I think this is a C# beginner question, but I can't seem to find a correct solution.
I have a ClassOne object, which defines an event.
I create a ClassTwo object, which is considered as a black box, which means I don't know whether it will register to any event or not. ClassTwo constructor registers to the event of ClassOne.
The problem comes, when ClassTwo object goes out of scope. The garbage collector never deletes this object, because it never deregistered the event.
So I have two questions:
Is there a way for ClassTwo object to know, when it goes out of scope? For an old C++ programmer this would be in the destructor, but with C# this doesn't work.
Is there a debug tool which helps me to find such objects?
Here is a sample code to reproduce the issue:
public partial class MainWindow : Window
{
static public ClassOne classOne = new ClassOne();
public MainWindow()
{
InitializeComponent();
ClassTwo classtwo = new ClassTwo();
}
private void buttonTest_Click(object sender, RoutedEventArgs e)
{
GC.Collect();
}
}
public class ClassOne
{
public ClassOne()
{
Trace.WriteLine(this + " constructor");
}
~ClassOne()
{
Trace.WriteLine(this + " destructor");
}
public delegate void UpdateFunc(object sender, EventArgs args);
public event UpdateFunc OnUpdate;
}
public class ClassTwo
{
public ClassTwo()
{
Trace.WriteLine(this + " constructor");
MainWindow.classOne.OnUpdate += new ClassOne.UpdateFunc(classOne_OnUpdate);
}
void classOne_OnUpdate(object sender, EventArgs args)
{
throw new NotImplementedException();
}
~ClassTwo()
{
Trace.WriteLine(this + " destructor");
}
}
I would implement IDisposable on an object like this and unregister from the event in the Dispose method.
You would use your object like this:
using(var two = new ClassTwo(classOne))
{
// Do something with two
}
// object can now be garbage collected.
If the caller fails to call Dispose, you are out of luck.
Not unless it implements IDisposable and the caller cooperates by calling Dispose correctly.
(Of course, why wouldn't the caller cooperate?)
Not that I've of. :( I think your best bet is to implement IDisposable and unregister on Dispose.
As other people have mentioned, the Dispose pattern is the way to address this issue. If the ClassTwo object lives for a short time, you can use the C# using statement to ensure Dispose is called after you are finished using the object:
using (var foo = new ClassTwo())
{
foo.Bar();
}
In order to find the root cause of such issues, you need to use a memory profiler. dotTrace has already been mentioned, and another good one is the SciTech Memory Profiler. What you need to find is the Root Path to the object that you think should be garbage collected, but isn't. The root paths are the reason why the object is not being collected - because through transitive references, an object that's guaranteed to be alive (a GC Root) is referencing your object that you want to be dead.
These memory profilers are very helpful in identifying which GC roots are causing you trouble, and what the reference paths are from the root to your object. Somewhere along that root path will be a reference that is out of place, and is the cause of the problem.
In your code, the cause of the ClassTwo object not being collected could be either the fact that MainWindow has a static reference to the ClassOne object, or that the event handler from ClassOne to ClassTwo has not been unhooked. Static references are one example of GC Roots - so everything classOne references will be alive until that static reference in MainWindow is changed.
Whether the static or the event handler is the problem depends on the scenario of your application - should the ClassOne object also be collected - is it wrong to have a static reference to it? Or is the static reference desired behaviour - is classOne a long lived object and classTwo a short lived one, in which case, classTwo should be Disposed when its life is over, and the Dispose should unhook the event handler.
This is a good article to learn about the .Net GC, written by Jeffrey Richter: http://msdn.microsoft.com/en-us/magazine/bb985010.aspx. It's a bit old now, there have been new additions to the GC over recent years, but it's a great place to start.
Related
I have the following code:
public abstract class State
{
public abstract void HandleState();
}
public class StateA : State
{
public override void HandleState()
{
// do stuff here
....
// this returns a new state object, and there are multiple, like StateB, StateC etc.
State newState = GetNewState();
newState.HandleState();
}
}
Assuming that state objects keep getting returned ad-infinitum, will the garbage collector ever be able to collect objects generated by this code?
