We Keep Coding

Why does the recommended dispose pattern adds a disposed field at each level of the hierarchy?

The Dispose pattern is notoriously complicated to get right, especially when we have a hierarchy of classes that needs to dispose things at different levels. The recommended implementation is as follow, took from Implement a Dispose method - Microsoft Docs.
using System;
class BaseClass : IDisposable
{
// To detect redundant calls
private bool _disposed = false;
~BaseClass() => Dispose(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)
{
// 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;
}
}
using Microsoft.Win32.SafeHandles;
using System;
using System.Runtime.InteropServices;
class DerivedClass : BaseClass
{
// To detect redundant calls
private bool _disposed = false;
// Instantiate a SafeHandle instance.
private SafeHandle _safeHandle = new SafeFileHandle(IntPtr.Zero, true);
// Protected implementation of Dispose pattern.
protected override void Dispose(bool disposing)
{
if (_disposed)
{
return;
}
if (disposing)
{
// Dispose managed state (managed objects).
_safeHandle?.Dispose();
}
_disposed = true;
// Call base class implementation.
base.Dispose(disposing);
}
}
What I don't get in this implementation is what advantage we have in adding a _disposed field at each level of the hierarchy? Instead, we could take care of the _disposed only in the top level of the hierarchy (the one implementing directly IDisposable and not care about it in the derived classes.
Something like this:
using System;
class BaseClass : IDisposable
{
// To detect redundant calls
private bool _disposed = false;
~BaseClass()
{
if (_disposed)
{
return;
}
Dispose(false);
_disposed = true;
}
// Public implementation of Dispose pattern callable by consumers.
public void Dispose()
{
if (_disposed)
{
return;
}
Dispose(true);
GC.SuppressFinalize(this);
_disposed = true;
}
// Protected implementation of Dispose pattern.
protected virtual void Dispose(bool disposing)
{
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.
}
}
using Microsoft.Win32.SafeHandles;
using System;
using System.Runtime.InteropServices;
class DerivedClass : BaseClass
{
// Instantiate a SafeHandle instance.
private SafeHandle _safeHandle = new SafeFileHandle(IntPtr.Zero, true);
// Protected implementation of Dispose pattern.
protected override void Dispose(bool disposing)
{
if (disposing)
{
// Dispose managed state (managed objects).
_safeHandle?.Dispose();
}
// Call base class implementation.
base.Dispose(disposing);
}
}
This is such a widely used code sample that there must certainly be a good reason for it to be implemented with the repeated _disposed at each level, but I just can't find any.
It's a tiny bit more code in the base class, but less to worry about in the derived class, and some repeated information removed.
What else am I missing?
Edit:
As #InBetween correctly says that one drawback of my implementation is that if you'd need to check if your object is disposed in one method of the derived class you won't be able to check it. Let's correct that issue by making it a protected property with a private set.
using System;
class BaseClass : IDisposable
{
// To detect redundant calls
protected bool Disposed { get; private set; } = false;
~BaseClass()
{
if (Disposed)
{
return;
}
Dispose(false);
Disposed = true;
}
// Public implementation of Dispose pattern callable by consumers.
public void Dispose()
{
if (Disposed)
{
return;
}
Dispose(true);
GC.SuppressFinalize(this);
Disposed = true;
}
// Protected implementation of Dispose pattern.
protected virtual void Dispose(bool disposing)
{
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.
}
}
using Microsoft.Win32.SafeHandles;
using System;
using System.Runtime.InteropServices;
class DerivedClass : BaseClass
{
// Instantiate a SafeHandle instance.
private SafeHandle _safeHandle = new SafeFileHandle(IntPtr.Zero, true);
public void DoSomething()
{
if(Disposed)
{
throw new ObjectDisposedException("Cannot access disposed resource");
}
}
// Protected implementation of Dispose pattern.
protected override void Dispose(bool disposing)
{
if (disposing)
{
// Dispose managed state (managed objects).
_safeHandle?.Dispose();
}
// Call base class implementation.
base.Dispose(disposing);
}
}

If you are inheriting from a disposable class, one of two conditions must be true.
Your subcless does not introduce a new disposable resource. In this case you don't need to override Dispose and the question is moot.
Your subclass introduces a new disposable resource. In this case, you're going to override Dispose, insert your own disposal code, then call base.Dispose. The _disposed flag is there to help you remember to prevent your disposal code from executing twice.
You certainly can remove _disposed if you want. But you probably do not care much about the base class' _disposed flag, if it even has one. It worries about what it is disposing, and you worry about yours.

