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How to do Sentry pattern in C#?

In C++, I'm used to using the sentry pattern to ensure that resources acquired are properly released when the block or function exits (see here for example). I used this, for example, for grabbing the graphics state, and then I can do whatever I want to that state, and it gets put back when the sentry object (reference) goes out of scope.
Now I'm using C#, and the same trick doesn't work, because the destructor doesn't get called until who-knows-when later.
Is there some OTHER method that is guaranteed to fire when an object's last reference is released? Or do I just have to remember to call some Restore() or Apply() or DoYourThing() method before returning from any method where I would otherwise use a sentry?

C# has finalizers, like C++, that are called when an object is destroyed. They are in the same form as C++, that is:
~ClassName()
{
}
As you know, though, C# does not have deterministic memory management, and so this isn't really a great guarantee. AS a direct result, you should not use finalizers for releasing unmanaged resources in C#. Instead, we use the IDisposable interface. This exposes a single method on the implementor, Dispose, which is intended to be called when you want to release unmanaged resources.
public class MyDisposable : IDisposable
{
public void Dispose()
{
// get rid of some expensive unmanaged resource like a connection
}
}
We can also use the using sugar to allow semantic invocation of Dispose() when the using block terminates (gracefully or not).
using(var disposable = new MyDisposable)
{
// Do something with the disposable
} // Dispose is invoked here
Should you find yourself using finalizers and Dispose, you can consider using the GC.SuppressFinalize method, although that's a bit out of my scope. There was a really good discussion elsewhere on StackOverflow about this here
This can be used to perform RAII-esque trickery, of course, such as
using(var guard = new Guard(resource))
{
}

C# provides the using block for this case, which works on any object that implements IDisposable.
For example, if you have a type class Foo : IDisposable then you can do this:
using (new Foo(/* ... */)) {
// Your code that depends on this resource.
}
This is equivalent to:
Foo x = new Foo(/* ... */);
try {
// Your code
} finally {
if (x != null) {
((IDisposable)x).Dispose();
}
}
Except, of course, that you don't have access to x. You can, however, gain such access if you need it by creating a variable in the using block:
using (Foo foo = new Foo(/* ... */)) {
// Your code that uses "foo"
}
The standard way of creating a class that can be disposed of this way or by the garbage collector is:
class Foo : IDisposable {
protected virtual void Dispose(bool disposing) {
// Your cleanup code goes here.
}
public void Dispose() {
GC.SuppressFinalize(this);
Dispose(true);
}
~Foo() {
Dispose(false);
}
}

To implement deterministic deallocation of unmanaged resources, you would use a Dispose pattern. For many cases you can combine this with a using block if you want this to occur within a specific scope.
File handles or handles to a graphics device would usually be considered a unmanaged resource. These are resources that the .NET framework does not track references for and does not automatically deallocate.
If it is managed resource, just references to C# objects/lists, then usually deterministic deallocation is not necessary, and the garbage collector can usually handle these more efficiently than you can. Don't worry so much about when managed resources are deallocated. If you no longer have a reference to them, then they should be of no concern. The concern comes when you are done with something but somehow have a lingering reference to it that is preventing the GC from deallocating it.

It sounds like what you're describing is objects that implement the IDisposable interface for C#. the IDisposable has a Dispose() method, that when defined properly on your own objects, is used to release resources.
Additionally C# does have destructors defined as such:
public class Foo
{
public Foo()
{
// constructor
}
~Foo() // destructor
}
when wrapping IDisposable objects in a using block, the Dispose() method is automatically called, the other option is to throw it in a finally block (when not using... using)

The easiest way to do this is enclosing the sentry object in a using statement. So something like this.
using (sentry = new Sentry())
{
//do your stuff here
}
After this block, sentry will have been released and out of scope.

Dispose() for cleaning up managed resources?

