I have a COM object implemented in C# (see below). It owns some IDisposable resource. Where should I dispose that resource?
Looks like finalizer is never called and I cannot overload IUnknown::Release method.
[ComVisible(true)]
[Guid("1992EC2F-087A-4264-B5B2-5E2E757F1A75")]
public class ComServer
{
IDisposable disposableResource; //where to dispose IDisposable resource?
public ComServer()
{
disposableResource = File.Open(#"c:\somefile.txt", FileMode.OpenOrCreate);
Console.WriteLine("ComServer.ComServer");
}
~ComServer() //finalizer is never called!
{
disposableResource.Dispose();
Console.WriteLine("ComServer.~ComServer");
}
}
Edited: the COM server is used in a native third party application, it's impossible to call Dispose on client side or make any changes there.
There is no guarantee when of if the finalize will be called. Your ComServer should implement the IDiposable interface itself and release it's disposable members within it.
From MSDN (if you override Object.Finalize)
If you are defining a base class that uses unmanaged resources and
that either has, or is likely to have, subclasses that should be
disposed, you should implement the IDisposable.Dispose method and
provide a second overload of Dispose, as discussed in the next
section.
class BaseClass : IDisposable
{
// Flag: Has Dispose already been called?
bool disposed = false;
// Public implementation of Dispose pattern callable by consumers.
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
// Protected implementation of Dispose pattern.
protected virtual void Dispose(bool disposing)
{
if (disposed)
return;
if (disposing) {
// Free any other managed objects here.
//
}
// Free any unmanaged objects here.
//
disposed = true;
}
~BaseClass()
{
Dispose(false);
}
}
Then call Dispose explicitly if you need it disposed or use using where ever possible.
using(ComServer com = new ComServer()) {
// operate on the ComServer
} // Dispose will be called automatically when the leaving the using block
Finalizer will never be called if your object is not marked for garbage collection. Go and look for the reason.
As to Dispose, only you can decide when to dispose your objects. It depends on the architecture, lifecycle of your objects. Generally speaking, you should call Dispose() when the object is no longer needed.
The best thing you can do is to implement IDisposable on your ComServer and invoke Dispose where your objects are no longer needed, or you can just wrap them in using constructions.
Tseng provided an example of standard implementation of IDisposable pattern, though consider that a good program does not need finalizers. Generally, it's a bad practice to leave your objects with intention to rely upon the mechanism of finalizing.
If you want to test your finalizer, than you can create your object, do not dispose it, call GC.Collect(). This call will force the garbage collection and if your object is eligible for garbage collection your finalizer will be invoked. But just remember never to use GC.Collect() as part of your production code.
Related
If Finalizer (destructor) is implemented in a class and GC.SupressFinalizer() is called from the overriden Dispose() method will the Garbage Collector still take care of any managed resources the instances of that class might have?
What confuses me is Microsofts documentation on this matter. For example the implemenation of IDisposable pattern in the virtual Dispose method takes care of the managed resources as well as the unmanaged. Why is that being done if the GC is taking care of the managed resources by default?
If I define a class like this:
public class Car
{
string plateNum;
}
and use this type as a filed in a class that also deals with unmanaged resources, according to the Microsofts documentation, the proper way to handle the disposal of the objects would be to call Dispose on the Car as well. For one to do so Car has to implement the IDisposable interface. Car is only dealing with the managed resources, there is no reason for doing so in the Car class, I have no idea what Dispose() would even do there, maybe give null to the plateNum? Also why would anyone want to implement IDisposable on the class that deals with the managed resources only?
Having that in mind, why is there a section in the code in the virtual Dispose() method (in the example in the MS documentation) in which managed resources are disposed?
protected virtual void Dispose(bool disposing)
{
// Check to see if Dispose has already been called.
if(!this.disposed)
{
// If disposing equals true, dispose all managed
// and unmanaged resources.
if(disposing)
{
// Dispose managed resources.
component.Dispose();
}
// Call the appropriate methods to clean up
// unmanaged resources here.