The reason is simple. In your implementation you cant use _disposed in the derived type to check if any method is invoked when the object is already disposed and take the necessary actions. In your implementation you'd need to create your own redundant flag isDisposed which defeats the purpose; you already have one "for free" from the pattern itself following the guidelines.
A case could be made though about making _disposed protected.

Implementing C# IDisposable [duplicate]

This question already has answers here:
Proper use of the IDisposable interface
(20 answers)
Closed 3 years ago.
I am trying to work out exactly what I need to do to dispose of a property in C# when using IDisposable. I can see the template, but I am not sure about actually disposing of objects.
I am working with the example given on this site:
https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/implementing-dispose
I have added 3 properties:
privInt: Private value type. Would I dispose of this as it could be saved on the heap so may require GC? If I would then how, cannot set to null or call dispose? If not then I assume it has to be left to GC - but is it not the point of Dispose to free resources?
NonIDisClassInstance - Do I set to null? Is that enough
Can anyone comment on my implementation below and advise on what is correct/wrong.
IDisClass - Just call dispose
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);
}
//Private value type
private int privInt;
//Private class that does not implement IDisposable
Private NonIDisClass NonIDisClassInstance;
//Private class that does implement IDisposable
Private IDisClass IDisClassInstance;
// 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.
DisposeOfThis = ????;
NonIDisClassInstance = null;
IDisClassInstance.Dispose();
disposed = true;
}
~BaseClass()
{
Dispose(false);
}
}

You only need GC.SuppressFinalize(this); and a destructor if you are working with unmanaged memory. If not you can safely omit this.
In your Dispose implementation call this.IDisClassInstance?.Dispose();, that is all you need.
Your code can be condensed down to this.
class BaseClass : System.IDisposable
{
public virtual void Dispose()
{
// never throw an exception from Dispose so prevent an NRE by using ?.
this.IDisClassInstance?.Dispose();
}
//Private class that does not implement IDisposable
private object NonIDisClassInstance;
//Private class that does implement IDisposable
private IDisposable IDisClassInstance;
}
Do keep in mind that using Dispose is not necessarily about freeing up memory, it is about cleaning up / freeing up unmanaged resources which might also include memory. Things like open file streams or database connections or network connections.
Note that your derived types can override Dispose but those overriden methods should include a call to base.Dispose(); so the managed resources of the base class are released.

CA1063 Modify Dispose() so that it calls Dispose(true), then calls GC.SuppressFinalize on the current object instance and then returns

I have this class:
public abstract class ImplementsIDisposable : IDisposable
{
public abstract void Dispose();
private class InnerClass : ImplementsIDisposable
{
private bool disposedValue;
public override void Dispose()
{
if (!disposedValue)
{
doSomething();
disposedValue = true;
}
GC.SuppressFinalize(this);
}
}
}
And Code Analysis is throwing this message:
CA1063 Modify Dispose() so that it calls Dispose(true), then calls
GC.SuppressFinalize on the current object instance and then returns.
And this one:
CA1063 Ensure that Dispose() is declared as public and sealed.
Both in the this line:
public abstract void Dispose();
Can it be that it wants Dispose() to be implemented in ImplementsIDisposable instead of InnerClass?

There is no reason why public Dispose() should be virtual and much less abstract.
You need to check out the dispose pattern and implement it correctly. The warnings are hinting how you should do it but they can be rather cryptic unless you know how the pattern goes to begin with.
You can read about the Dispose Pattern here, it’s rather simple.
And of course, don’t miss this canonical SO answer on the subject.

