
{
src.DataReady += HandleDataReady;
}
public void HandleDataReady(object sender, DataReadyEventArgs e)
{
Trace.WriteLine("Handling DataReady");
}
}
To avoid writing out further code to give the full picture, you can also make the following assumptions:
There is always a strong reference to the DataSource instance so it is not being garbage collected while the DataConsumer is expecting an event
The DataConsumer attaches its handler method once and does not remove it until the instance is garbage collected.
Here is the weird part; When I attach a debugger to the software in its broken state, I can see that the evt variable in OnDataReady() references an EventHandler instance and is not null. It actually calls BeginInvoke but the HandleDataReady method is never entered. The debugger shows that the EventHandler base class's Method property is null though. I can't seem to figure out how it is possible that the EventHandler is being changed.
The (First) Question:
What can cause the event in the above code to stop being invoked after several hours of runtime?
My Best Guess:
Could something completely unrelated in my application be corrupting the memory for the EventHandler? I am using both native and managed c++ libraries.
Update:
My best guess turns out to be unlikely, as I have run some other tests that make me reasonably sure the native libraries are not leaking or corrupting memory.
After reproducing the problem another time with logging, I can see that the event is not stopping entirely but being invoked on a ~19 hour interval! Obviously there is a long-running operation or blocking somewhere in the handler delegate.
To be certain that it's my code and not .NET, I have an updated question...
The (Second) Question:
Is there any behaviour built in to .NET (4.5) or its garbage collector that can block invocation of an EventHandler?
I'm working on a database and I need to write the database to file when the database class is destroyed along with every else when the form is closed
This is how I'm currently calling it:
class database: IDisposable
{
List<databaseEntry> dDatabase;
public database()
{
dDatabase = new List<databaseEntry>;
}
protected virtual void Dispose(bool disposing)
{
if (!this.disposed)
{
StreamWriter sw = new StreamWriter(path);
string toWrite;
foreach (databaseEntry dE in dDatabase)
{
toWrite = dE.rfid.ToString() + " " + dE.currentArea.ToString() + " " + dE.itemName;
sw.WriteLine(toWrite);
}
sw.Close();
disposed = true;
}
}//destructor for database (saves database to file)
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
}
The Dispose method isn't called when I close the windows form that this class is open in.
This is for an assignment and I'm not allowed to use SQL for this.
You do not have control over when the Dispose method is called by the garbage collector. If you want to dispose resources when the form is closed, you should manually call the Dispose method when the form is closed.
This is achieved by listening to the FormClosed event of the form.
So you would do something similar to this:
Create this method:
private void Form1_FormClosed(Object sender, FormClosedEventArgs e)
{
if(this.database != null)
this.database.Dispose();
}
Put this line in your form's constructor:
this.FormClosed += Form1_FormClosed;
Since you're implementing IDisposable, another solution would be to wrap your database object instantiation in a using statement - this would automatically call Dispose once the code in the using statement completes.
Syntax for your use case is as follows:
using(database db = new database())
{
//use your db object
} //db is disposed here
Edit:
Matthew Watson’s comment made me realize my answer is over simplified and could be confusing (thanks for the comment Matthew). Here’s (most of) the real story:
All objects have a constructor and a destructor. If these are not specified the default constructor and destructor are used.
Constructor:
ClassName()
Destructor:
~ClassName()
The Constructor is called when you use the new keyword. The Destructor can be called once there is no longer a reference to that specific instance of an object. Notice I said can be called, not is called. Meaning at this point the object is eligible for garbage collection and could be freed when the garage collector runs. I will not go into detail here but just know that the garage collector does not always collect all the unreachable objects for performance reasons. This is one of the key reasons you would want to the IDisposable interface. You can immediately clean up resources when you know they are no longer being used vs. waiting for the GC to clean up the resources. When working with streams or database connections it is a good idea to implement the IDisposable interface.
With that said here is the updated answer (replacing the previous "default Dispose()" references with the destructor reference).
Since this is an assignment I am not going to give you the answer per se, but I'll get you close. The destructor (~database) is normally called by the garage collector when an instance of a database object is no longer reachable. By implementing the IDisposable interface you have provided an alternative to allowing the garage collector to dispose of the object. You are telling the garage collector "I know when and how these resources need to be freed" vs. free these resources when there are no longer references to the objects. But that only applies if you call your Dispose method; more specifically the line:
Dispose(true);
GC.SuppressFinalize(this);
This means "Garage collector, do not call the destructor (~database()) method, I have already handled it".
What you should do in this method is close any streams (StreamWriter) you have open.
Usage: The IDisposable interface has a nice construct in C# the using statement. This is in effect a try catch block where the Dispose method is called in the finally block. StreamWriter also implements IDisposable so you should think about implementing the using statement in your database class. Example:
using(StreamWriter writer = new StreamWriter()){
//do stuff with writer here
}
This is the pattern you should use for your database class in your form. I will let you figure out how to code that up.