// If disposing is false,
// only the following code is executed.
CloseHandle(handle);
handle = IntPtr.Zero;
// Note disposing has been done.
disposed = true;
}
}
The only reason that I can think of is that GC.SupressFinalize(Object) is telling to the GC that it doesn't need to take care of anything related to the object argument that is given. But this shouldn't be the case because implementing the finalizer should only mean that the object that implements it should be put on the Finalizer queue only after the object has been dealt with by the GC because the Finalizer method, that is explicitly implemented by the user, should be called.
Also if a finalizer is defined does that change the way the GC collects the managed resources that the instance contains or does it just mean that additional code contained in the finalizer will be executed?
There are many Q+A about this already on SO so I'll give you a very practical answer: You won't ever need to write a finalizer.
Summary: In the rare case that you have an unmanaged resource, use the SafeHandle class to make it a managed resource.
When you inspect the full pattern carefully you can see that without unmanaged resources the destructor (aka the Finalizer, ~MyClass() {}) code path does exactly nothing.
And actually having that Finalizer without SuppressFinalize() is very expensive, it will delay the cleanup of your object to the next GC. Promoting its data to gen 1.
The remaining reason for the full pattern with virtual void Dispose(bool) is inheritance. A resource holding class should almost never need that. So make it sealed and all you need (want) is:
public sealed MyClass : IDisposable
{
private SomeResource _myResource;
public void Dispose()
{
_myResource?.Dispose();
}
}
And when you need inheritance then this is the official pattern.
This question already has answers here:
Use of Finalize/Dispose method in C#
(13 answers)
Closed 5 years ago.
From the MSDN article for implementing Finalize;
You should override Finalize for a class that uses unmanaged resources such as file handles or database connections that must be released when the managed object that uses them is discarded during garbage collection.
From the MSDN article for implementing IDisposible.Dispose;
Use this method to close or release unmanaged resources such as files, streams, and handles held by an instance of the class that implements this interface. By convention, this method is used for all tasks associated with freeing resources held by an object, or preparing an object for reuse.
Which, even in the context of each (thorough) article, seems to be a pretty ambiguous definition.
Where I really lose the purpose of Finalize though, is here;
Because garbage collection is non-deterministic, you do not know precisely when the garbage collector performs finalization. To release resources immediately, you can also choose to implement the dispose pattern and the IDisposable interface. The IDisposable.Dispose implementation can be called by consumers of your class to free unmanaged resources, and you can use the Finalize method to free unmanaged resources in the event that the Dispose method is not called.
Am I supposed to implement both, and force a finalize myself in the event that the consuming application forgets to dispose my object?
Dealing so closely with the GC is new to me...and I want to make sure I drop my resources properly. However, I don't fully understand why both of these exist, or when to use each one.
Implement both using the Dispose pattern. Visual Studio will even put in the scaffolding for you. Dispose allows you to divest scarce resources as soon as you have finished with them. Finalizer allows you to dispose of them if for some reason the consumer forgets to call your Dispose method but their class instance is allowed to go out of scope.
Just implement IDisposable pattern like that if you have unmanaged resources:
public class ApiClient : IDisposable
{
private readonly HttpClient http = new HttpClient();
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (disposing)
{
http?.Dispose();
}
}
}
Read more about disposable pattern here. I never forget to use using statement for my disposable objects to make sure the dispose method gets called:
using (var api = new ApiClient())
{
// use api
}
Trying to see if I'm understanding this correctly. Regretfully, I have not had much experience with IDisposable. We're taking a static class and making it non-static, and it's a class that is responsible for making web service calls in our Xamarin app. This is an object that I would definitely not want lying around. So I thought of having this class inherit the IDisposable interface, and I implemented the methods as shown:
private bool disposedValue = false; // To detect redundant calls
protected virtual void Dispose(bool disposing)
{
if (!disposedValue)
{
if (disposing)
{
// TODO: dispose managed state (managed objects).