Well, that depends on what GetNewState returns. If you are creating stack overflow (as Frédéric Hamidi pointed out) by creating new instances of StateA, then you'll get a StackOverflowException (most likely before the GC would try to run).
However, provided GetNewState() returns an instance of something else that doesn't create objects that are in turn creating new State objects recursively, then they will eventually get garbage collected at some point after they are no longer referenced (garbage collection is non deterministic and will clean up unused memory when it gets around to it).
Example of an implementation that wouldn't cause a stack overflow:
public class StateB : State
{
public override void HandleState()
{
// do stuff here - that doesn't keep creating new State objects.
....
}
}
As Eugene Podskal pointed out, those created classes could create a reference to themselves that would cause them to not get garbage collected. Such as:
public class StateC : State
{
private static List<StateC> _myStates = new List<StateC>();
public StateC()
{
//Unless items are removed at some point from _myStates, they
// won't get garbage collected.
_myStates.Add(this);
}
public override void HandleState()
{
// do stuff here
}
}
No. You're holding onto all of the references to these old State objects in the stack, which you continually delve down in without (apparently) ever coming back up from. Since you're holding onto references to the old state objects, they won't be GCed.
Once (if) you get to whatever point at which you don't generate a new state and "go back up the stack" then the objects will be able to be reclaimed.
Lets say we have the following model:
public class Bar<T>:List<T>
{
public delegate void CollectionChangedDelegate();
public event CollectionChangedDelegate CollectionChanged;
}
public class Foo
{
Bar<object> MyCollection = new Bar<object>();
public Foo()
{
MyCollection.CollectionChanged += new Bar<object>.CollectionChangedDelegate(MyCollection_CollectionChanged);
}
public void MyCollection_CollectionChanged()
{
//Do Something
}
~Foo() //Would this work ?
{
MyCollection.CollectionChanged -= MyCollection_CollectionChanged;
}
}
Could the destructor of Class Foo be called in this case ?
It is pointless to do so. Let's assume that there is such a subscription. Now, for us to get to the ~Foo method we must be being collected, so we must be unreachable. Because of how events work, subscriptions make us reachable (the publisher has a reference to the subscriber) - so we can deduce that MyCollection is also unreachable. If it wasn't, we wouldn't be being collected.
And if MyCollection is unreachable, then either it has already been collected, or it is about to be collected. There is no need to unsubscribe.
Remove ~Foo: it is pointless here; actually, it is worse than pointless - in addition to serving no useful purpose, it forces the garbage collector to push this object through an extra step (the finalizer queue).
You might, however, want to add some kind of deterministic clean-up that does this; as Joroen notes, IDisposable may be of use - however, it is hard to say whether it is appropriate in the case without knowing more context.
first of all, I believe that you are trying to write ObservableCollection
You can find info here: http://msdn.microsoft.com/en-us/library/ms668604.aspx.
Now, if my object holds data-members that have a need to dispose, I would implement IDisposable and there I would dispose them, or at least remove the event subscription.
implment it like that:
class Foo:Idisposable
{
public void Dispose(bool b)
{
MyCollection.CollectionChanged -= MyCollection_CollectionChanged;
}
}
I would like to ask what is the efficiency way the clean and dispose the object collected in the List (Of Object)?
Is that by calling the List.Clear() method will automatic clean all the object collected by it?
For Example consider the below example
public partial class Form1 : Form
{
FontCollection m_fontCollection;
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
m_fontCollection = new FontCollection();
for (int i = 0; i < 5000; i++)
{
Font font = new Font("Arial", 23);
FontImpl impl = new FontImpl(font, Color.Black);
impl.AfterChange +=
new FontImpl.AfterChangeHandler(impl_AfterChange);
m_fontCollection.Add(impl);
}
m_fontCollection.Dispose();
MessageBox.Show("TakeSnap");
}
void impl_AfterChange()
{
throw new NotImplementedException();
}
}
public class FontCollection : IEnumerable, IDisposable
{
IList<FontImpl> m_Implementation = new List<FontImpl>();
internal void Add(FontImpl impl)
{
this.m_Implementation.Add(impl);
}
public IEnumerator GetEnumerator()
{
return this.m_Implementation.GetEnumerator();
}
public void Dispose()
{
m_Implementation.Clear();
m_Implementation = null;
}
}
public class FontImpl
{
private Font m_Font;
private Color m_color;
public FontImpl(Font newFont, Color newColcor)
{
m_Font = newFont;
m_color = newColcor;
}
public event AfterChangeHandler AfterChange;
public delegate void AfterChangeHandler();
}
When i run the above application ANTS memory profiler i could see the memory leaks Font and FontFamily (I could not able to upload the screen) and how to remove these leaks
In general, you shouldn't need to worry about garbage collection, and when it occurs. An object is eligible for GC when there are zero references to it.
