Part of the disposable pattern includes the following method.
protected virtual void Dispose(bool disposing)
{
if (!disposed)
{
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.
disposed = true;
}
}
This method has different handling to clean up managed and unmanaged resources. But what if I want to clean up a class member that implements IDisposable?
Normally, I don't know if that member cleans up managed or unmanaged resources. So if I want my Dispose method to clean up a class member that implements IDisposable, would I call Dispose() on that member in the managed or unmanaged sections in the code above?
You should call Dispose in the managed section (i.e., only when disposing is true).
Part of the guidance in Dispose Pattern under Framework Design Guidelines is:
The Boolean parameter disposing indicates whether the method was invoked from the IDisposable.Dispose implementation or from the finalizer. The Dispose(bool) implementation should check the parameter before accessing other reference objects [...]. Such objects should only be accessed when the method is called from the IDisposable.Dispose implementation (when the disposing parameter is equal to true). If the method is invoked from the finalizer (disposing is false), other objects should not be accessed. The reason is that objects are finalized in an unpredictable order and so they, or any of their dependencies, might already have been finalized.
Another useful resource is Implementing a Dispose Method
The standard way of disposing class members that implements IDisposable is like this:
public class Foo : IDisposable
{
// MemoryStream implements IDisposable
private readonly Stream _stream = new MemoryStream();
private bool _disposed;
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (_disposed)
{
return;
}
if (disposing)
{
_stream.Dispose();
}
_disposed = true;
}
}
To clarify, cleaning up managed resources means the resource itself implements IDisposable (as MemoryStream does above). MemoryStream/Stream holds the underlying unmanaged resource and its cleanup logic is implemented for you already. There is no need to clean up unmanaged resources in Foo.
Related
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.
Documentation of BlockingCollection<T> class has the following note:
Always call Dispose before you release your last reference to the
BlockingCollection<T>. Otherwise, the resources it is using will not be freed
until the garbage collector calls the BlockingCollection<T> object's Finalize
method.
And internal implementation of BlockingCollection<T> in C# has the following method:
/// <summary>
/// Releases resources used by the <see cref="T:System.Collections.Concurrent.BlockingCollection{T}"/> instance.
/// </summary>
/// <param name="disposing">Whether being disposed explicitly (true) or due to a finalizer (false).</param>
protected virtual void Dispose(bool disposing)
There is only one call of this Dispose with argument disposing: true and finalization suppress after the call. But, surprisingly for me, there is no explicit finalizer in the class, and even no call of Dispose(false).
It looks like Dispose function is relatively simple and it just removes references to different objects for GC speed up. In this case, GC will do this work for us in the case when we forget to call Dispose() explicitly.
Can someone spot the light on the internals of this class? What is the purpose of a method Dispose(bool disposing)? Is it common practice to implement this method for .NET core libraries even in cases when it is not needed?
I think the MSDN documentation for how to properly implement the dispose pattern is worth a read. However, the answer to your question is that BlockingCollection is not sealed. This means that it can be derived. The reason for the void Dispose(bool disposing) is to allow derived classes to properly de-allocate the resources of the base class. So, for example, I could implement
class Foo : BlockingCollection<string>
{
private bool disposed = false;
~Foo()
{
Dispose(false);
}
protected override void Dispose(bool disposing)
{
if (disposed)
{
return;
}
if (disposing)
{
// free managed resources
}
// free unmanaged resources
disposed = true;
base.Dispose(disposing);
}
}
By doing this, when Dispose() is called, the BlockingCollection will call Dispose(true) on Foo, which will eventually call Dispose(true) on BlockingCollection, and you get the benefit that the ~Foo() finalizer is suppressed. If the Dispose() method is not called, the finalizer is not suppressed, and it is called, allowing Foo to still deallocate its unmanaged resources.
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.
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.
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