What is the correct way to implement the IDisposable-Pattern, given that the implementing class contains a GCHandle-Member?
I came up with this, but it causes my application to leak memory:
public class foo : IDisposable
{
private bool disposed = false;
private readonly byte[] bytes;
private readonly GCHandle hBytes;
private readonly IDisposable someWrapperForUnmanagedData;
public foo(byte[] bytes)
{
this.bytes = bytes;
hBytes = GCHandle.Alloc(bytes, GCHandleType.Pinned);
someWrapperForUnmanagedData = new bar(..., bytes, ...);
}
protected virtual void Dispose(bool disposing)
{
if (disposed) return;
if (disposing)
{
hBytes.Free();
}
someWrapperForUnmanagedData.Dispose();
disposed = true;
}
public void Dispose()
{
Dispose(true);
GC.SupressFinalize(this);
}
~foo() => Dispose(false);
}
Microsoft-documentation does not mention how GCHandles are supposed to be used with IDisposable, and I can't seem to find anything on the net.
Should the finalizer for foo call hBytes.Free()?
Following the documentation, the finalizer is only supposed to call cleanup-routines for unmanaged resources, which in this case is only someWrapperForUnmanagedData.
You've got your Dispose method backwards.
When your type's finalizer is called, you should release managed resources that you own.
When your type's IDisposable.Dispose() method is called, you should call Dispose on the things you own, and also release managed resources that you own.
The reason for this is straightforward. The happy case is that your type is disposed (someone calls .Dispose() on it). This gives you the opportunity to release unmanaged resources that you own, and you also need to propagate this Dispose() call down to your children so that they can release their unmanaged resources as well.
If someone forgets to call .Dispose() on you, then the GC will probably save your back by calling your finalize method. In this case, you should release any unmanaged resources you own, but you should not call into any of your children. If any of your children have their own finalizers, then:
The GC will call their finalizer separately, so you should not call it as well.
There's no guarantee of the order that types are finalized, so your children might already have been finalized.
Note the use of the term ownership. An unmanaged resource should have exactly one thing which owns it, and that is the thing which is responsible for releasing it.
Note that your someWrapperForUnmanagedData is not an unmanaged type. It's a C# class, which is very much managed. Unmanaged types are things like raw pointers. If someWrapperForUnmanagedData itself owns some other unmanaged type(s), it should implemenet IDisposable and define a finalizer in order to make sure that those are freed.
GCHandle doesn't have its own finalizer, so there's no way for it to be Free()ed by the GC. Therefore it counts as an unmanaged resource here: you'll need to manually call Free() on it.
So:
protected virtual void Dispose(bool disposing)
{
if (disposed) return;
if (disposing)
{
someWrapperForUnmanagedData.Dispose();
}
hBytes.Free();
disposed = true;
}
Note, as this doc makes clear, you should use a SafeHandle (or one of its subclasses) if possible. SafeHandle implements its own finalizer, so you don't have to.
The runtime also knows about SafeHandle, and this lets it avoid some really nasty race conditions where e.g. your type can be finalized at the same time as it is making an unmanaged call, leading to a nasty crash. If you don't understand this race, this means you should definitely be using a SafeHandle.
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.
I am at a loss on how to handle a class that contains variables with Dispose/Finalize methods. I wish for this class to contain its own Dispose/Finalize methods that call upon the Dispose for each variable; however, both the C# documentation and all other answers/examples on StackOverflow are causing some confusion.
The primary confusion comes from the lack of clarification between what's a "managed" or "unmanaged" object. For example, the documentation here that explains how to implement Dispose simply uses placeholder comments that simply state "Free any unmanaged objects here." or "Free any other managed objects here."
Is a class variable with Dispose/Finalize fall under the category of managed or unmanaged? (Furthermore, should I be concerned about class variables that do NOT contain any sort of Dispose/Finalize implementations? Considering how there's two types of "management", does that mean those without "Dispose" still need to be disposed somehow?)
I.e., what would be the correct way to handle this class?
class BaseClass : IDisposable {
MyDisposeableObject disposeMe; // object with Dispose/Finalize
Dictionary<string,int> anotherObject; // just some arbitrary other object
bool disposed = false;
public BaseClass() {
disposeMe = new MyDisposeableObject();
anotherObject = new Dictionary<string,int>();
}
public void Dispose() {
Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing) {
if (disposed)
return;
if (disposing) {
// Free any other managed objects here.
// Should I use "disposeMe.Dispose()" here?
}
// Free any unmanaged objects here.
// OR should I use "disposeMe.Dispose()" here?
// Also should I be doing anything with "anotherObject"?
disposed = true;
}
~BaseClass() {
Dispose(false);
}
}
That was also a confusion for me but when I read more about memory management and GC mechanism in .Net everything became clear.
You should call disposeMe.Dispose() only if "disposing=true". Because it is a managed class/resource. I assume that it also implements this dispose and destructor pattern correctly.