}
// TODO: free unmanaged resources (unmanaged objects) and override a finalizer below.
// TODO: set large fields to null.
disposedValue = true;
}
}
// TODO: override a finalizer only if Dispose(bool disposing) above has code to free unmanaged resources.
//~WebServiceCallThread()
//{
// // Do not change this code. Put cleanup code in Dispose(bool disposing) above.
// Dispose(false);
//}
// This code added to correctly implement the disposable pattern.
public void Dispose()
{
// Do not change this code. Put cleanup code in Dispose(bool disposing) above.
Dispose(true);
// TODO: uncomment the following line if the finalizer is overridden above.
//GC.SuppressFinalize(this);
}
So all this appears to be doing, is when the Dispose method is called, it then calls a virtual method declared above Dispose(bool disposing). With the current code, this does nothing (or so it seems). It just calls the Dispose method, in which that calls an overloaded Dispose method. But nothing is actually "disposing" here.
Upon some research, it SEEMS (which is why I'm asking, I think I am understanding correctly). It says to only call a finalizer and the GC.SuppressFinalize method IF you dispose managed state. I'm not sure what dispose manage state means, but there's really nothing in my class that needs to get disposed. I just have some global int variables but that's it. The meat of this class is a HttpCall, which is already disposing of the objects automatically, as such:
public async void CallUrlHttpClient(string URL, System.Action<string> Handler, System.Action<XamarinMobile.Classes.ErrorInfo> ErrorHandler, int Tries)
{
var result = string.Empty;
didMakeSuccessfulHttpRequest = false;
using (var client = new HttpClient())
{
... Code
}
...
So here's my question: Do I need to make this class inherit IDisposable? I'm wondering if anything under the hood is being done, but it seems as if my application can just utilize the GC to get rid of my object that uses this class. This is how I call it, via a static method in a class:
public static void WebServiceCall(string URL, System.Action<string> Callback)
{
//using (XamarinMobile.Classes.WebServiceCallThread thread = new Classes.WebServiceCallThread())
//{
XamarinMobile.Classes.WebServiceCallThread thread = new Classes.WebServiceCallThread();
thread.Call(URL, Callback);
//}
}
If I understand GC correctly, once this method is done, the GC will automatically get rid of the object. So do I really need to make the WebServiceCallThread inherit from IDisposable? Again I'm asking if I'm understanding all of this correctly. If not, please correct me and let me know where I'm confused.
Thanks.
If your class created the HttpClient and maintained that instance throughout its own lifetime (like an instance variable) then you would implement IDisposable, and your Dispose() method would dispose the HttpClient.
That would be this part:
if (disposing)
{
// TODO: dispose managed state (managed objects).
_httpClient.Dispose();
}
In this case you don't need to implement IDisposable because you're already disposing your HttpClient.
using (var client = new HttpClient())
{
... Code
}
However, it's actually recommended to create one instance of HttpClient per endpoint and reuse it instead of creating and disposing over and over. (If your load isn't that heavy then you likely won't see any bad effect from creating and disposing HttpClient.)
If you wanted to avoid creating and disposing HttpClient then you could implement IDisposable. Your class either creates or receives (in its constructor) an instance of HttpClient. Now that your class "owns" that instance which needs to be disposed when your class is disposed, you make your class IDisposable. That way others know that your class needs to be disposed. They might not know why, but that's not important. By making it IDisposable you're letting them know that they need to dispose it. And then when your class is disposed it cleans up its HttpClient.
Just to provide a little extra clarification:
Many people, if asked "What is IDisposable for?" will answer that it causes garbage collection. It actually has nothing to do with garbage collection. Objects always get garbage collected when they're out of scope (there are no more references to them) whether or not they implement IDisposable.