In this answer I found,
Cleanup the unmanaged resources in the Finalize method and the
managed ones in the Dispose method, when the Dispose/Finalize pattern
has been used in your code.
And later I found this nice article about finalize and dispose and got a clear idea about them. The article has the following code(Page 3), to explain the concepts:
class Test : IDisposable
{
private bool isDisposed = false;
~Test()
{
Dispose(false);
}
protected void Dispose(bool disposing)
{
if (disposing)
{
// Code to dispose the managed resources of the class
}
// Code to dispose the un-managed resources of the class
isDisposed = true;
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
}
But below that, the same note (which I included in the beginning of this question) appears.
The Dispose/Finalize Pattern
Microsoft recommends that you implement both Dispose and Finalize when working with unmanaged resources. The correct sequence then would
be for a developer to call Dispose. The Finalize implementation would
run and the resources would still be released when the object is
garbage collected even if a developer neglected to call the Dispose
method explicitly. Francesco Balena writes in his blog "the
Dispose/Finalize pattern should be used only when your type invokes
unmanaged code that allocates unmanaged resources (including unmanaged
memory) and returns a handle that you must use eventually to release
the resource. Both dispose and finalize must chain up to their parent
objects by calling their parent's respective methods after they have
disposed or finalized their own members".
Simply put, cleanup the unmanaged resources in the Finalize method and the managed ones in the Dispose method, when the
Dispose/Finalize pattern has been used in your code.
Now I am confused again. In the entire article and in the code sample, it is shown that unmanaged resources should be freed in Dispose(). But then what is the relevance of that comment?
Edit:
As it is confirmed that this line :
Simply put, cleanup the unmanaged resources in the Finalize method and
the managed ones in the Dispose method, when the Dispose/Finalize
pattern has been used in your code
is erroneous, I edited this answer.

See its very simple.
If you are dealing with unmanaged resources - Implement both Dispose and Finalize. Dispose is to be called by developers to free up the resources as soon as they see it that its no longer needed for them. If they forget to call Dispose then Framework calls the finalize in its own GC cycle (usually will take its own sweet time).
If your object uses disposable objects internally - You implement Dispose() if you created and retained a reference to any object of a type which implements Dispose() and which you haven't already disposed.
If neither of the above is the case (you are NOT dealing with unmanaged resources nor your object uses disposable objects internally) - Then don't do anything. Don't implement Finalize nor Dispose.
Some classic examples:
System.IO.FileStream object manages the lock/stream handles to files. So it implements both dispose and finalize. If the developer disposes it then the other program can access it right away. If he forgets to dispose it then Framework finalize it and close the handles later in its GC cycle.
System.Text.StringBuilder dose not have any unmanaged resource. So no dispose no finalize.
As far as the pattern is concerned what it means to
// Code to dispose the managed resources of the class
is that call the Dispose methods of any .NET objects that you have as components inside that class
And
// Code to dispose the un-managed resources of the class
Means to close the raw handles and pointers. Here is your updated code with examples
class Test : IDisposable
{
private bool isDisposed = false;
~Test()
{
Dispose(false);
}
protected void Dispose(bool disposing)
{
if (!isDisposed)
{
if (disposing)
{
// Code to dispose the managed resources of the class
internalComponent1.Dispose();
internalComponent2.Dispose();
}
// Code to dispose the un-managed resources of the class
CloseHandle(handle);
handle = IntPtr.Zero;
isDisposed = true;
}
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
}
Here is an old question explaining it

If a Foo has resources which will benefit from deterministic cleanup, but none that can be usefully cleaned up in a finalizer, it should implement IDisposable but should not override Finalize or have a destructor. If a class holds multiple resources, and at least one can be cleaned up in a finalizer, then each discrete resource that could be cleaned up in a finalizer should be encapsulated into its own Finalizer/destructor-equipped object (which may be defined in a protected nested class), and the class that would contain those resources should hold references to the wrapper objects. Once that is done, the outer class will fit the pattern for classes with a Dispose method but no finalizer/destructor.

What is the difference between Dispose and setting reference to null in C#?

I have two code samples and I want to know what is the deifference between them and which is better to use as best practice and performance wise:
using (TestForm test = new TestForm())
{
test.ShowDialog();
}
and the other is:
TestForm test = null;
try
{
test = new TestForm();
test.ShowDialog();
}
catch(Exception ex)
{
}
finally
{
test = null;
}

The IDisposable interface defines the Dispose method, as well as the possibility to use the "using" syntax. The dispose method of a class can be implemented to release resources, close database connections and any sort of finalizing and cleanup. Just setting the class instance to null won't execute any of the code defined in the dispose method. As a generic rule, if a class implements IDisposable, dispose should be called when you're finished with the class instance.