Why you should not try to use any managed object outside of the if(disposing) block?
Because GC may not and does not collect your objects by following a graph from owner to owned. So, when the Dispose method is called by Destructor, disposeMe object may already be collected and unreachable. So you can not/should not dispose it in this area.
But you can free unmanaged resources, like unmanaged memory spaces you allocate, handles you opened... Since GC does not know anything about them, it can not collect and free them unless you intentionally free them. If you don't, there will be memory and handle leaks, which will eventually crash the application.
I want to know should we use dispose and finalize if we are to deal with an object that is holding an unmanaged resource.
second thing if we dispose an object does that object release memory along with the unmanaged resource on that moment only or that object's memory will be released by garbage collector later. same thing i want to know in fianilize context, once we finalize an object does that release memory for that object on the definite time or we have wait until garbage collector frees it's memory.
You should prevent users of your application from calling an object's Finalize method directly by limiting its scope to protected. In addition, you are strongly discouraged from calling a Finalize method for a class other than your base class directly from your application's code. To properly dispose of unmanaged resources, it is recommended that you implement a public Dispose or Close method that executes the necessary cleanup code for the object. The IDisposable interface provides the Dispose method for resource classes that implement the interface. Because it is public, users of your application can call the Dispose method directly to free memory used by unmanaged resources. When you properly implement a Dispose method, the Finalize method becomes a safeguard to clean up resources in the event that the Dispose method is not called.
// Design pattern for a base class.
public class Base: IDisposable
{
private bool disposed = false;
//Implement IDisposable.
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (!disposed)
{
if (disposing)
{
// Free other state (managed objects).
}
// Free your own state (unmanaged objects).
// Set large fields to null.
disposed = true;
}
}
// Use C# destructor syntax for finalization code.
~Base()
{
// Simply call Dispose(false).
Dispose (false);
}
}
// Design pattern for a derived class.
public class Derived: Base
{
private bool disposed = false;
protected override void Dispose(bool disposing)
{
if (!disposed)
{
if (disposing)
{
// Release managed resources.
}
// Release unmanaged resources.
// Set large fields to null.
// Call Dispose on your base class.
disposed = true;
}
base.Dispose(disposing);
}
// The derived class does not have a Finalize method
// or a Dispose method without parameters because it inherits
// them from the base class.
}
Source: MSND
As the implementer of a class, if you hold managed resources that ought to be disposed, you implement Dispose. If you hold native resources, you implement both Dispose and Finalize, and both call a common method that releases the native resources.
This is very well described in msdn Implementing Finalize and Dispose to Clean Up Unmanaged Resources
Both. The Dispose method is where resources are released and it's Dispose that you call explicitly in code when you're done with an object. Finalize doesn't do any work itself but rather just calls Dispose. If you don;t call Dispose yourself in your code then the garbage collector will call Finalize at some point in order to release the resources held by the object.
Memory is not considered to be a resource for the purposes of disposal. Once an object has been disposed and there are no more references to it, then the garbage collector can free the memory it occupies. Think about it. Calling Close on a form disposes it but the form still exists, so obviously the memory it occupies has not been released. Only the window handle is released.
I want to know should we use dispose and finalize if we are to deal with an object that is holding an unmanaged resource
Not necessarily, Dispose itself enough if you will make sure you call Dispose for sure. If not it is recommended to implement finalizer which is called by GC at the time of reclaiming memory.(Note: It is not reliable, meaning it may not be called also!).
second thing if we dispose an object does that object release memory
along with the unmanaged resource on that moment only or that object's
memory will be released by garbage collector later
No, disposing won't release memory. GC will do that once it sees it is time to reclaim. It is in-deterministic. Keep in mind Disposing means just calling some method named Dispose that's it, nothing else.
once we finalize an object does that release memory for that object on
the definite time or we have wait until garbage collector frees it's
memory.
Hypothetical question, It is expected that immediately after Finalizer called object will be released from memory, but Eric says A destructor can "resurrect" an object, making a dead object alive again. That's really weird. Don't do it. So I think hard to answer this question.
Hope this helps.
We should use Dispose whenever we operate with unmanaged resources, the typical scheme is
public class Wrapper: IDisposable {
...
// Let's acquire resource in constructor
public Wrapper(...) {
...
AcquireResource();
...
}
public Boolean IsDisposed {
get;
protected set; // <- Or even "private set"
}
protected virtual Dispose(Boolean disposing) {
if (IsDisposed)
return;
if (disposing) {
ReleaseResource();
}
IsDisposed = true;
}
public Dispose() {
Dispose(true);
GC.SuppressFinalize(this);
}
}
Do not use finalize: since finalization is called by GC and that's why
in unpredictable moments of time each error/leak in Dispose will be
floating error (and harder to detect) if you call Dispose(false) from ~Wrapper. The only, IMHO, case when you may want to implement finalization is when you acquire
unmanaged memory chunk:
public class MemoryWrapper: IDisposable {
private IntPtr m_Handle;
private int m_Size;
public void AcquireMemory(int size) {
...
m_Size = size;
m_Handle = UnmanagedAcquireMemory(size);
// Let GC know that we acquire memory in some weird way
GC.AddMemoryPressure(size);
}
private void ReleaseMemory() {
...