The confusion comes up because the recommended pattern for IDisposable (which you referenced) includes a finalizer (~WebServiceCallThread()) which is called when the object is garbage collected.
When the garbage collector removes an item it calls the finalizer if there is one. This pattern causes the finalizer to call Dispose() if it hasn't been called already, just as a backup in case someone didn't call Dispose() like they were supposed to. But it's no substitute for calling Dispose() because there's no telling when the garbage collector will collect the item. There's a chance the finalizer will never get called. It's just a backup to try to mitigate what happens if consumers don't call Dispose().
I have this 'PlayerClass' class and every time I instantiate it I need to dispose of it, including the fields inside that class. Is there a way to do it? This is my class:
internal class PlayerClass
{
public WindowsMediaPlayer _wplayer;
}
How do I get to dispose of the class after using it?
I have tried to find a way on the internet but none of them are working after testing it.
I've tried this:
internal class PlayerClass : IDisposable
{
public WindowsMediaPlayer _wplayer;
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
/// <summary>
/// Is this instance disposed?
/// </summary>
protected bool Disposed { get; private set; }
/// <summary>
/// Dispose worker method. See http://coding.abel.nu/2012/01/disposable
/// </summary>
/// <param name="disposing">Are we disposing?
/// Otherwise we're finalizing.</param>
protected virtual void Dispose(bool disposing)
{
Disposed = true;
}
}
What am I doing wrong?
It looks as if you're implementing IDisposable just for the heck of it which is not necessary for managed code. The garbage collector will automatically clean up behind you.
You may need to clarify in your post why you are implementing IDisposable.
However I believe your mistake is that Dispose() is not automatically called by the garbage collector. If you do implement IDisposable you must make sure that the code using your class either instantiates inside a using() statement or manually calls .Dispose. Otherwise your dispose method will never fire.
using(var player = new PlayerClass()){
// Do something with player
// player.Dispose() is called automatically when you exit this using statement.
}
Since you're relying on the caller to make sure Dispose is called you may also want to look into a SafeHandle (preferred) or Finalize.
Because the IDisposable.Dispose implementation is called by the
consumer of a type when the resources owned by an instance are no
longer needed, you should either wrap the managed object in a
SafeHandle (the recommended alternative), or you should override
Object.Finalize to free unmanaged resources in the event that the
consumer forgets to call Dispose.
Source: IDisposable Interface
Using Statement
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/using-statement
Contrary to the advice of the other answers, it is necessary and essential to Dispose of all created objects that implement a Dispose() method (ie implement IDisposable). These objects are unmanaged resources, and thus are invisible to the GC. Not disposing of them is a guaranteed memory leak.(Note that some terminology prefers a Close() method to a Dispose method, in which case the Close() method should be identical.)
Here is a best practice implementation of the IDisposable interface.
#region IDisposable implementation with finalizer
private bool isDisposed = false;
public void Dispose() { Dispose(true); GC.SuppressFinalize(this); }
protected virtual void Dispose(bool disposing) {
if (!isDisposed) {
if (disposing) {
if (_wplayer != null) _wplayer.Dispose();
}
}
isDisposed = true;
}
#endregion
Here are the Best Practices Dos and Donts from the MSDN link above:
DO declare a protected virtual void Dispose(bool disposing) method to
centralize all logic related to releasing unmanaged resources.
DO implement the IDisposable interface by simply calling Dispose(true) followed by GC.SuppressFinalize(this).
DO NOT make the parameterless Dispose method virtual.
DO NOT declare any overloads of the Dispose method other than Dispose() and Dispose(bool).
DO allow the Dispose(bool) method to be called more than once. The method might choose to do nothing after the first call.
AVOID throwing an exception from within Dispose(bool) except under critical situations where the containing process has been corrupted (leaks, inconsistent shared state, etc.).
CONSIDER providing method Close(), in addition to the Dispose(), if close is standard terminology in the area.