Dispose() is for freeing unmanaged resources. This may be done in a finalizer as well (which might call Dispose()) but don't rely on it. If it isn't done, then you leak unmanaged resources.
Setting a reference to null only means that a particular reference no longer points to that object. It can live on quite a while after that (or even indefinitely if you have another reference – well, if you have multiple reference to an object you Dispose()d of, then it gets ugly, probably).
Generally, always call Dispose() on IDiposables when you're done with them. It's easier if you wrap them into a using statement:
using (var foo = new SomeDiposableObject()) {
// do something with foo
}

The IDisposable pattern is a mechanism to timely free unmanaged and managed resources that an object may be consuming.
The typical way the pattern is implemented is as follows:
public void Dispose() //Implementes the IDisposable interface
{
this.Dispose(true);
GC.SupressFinalize(this); //All resources have been released, no need to run the finalizer. We make the GC's life a little easier;
}
protected void Dispose(bool disposing)
{
if (disposing)
{
//Relesase managed resources.
}
//Release unmanaged resources.
}
~MyDisposableObject() //finalizer
{
this.Dispose(false)
}
The thing to note here is that the release of resources through the Dispose method is very similar to what you logically would expect to find in a finalizer. It is not done directly in the finalizer due to two main reasons:
the finalizer is not executed in a deterministic order. That is why we do not dispose managed resources from the finalizer (Dispose(false)), as some or all the managed resources held by the object might have been finalized before the object itself. This is not true with unmanaged resources as, by definition, they will never be finalized by the GC.
We do not know when the finalizer is run (it is up to the GC).
The basic idea is that an object implementing IDisposable is a sign for any consumer saying: "hey, I'm holding on to a certain amount of unmanaged and/or managed resources that will eventually be released when the GC decides I'm no longer useful, but if you need those resources back in a timely way, call Dispose() and I'll be happy to oblige.".
On the other hand, setting a reference variable to null is not freeing any resources at all. If the reference you have removed from the object was the only one to said object, then the object will eventually be collected by the GC and managed and unmanaged resources will be freed (when, is anyones guess).
If there were more live references still pointing to the object, then the object would live one and no resources whatsoever would be freed.

still trying to understand the dispose pattern

i've read msdn and various posts about dispose pattern, and there are still a couple of things i don't understand. I've written the following code to test the dispose pattern. Please note that there aren't unmanged resources, i'm using vs2008 and .net 3.5 :
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void tryDispose()
{
//test 1 : allocate resource and leave manage it to gc
BL.myclass df = new BL.myclass();
//test 2 : try to force garbage collecting
//GC.Collect();
//test 3 : call dispose myself
//using (BL.myclass df = new BL.myclass())
//{
//}
}
private void button1_Click(object sender, EventArgs e)
{
tryDispose();
}
this is my disposable class:
class myclass: IDisposable
{
private StronglyTypedDs myDS;
private bool _disposed;
public myclass()
{
using (myDSTableAdapter docDocadpt = new myDSTableAdapter())
{
myDS = new StronglyTypedDs();
docDocadpt.Fill(myDS.TheTable);
}
}
#region IDisposable Members
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
~myclass()
{
Dispose(false);
}
protected virtual void Dispose(bool disposing)
{
if (!_disposed)
{
if (disposing)
{
if (myDS != null)
myDS .Dispose();
myDS = null;
}
}
_disposed = true;
}
#endregion
}
The results are :
test 1a - just instantiating myclass, the destructor is commentend since myclass doesn't contains unmanaged resources : myclass.dispose is not called, even if i close the application (whose dispose is executed instead) . So what's the state of the dataset once i close the application ?
test 1b - uncommenting destructor, it's ok all disposes are called when i close the application.
test 2a and 2b - i do the above test just calling gc.collect : the behaviour is identical to test 1a and 1b respectively
test 3 - everything works fine (of course)
many posts says that, if myclass doesn't contains unmanaged resources, i don't need to add the destructor; yet in my tests , if i don't add the destructor , myclass.dispose doesn't get called when i close the application. I haven't waited for the gc to run by itself (if i remember correctly gc.collect doesn't guarantes the class instance is deallocated) to check if it will call myclass.dispose .
So what's the correct implemention : always define e destructor or avoid it if myclass contains only managed resources ?
If i had filled all generations levels, would had the gc called myclass dispose or not without having implemented a destructor?
Finally i've noticed that if i define a destructor , but don't declare the class as implementing IDisposable, the disposing chain works anyway. This could make sense since the destructor might be translated to finalize in IL. But i find it really confusing : is it some kind of "implicit" interface implementation i don't know? gc can dispose the item but users can't
thank you in advance
Stefano