I tried to follow the accepted answer, but I kept running into a wall because the last two lines of my method were indeed Dispose(true) and GC.SupressFinalize(this). It turns out that the warning did not make me understand that these should be the ONLY lines in the method, leaving something like the below. I believe the first link in the accepted answer actually does explain this difference, but I didn't read it as carefully as maybe I should have.
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
try
{
if (disposing)
{
// call Dispose on all your disposable fields/properties
}
}
finally
{
// free all unmanaged resources here. Note that this is not in an else block.
}
}
Credit to https://netvignettes.wordpress.com/2011/06/29/how-to-implement-dispose-properly/ for getting me on track.

Take on IDisposable pattern

OK, I've read a few things about best practices around IDisposable and I thing I basically get the idea (finally).
My question relates to inheriting from an IDisposable base class. All the examples i see write the same code blocks over and over in the sub-classes and i don't see the advantage.
Why not simply bake a virtual method into the base class, invoke it at the right time from inside the (privately implemented) IDisposable routine so that the sub-classes don't have all that clutter, yet still get a chance do manage their resources?
My proposed base class:
public abstract class DreamDisposableBase : IDisposable
{
private bool _disposed = false;
protected virtual void LocalDispose(bool disposing)
{
}
~DreamDisposableBase()
{
// finalizer being called implies two things:
// 1. our dispose wasn't called (because we suppress it therein)
// 2. we don't need to worry about managed resources; they're also subject to finalization
// so....we need to call dispose with false, meaning dispose but only worry about *unmanaged* resources:
dispose(false);
}
void IDisposable.Dispose()
{
dispose(true); // true argument really just means that we're invoking it explicitly
}
private void dispose(bool disposing)
{
if (!_disposed)
{
// give sub-classes their chance to release their resources synchronously
LocalDispose(disposing);
if (disposing)
{
// true path is our cue to release our private heap variables...
}
// do stuff outside of the conditional path which *always* needs to be done - release unmanaged resources
// tell .net framework we're done, don't bother with our finalizer -
GC.SuppressFinalize(this);
// don't come back through here
_disposed = true;
}
}
}

I wouldn't want every type to have a finalizer. The need to perform any work in a finalizer is very rare. Almost all implementations will not do anything if disposing is true. Finalizers hurt performance because they cause promotion to Gen2, need two collections to be cleaned and calling finalizers is single-threaded.
Most classes do not wrap unmanaged resources, and if they do they should use the SafeHandle types or others. That makes finalizers unnecessary as well.

I do not see any real improvement in your code but as the pattern is non-standard it may be harder for other developers to understand it. To create a derived class using your pattern you need something like this:
class DerivedDreamDisposable : DreamDisposableBase
{
protected override void LocalDispose(bool disposing)
{
if (disposing)
{
// Dispose aggregated objects that are disposable.
}
// Dispose unmanaged resources.
_disposed = true;
base.LocalDispose(disposing);
}
}
Using the standard IDisposable pattern your derived class is something like this:
class DerivedDisposable : DisposableBase
{
bool _disposed;
protected override void Dispose(bool disposing)
{
if (!_disposed)
{
if (disposing)
{
// Dispose aggregated objects that are disposable.
}
// Dispose unmanaged resources.
_disposed = true;
}
base.Dispose(disposing);
}
}
By deriving from DreamDisposable you avoid to replicate the field to keep track of the disposed state of the object. However, other than that the methods are effectively the same. Also, in your base class LocalDispose is empty and you have moved the code into the private Dispose method but that can be fixed by doing a small refactoring.
However, many classes will not have any unmanaged resources to release and because calling the Dispose method is idempotent (you can call it several times) you often do not have to keep track of the disposed state and your disposable code is simplified to:
protected override void Dispose(bool disposing)
{
if (disposing)
{
_child1.Dispose();
_child2.Dispose();
}
}
If you do not have any unmanaged resources in your inheritance hierarchy you do not need finalizer and the disposing parameter will always be true. Your Dispose method will then be:
protected override void Dispose(bool disposing)
{
_child1.Dispose();
_child2.Dispose();
}
In general avoiding finalizers are good for performance which is why you call GC.SuppressFinalize to undo the harm you did by implement a finalizer.
But in many situations you still need to keep track of the disposed state of an object because you have to throw ObjectDisposedException if methods are invoked after the object has been disposed, and in that case my simplified Disposed methods is too simple. Here is an example on how to handle that without replicating the _disposed flag in each subclass and still use the standard dispose pattern:
class DisposableBase : IDisposable
{
bool _disposed;
~DisposableBase()
{
Dispose(false);
GC.SuppressFinalize(this);
}
public void Dispose()
{
if (_disposed)
return;
Dispose(true);
_disposed = true;
}
public void DoStuff()
{
ThrowIfDisposed();
// Now, do stuff ...
}
protected virtual void Dispose(bool disposing)
{
// Dispose resources ...
}
protected void ThrowIfDisposed()
{
if (_disposed)
throw new ObjectDisposedException(GetType().FullName);
}
}
Any derived class do not need to keep track of the disposed state and instead should use ThrowIfDisposed in all public methods that depends on the object not being disposed.