UnmanagedReleaseMemory(m_Handle, m_Size);
// Let GC know that we release memory in some weird way
GC.RemoveMemoryPressure(m_Size);
}
private MemoryWrapper(int size) {
AcquireMemory(size);
}
public Boolean IsDisposed {
get;
protected set; // <- Or even "private set"
}
protected virtual Dispose(Boolean disposing) {
if (IsDisposed)
return;
ReleaseMemory();
IsDisposed = true;
}
public Dispose() {
Dispose(true);
GC.SuppressFinalize(this);
}
// Possible finalizer
~MemoryWrapper() {
Dispose(false);
}
}
For typical resources wrappers the best way is to use special classes like
SafeHandle
SafeHandleZeroOrMinusOneIsInvalid
http://msdn.microsoft.com/query/dev11.query?appId=Dev11IDEF1&l=EN-US&k=k(System.Runtime.InteropServices.SafeHandle);k(TargetFrameworkMoniker-.NETFramework,Version%3Dv4.5);k(DevLang-csharp)&rd=true
I have a couple of unmanaged memory structures used to communicate with c++ dlls.
Each such structure has to be freed manually, so I wrap it in a MyUnmanagedStructure which implements IDisposable.
I always need a variable number of these structures together, so I have a collection MyUnmanagedStructureCollection which also implements IDisposable.
(see below for the minimal example code)
As long as the user of my library always calls Dispose() or wraps the collection with using() {} there is no problem, but I cannot assure that. I do not want to leak memory even if the user does not dispose the collection manually.
When the MyUnmanagedStructureCollection.Dispose() method is called by the garbage collection via finalizer instead, then as far as I understand I cannot be sure that my private List<MyUnmanagedStructure> has not been garbage collected already, so how can I dispose of each structure in that case?
In my finalizing code, should I attempt to iterate over the list, hoping that it has not been garbage collected yet?
Is it good practise to do this in a try/catch block, catching ObjectDisposedException?
Or should I let each unmanagedStructure "fend for itself", relying on the individual finalizers, and simply do nothing in the finalizer of my collection?
public class MyUnmanagedStructureCollection : IDisposable
{
private List<MyUnmanagedStructure> structures;
private bool disposed = false;
#region standard IDIsposable pattern
public ~MyUnmanagedStructureCollection()
{
this.Dispose(false);
}
public void Dispose()
{
this.Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (!disposed)
{
// Dispose unmanaged resources
// Should not access managed resources,
// the garbage collection may have claimed them already!
// PROBLEM!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
// this.structures is a List<MyUnmanagedStructure>; so is a managed resource!!!
foreach (var structure in this.structures)
structure.Dispose(disposing)
this.removeAllMemoryPressure();
if (disposing)
{
// Dispose managed resources.
this.structures.Clear();
this.structures = null;
}
}
disposed = true;
}
}
public class MyUnmanagedBuffer : IDisposable
{
...
}
The way the GC works is:
Find all reachable objects
Enqueue all unreachable objects with a finalizer into the finalization queue
Mark all objects that are reachable from from the finalization queue as reachable
Free the remaining unreachable objects
An object that's referenced from an object whose finalizer is running cannot have been garbage collected yet.
The only thing you need to be careful about is that the order of finalization is undefined. So the elements of the list might have been finalized yet, but not collected. Finalization is guaranteed to be single threaded, so you need locking too.
One generally tries to avoid such finalization chains, since independent finalization is simpler. But if some objects need to be disposed before others, such a construction is unavoidable.
You should also consider critical finalization using SafeHandles.
Reachability
One of the guidelines for finalization is that a Finalize method shouldn't touch other objects. People sometimes incorrectly assume that this is because those other objects have already been collected. Yet, as I have explained, the entire reachable graph from a finalizable object is promoted.
The real reason for the guideline is to avoid touching objects that may have already been finalized. That's because finalization is unordered.
Chris Brumme on finalization
Since you're MyUnmanagedBuffer class is a managed resource from point-of-view of your MyUnmanagedStructureCollection class, I think it should handle it as such. This would mean that the MyUnmanagedStructureCollection.Dispose(bool) method would be as below.
protected virtual void Dispose(bool disposing) {
if (!disposed) {
// Dispose unmanaged resources
// Should not access managed resources,
// the garbage collection may have claimed them already!
if (disposing) {
// Dispose managed resources.
// This means that we try to dispose all items in the structures collection.
if (this.structures != null) {
foreach (var structure in this.structures) {
structure.Dispose(disposing);
this.removeAllMemoryPressure(); // What does this?
}
this.structures.Clear();
this.structures = null;
}
}
}
disposed = true;
}
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