From the MSDN documentation.
Provides a mechanism for releasing unmanaged resources.
You do not need to dispose managed objects. .Net will lead with this for you. It is usefull when you create a interop classes which will lead with unmanaged resources, then you can dispose all objects.
In .NET, under which circumstances should I use GC.SuppressFinalize()?
What advantage(s) does using this method give me?
SuppressFinalize should only be called by a class that has a finalizer. It's informing the Garbage Collector (GC) that this object was cleaned up fully.
The recommended IDisposable pattern when you have a finalizer is:
public class MyClass : IDisposable
{
private bool disposed = false;
protected virtual void Dispose(bool disposing)
{
if (!disposed)
{
if (disposing)
{
// called via myClass.Dispose().
// OK to use any private object references
}
// Release unmanaged resources.
// Set large fields to null.
disposed = true;
}
}
public void Dispose() // Implement IDisposable
{
Dispose(true);
GC.SuppressFinalize(this);
}
~MyClass() // the finalizer
{
Dispose(false);
}
}
Normally, the CLR keeps tabs on objects with a finalizer when they are created (making them more expensive to create). SuppressFinalize tells the GC that the object was cleaned up properly and doesn't need to go onto the finalizer queue. It looks like a C++ destructor, but doesn't act anything like one.
The SuppressFinalize optimization is not trivial, as your objects can live a long time waiting on the finalizer queue. Don't be tempted to call SuppressFinalize on other objects mind you. That's a serious defect waiting to happen.
Design guidelines inform us that a finalizer isn't necessary if your object implements IDisposable, but if you have a finalizer you should implement IDisposable to allow deterministic cleanup of your class.
Most of the time you should be able to get away with IDisposable to clean up resources. You should only need a finalizer when your object holds onto unmanaged resources and you need to guarantee those resources are cleaned up.
Note: Sometimes coders will add a finalizer to debug builds of their own IDisposable classes in order to test that code has disposed their IDisposable object properly.
public void Dispose() // Implement IDisposable
{
Dispose(true);
#if DEBUG
GC.SuppressFinalize(this);
#endif
}
#if DEBUG
~MyClass() // the finalizer
{
Dispose(false);
}
#endif
SupressFinalize tells the system that whatever work would have been done in the finalizer has already been done, so the finalizer doesn't need to be called. From the .NET docs:
Objects that implement the IDisposable
interface can call this method from
the IDisposable.Dispose method to
prevent the garbage collector from
calling Object.Finalize on an
object that does not require it.
In general, most any Dispose() method should be able to call GC.SupressFinalize(), because it should clean up everything that would be cleaned up in the finalizer.
SupressFinalize is just something that provides an optimization that allows the system to not bother queuing the object to the finalizer thread. A properly written Dispose()/finalizer should work properly with or without a call to GC.SupressFinalize().
Dispose(true);
GC.SuppressFinalize(this);
If object has finalizer, .net put a reference in finalization queue.
Since we have call Dispose(true), it clear object, so we don't need finalization queue to do this job.
So call GC.SuppressFinalize(this) remove reference in finalization queue.
If a class, or anything derived from it, might hold the last live reference to an object with a finalizer, then either GC.SuppressFinalize(this) or GC.KeepAlive(this) should be called on the object after any operation that might be adversely affected by that finalizer, thus ensuring that the finalizer won't run until after that operation is complete.
The cost of GC.KeepAlive() and GC.SuppressFinalize(this) are essentially the same in any class that doesn't have a finalizer, and classes that do have finalizers should generally call GC.SuppressFinalize(this), so using the latter function as the last step of Dispose() may not always be necessary, but it won't be wrong.
That method must be called on the Dispose method of objects that implements the IDisposable, in this way the GC wouldn't call the finalizer another time if someones calls the Dispose method.
See: GC.SuppressFinalize(Object) Method - Microsoft Docs