The thing you should pay attention to, however, are IDisposable objects. When you are finished with an IDisposable object, you really need to be calling Dispose() on it. If the object lives within the local scope of a functinon, then a using block makes this easy:
using (var resource = new SomeIDisposable()) {
// use resource
// resource.Dispose() is automatically called, *even if* an exception
// is thrown.
}
You can cause memory leaks by ending up with "dangling references" to objects, which prevent them from being garbage collected. Most often the cause of this is with event handlers. When you subscribe to an event exposed by object A, with an event handler on object B, A gets a reference to B:
class B {
void hook_up_to(A a) {
a.SomeEvent += Handler; // `a` gets a reference to `this`
}
void Handler(object sender, EventArgs e) {
}
}
class Program {
private A _a = new A();
static void SomeMethod() {
var b = new B();
b.hook_up_to(_a); // _a now has a reference to b
// b goes out of scope.
}
}
Normally, when b goes out of scope, the object it references would be eligible for garbage collection. In this case though, b hooked up to an event published by member variable _a, which caused _a to gain a reference to b. Now there is one outstanding reference to b, which is impossible1 to clear, and b is ineligible for GC. This is a memory leak.
1 The only way reference to b in this case is this pointer if/when the event handler is fired.
You don't need to implement IDisposable on your class. In general the only time your class needs to implement IDisposable is when it contains other classes that need to be disposed of (things like database and network connections, and unmanaged objects).
In your example, you might want to consider making m_fontCollection a local variable within button1_Click, since you're creating and disposing of it entirely within that method. If you make it local, then it will be garbage-collected once button1_Click exits ... provided there are no remaining references to it (which indeed there are not in this case).
Clear just removes everything from the list. In your case you have a list of IDisposable objects and you need to call Dispose on all the items in the list. Calling Clear does not do that.
Your FontImpl should implement IDisposable since it manages an IDisposable object:
public void Dispose() {
if (m_Font != null) {
m_Font.Dispose();
m_Font = null;
}
}
And your FontCollection Dispose should look like this:
public void Dispose()
{
foreach(FontImpl font in m_Implementation) {
font.Dispose();
}
m_Implementation.Clear();
m_Implementation = null;
}
In the case of the code you have shown you have Font objects which implement IDisposable and thus should be disposed. Those objects are being managed by your FontImpl class and so FontImpl should implement IDisposable. Your FontCollection class contains a list of FontImpl objects which should now be disposable and so FontCollection needs to implement IDisposable.
You should read up on the IDisposable pattern (this SO answer gives some great information - https://stackoverflow.com/a/538238/416574).
That being said, from the code snippet you provided it does not appear that you gain anything from your FontCollection other than a class to wrap a list. Unless there are other things you are going to do with that class I would just have a list as a member variable. That way in the Dispose method of your form you can walk your list and dispose the FontImpl objects. If you want to protect from double disposing, then just empty the list after disposing everything.
I have problem with destructor after going out of scope(it is calling but after some time and need to make an action on form, for example change radio button), maybe there's mistake in my code. Take a look:
namespace WindowsFormsApplication2
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
EventLogger.Print += delegate(string output)
{ if (!textBox1.IsDisposed) this.Invoke(new MethodInvoker(() => textBox1.AppendText(output + Environment.NewLine)), null); };
}
private void button1_Click(object sender, EventArgs e)
{
TestClass test = new TestClass();
}
}
public static class EventLogger
{
public delegate void EventHandler(string output);
public static event EventHandler Print;
public static void AddLog(String TextEvent)
{
Print(TextEvent);
}
}
public class TestClass
{
public TestClass()
{
EventLogger.AddLog("TestClass()");
}
~TestClass()
{
EventLogger.AddLog("~TestClass()");
}
}
}
Right, because this isn't C++. The finalizer (not destructor as in C++) is not guaranteed to be called immediately after an object has left its declaring scope, it is called when the GC decides to swoop in and clean up after you.