You say:
My question relates to inheriting from an IDisposable base class. All
the examples i see write the same code blocks over and over in the
sub-classes and i don't see the advantage.
That is not true, in the IDisposable pattern the method:
protected virtual void Dispose(bool disposing)
should be overrided by inheriting classes.
You need to note that the Dispose(bool disposing) method is actually your
LocalDisposing(bool disposing) method. And this fact, I think, is the source of your confusion.
Please read the related section on the seminal book:
Framework Design Guidelines, Second edition
Quoting the book:
If you are inheriting from a class that already implements the
pattern, simply override the Dispose(bool) method to provide aditional
resource cleanup logic
In derived classes the code looks like:
protected override void Dispose(bool disposing)
{
if(disposing)
{
//release own resources
}
}
Also note that in this case you should call GC.SuppressFinalize(this) only in the non-virtual Dispose method.
Also in your code you are implementing the IDisposable interface implicitly, important to be aware of this.

Properly disposing an object hierarchy in C#

I am working on a project where a certain class hierarchy implements IDisposable.
Some of the classes also implement a Finalize method ( ~MyClass() )
In general it looks something like this:
public class BaseClass : IDisposable
{
}
public class SomeClass : BaseClass
{
~SomeClass()
{
Dispose();
}
Dispose()
{
// Do some stuff.
base.Dispose();
}
}
public class AnoterClass : SomeClass
{
~AnoterClass()
{
Dispose();
}
Dispose()
{
// Do some stuff.
base.Dispose();
}
}
I'd like to know what is the proper way of handling calls to Dispose, as it seems that these objects are being disposed by calling Dispose(), and later on crash since the Finalize method is called.
Is it best to keep a flag at the lowest class in the hierarchy (protected bool disposed), and check that in every level of the class hierarchy?
I find that every possible solution requires some code duplication, which is not what i'm after.

You need to suppress finalization if you are manually disposing of your objects.
The pattern to follow is here
Edit:
I think you only need to implement at a particular level of class hierarchy if you have something new to dispose at that level, otherwise I believe that the disposal in the base class will do everything that you need. If your implementation in any particular class just calls base.Dispose(disposing) then it is not needed, if it has to do some clean uop, then call base.Dispose() then you need it.
Having a protected flag as you suggest should be fine.

If your base class implements the standard IDisposable pattern, all you need to do is add the Dispose(bool disposing) override to each derived class that itself owns IDisposable or unmanaged resources:
protected override void Dispose(bool disposing)
{
try
{
if (disposing)
{
// Release managed resources
}
// Release unmanaged resources
}
finally
{
base.Dispose(disposing);
}
}
You shouldn't implement finalizers in any of the derived classes.
And, of course, any classes in the hierarchy that don't have their own IDisposable resources don't need this override.