I see this piece of code written by someone else:
public class DynamicPageContent : IHttpModule
{
public void Dispose() { }
public void Init(HttpApplication context)
{
// it is necessary to
context.BeginRequest += new EventHandler(DynamicPageContent_BeginRequest);
}
//actual handler not pasting as it's meaningless for this question
}
As you can see the handler is registered but never deregistered. Wouldn't that create memory leak?
Event handling can create leaks but only under certain conditions.
The thing to watch out for is when the subscribing object has a larger lifespan than the publishing object.
The thing to watch out for is when the publishing object has a larger lifespan than the subscribing object.
In this case they appear to be the same object and then it is not necessary or useful to unsubscribe. That would only clutter your code.
I have a static class which publishes a few events. So I have many different small classes that have different lifetimes that subscribe to this event.
I found out now that this leads to memory "leaks" because of the subscribing classes staying alive when they subscribed a longer-living event. I know that this is happening by using a memoryprofiler and I read about this problem.
I am not able to manually unsubscribe, as I might have hundreds of "clients" in a list. This list will just get cleared. So I can't (and don't want to) unsubscribe by hand.
I read that the "weak event pattern" might help here. Could someone please lead me to an "easy" way to implement this? All I found until now is either too simple to use in practice or too complicated to understand it in the beginning.
Or is there any "best practice" for this case?
Thanks in advance!
UPDATE:
Based on jbl's answer I found this (http://blogs.msdn.com/b/greg_schechter/archive/2004/05/27/143605.aspx) as a possible solution. Any comments here? It's rather old (2004), so there might be better solutions out there?
Never implemented something like that, but I would try (with a static class or a singleton, your choice) :
having the static class maintain a static collection of WeakReference to the client event handlers
the clients do not subscribe directly to the event. The static class exposes subscribe and unsubscribe methods which add/remove the handlers from the weak references collection
the static class is the only one subscribing directly to the event
upon event triggering, the static class enumerates the weak references collection and runs the handlers for the references which are still alive (removing the null ones)
Hope this will help
Best practice: always implement the Dispose pattern when a class subscribes to an event generated by an object not constructed by this class.
Then in the Dispose method remove the handler.
public NotificationServiceAccessor(ObjectWithEvent objectWithEvent)
{
_notificationService = new NotificationService();
_notificationService.StatusChanged += NotificationService_StatusChanged; // Local object, no Dipose
_objectWithEvent = objectWithEvent;
_objectWithEvent.AnEvent += AnEventHandler(); // Event that has to be disposed.
}
#region IDisposable Members
protected bool Disposed { get; private set; }
private void Dispose(bool disposing)
{
if (!this.Disposed)
{
this.InternalDispose(disposing);
}
this.Disposed = true;
}
protected virtual void InternalDispose(bool disposing)
{
if (disposing)
{
// Dispose here the event handlers
_objectWithEvent.AnEvent -= AnEventHandler()
}
// Dispose here only unmanaged objects
// Don’t use managed objects here because maybe
// they have been finalized already
}
public void Dispose()
{
this.Dispose(true);
GC.SuppressFinalize(this);
}
~NotificationServiceAccessor()
{
this.Dispose(false);
}
#endregion
Does the following psuedo code accomplish my goal of cleaning up after myself when my DLL is being hosted by code I don't control?
More specifically, how do I clean up my objects created in my static constructor?
Do I need to suppress Finalize in the Disposable?
Am I guaranteed that the compiler or something will call IDisposable even if the Host doesn't?
Psuedo code:
public class MyDLL : SomeHostICantControl, IDisposable
{
public SpinLock MyDictionarySpinlock = new Spinlock; // approximate syntax
public static Dictionary<string, string> MyDictionary = null;
public static Timer MyCleanUpTimer = null;
public static MyDLL()
{
// Set up Cache Here ... how do I dispose of it?
MyDictionary = new Dictionary<string, string>();
// remove old data from the dictionary, use spinlock to sync writes
// or use concurrent dictionary in .NET 4
MyCleanUpTimer = new Timer(); // run every hour
}
// many instances will be created and disposed of. I have no control when or how often
public void MyDll()
{
this.MyHostEvent += New Event Handler....
}
//.. some event handler code here
public void Dispose()
{
this.MyHostEvent -= Event Handler.;
// Do I need to suppressFinalize here?
}
//~MyDll()
//{
// Is this the right place to clean up after my constuctor?
//}
}
Answering your questions in order:
Static fields exist for the lifetime of the application, as such they are "cleaned up" as a result of the application exiting (memory is reclaimed, files are closed etc.). You don't appear to be doing anything that might require explicit action to be taken to clean-up (e.g. flushing buffered data in a StreamWriter to file), but perhaps there are details missing in your code snippet.
You need to suppress finalize in your Dispose() method if your class has a finalizer (yours appears not to as it's commented out), or if someone can derive from your class and may introduce unmanaged resources that may need cleaning up. The latter applies here as your class is not marked sealed, so you should suppress finalize in Dispose().
The runtime will not call Dispose(), however it will call the finalizer if one is present. Note however that as you class instances don't appear to be using any unmanaged resources, a finalizer should not be required.