May I ask why you are using a finalizer to begin with? Are you maintaining references to unmanaged resources which need to be deallocated as deterministically as possible (if so, read up on the IDisposable interface)? The use cases for C# finalizers are few and far between, it's not common to implement them.
C# is not C++. Destructors don't run synchronously.
There is no bugs in your code as is, but it looks like you may need to implement "IDisposable pattern" for your class to provide a way for caller to guaranty that some destruction of your object is executed synchronously.
For those of us who are more comfortable with the C++ pattern it's useful to check out the docs for the IDisposable interface:
http://msdn.microsoft.com/en-us/library/system.idisposable.aspx
Unfortunately I've just tried it and it's not working as I'd like. I used an automatic object to save the current state of the GUI cursor, switch to the wait cursor and restore the original cursor as the object goes out of scope. The IDisposable interface does result in the cursor being restored but not immediately - so the Wait Cursor is showing for too long. Too bad because this is a very useful pattern.
Update: The C# try/finally pattern is comfortable enough after a while:
public void do_something_time_consuming()
{
ShowBusyCursor cursor = new ShowBusyCursor();
try
{
...
return;
}
finally
{
cursor.done();
}
}
The finalizer is not designed to be called immediately after going out of scope. It is called when the object is garbage-collected, which be may anywhere from milliseconds to days after going out of scope.
The finalizer is NOT meant for this kind of code. It is only for resource cleanup.
You can't force it to do something immediately after going out of scope, but you can tell it to immediately before going out of scope, with a Close() or similar method to signal that the object is done being used.
For example:
private void button1_Click(object sender, EventArgs e)
{
TestClass test = new TestClass();
// do stuff
test.Close();
}
Note: You could implement IDisposable, as has been suggested, but this use doesn't exactly fit the intended use of IDisposable, so although it would work, it's a bit hackish.
If I do the following:
public class Test
{
public static void Main()
{
List<Person> persons = new List<Person> { new Person() };
persons[0].Sneezing += new EventHandler(Person_Sneezing);
persons = null;
}
public static void Person_Sneezing(object sender, EventArgs e)
{
(sender as Person).CoverFace();
}
}
Does the person that was in person[0] still exists in memory because it's Sneezing delegate has a reference to the Person_Sneezing method or does it get collected by the GC?
This will be collected by the GC. To be kept in memory an object must be referenced directly or indirectly by ...
A value on the stack
A value rooted in a strong GC handle
A corner case or two I'm not thinking of at the moment
This is not true for the object at persons[0]. So it will be collected.
That is of course assuming the constructor for Person() doesn't do anything funny like add itself to ThreadLocalStorage.
You're halfway there; this would be a memory leak if it were the other way around. That is, if it looked like this:
public class Test
{
public void HookupStuff()
{
List<Person> persons = new List<Person> { new Person() };
this.EventHappened += new EventHandler(persons[0].SomeMethod);
// persons[0].Sneezing += new EventHandler(Person_Sneezing);
persons = null;
}
}
Now persons[0] will stick around, even though you nulled out persons, since the parent class has a reference to a method on it.
Further to the answers you've already got, you need to be careful in more realistic examples.
If your application has a main window that survives as long as the program runs, and you frequently create "short-lived" objects that enlist their methods to events on the main window, then you must delist the objects from those events when you no longer need them, because otherwise those objects won't be "short-lived" at all - they'll survive as long as the main window, i.e. until the user closes the application. The effective result will be equivalent to a memory leak.
It can help if you make the short-lived object's class implement IDisposable, so you can delist from the events in Dispose, and then make sure you call dispose when you want to discard an object.