Trust your Garbage Collector. Managed resources will get cleaned up, eventually. There is no need for a finalizer or implementing IDisposable unless you have some external resource that needs to be released.
This typically means that you only should implement IDisposable when you either:
Are wrapping a native resource. (In this case, you probably also want a finalizer.)
Are encapsulating a class which implements IDisposable. (In this case, you want IDisposable, but don't need to implement a finalizer/destructor.)
Part of the confusion with IDisposable, in my opinion, is that it covers quite a few use cases, and the proper implementation changes depending on what you're doing (ie: wrapping native resources, wrapping other IDisposable classes, or using Factored types). To address this, I've written a multiple part series on IDisposable - it might help clarify some of this for you.

The correct pattern is to only use a finalizer when your class contains unmanaged resources. As to relying on the GC to dispose of your managed objects, don't. The IDisposable contract makes it clear that this object needs to be disposed.

Ok i think i've understood, referring to my example its correct to implement a dispose because the dataset is global to my class and implements IDisposable , while i don't need the finalizer because there aren't unmanaged resources.
Even if i "forget" to dispose some managed resource in my class dispose method, the gc will collect it at some point. The dispose method is just an utility i provide to other classes/developers for managed resources, a must with the finalizer if i wrap unmanaged resources .
i'll read the articles you have provided asap, but in the mean time i've the last question : when gc will free memory owned by my class and its resources ? when someone calls dispose or when it will run (it will free memory instead of moving it to next generation ) ?
thank you everybody for your answers and examples

I wrote a brief seris entitled How to Implement IDisposable and Finalizers: 3 Easy Rules. It describes a much simpler approach that Microsoft themselves have followed since the 2.0 version of the BCL.
The "official" pattern is needlessly complex and needlessly confusing.

Your code is correct, you've implemented it exactly like it is documented in the MSDN library.
You'll need to take a second look though. Reason what happens when the destructor (aka finalizer) runs. The disposing argument will be false, the protected Dispose method does nothing. This is entirely normal, finalizers should only ever release unmanaged resources. You don't have any. It is extraordinary rare to ever have an unmanaged resource in your own code. They belong in the nice wrapper classes available in .NET to turn an unmanaged operating resource into a nice managed class. If you find yourself thinking you need a finalizer, you'll be wrong 99.99% of the time. Even if you do wrap an unmanaged resource, you should use one of the SafeHandle wrappers. And rely on their finalizers.
Okay, you want to get rid of the destructor. It isn't healthy to leave it in, it keeps the object in memory longer than necessary. When you do, you'll cut it down to:
public void Dispose()
{
if (myDS != null) myDS.Dispose();
}
Which is the boiler-plate implementation of most any Dispose() method, just call the Dispose method of members of the class. You can completely omit it if you don't have any members with a Dispose() method.
Next, you do misunderstand how the Dispose() method gets called. It is not automatic. Which is the point of having it in the first place, releasing resources automatically is already taken care of by the garbage collector. The Dispose() method is there for you to call, either with the using statement or calling it directly. So that you can release the resource early instead of waiting for the garbage collector finalizer thread to get around to it. Which can take a while. Call it when you know that your program won't be using the object anymore.
If your DataSet is actively used by the form then you cannot dispose it until the form closes. Call the class' Dispose() method in a FormClosed event handler. Or, better, open the form's Designer.cs file, cut-and-paste the Dispose() method you find in there and move it to the form's source code file. And add the dispose call. I know that's a bit confuzzling, but the only time it's okay to edit the designer file.

The main purpose of IDisposable is to have a consistent standard interface you can dispose of unmanaged resources with, that is if you didn't do something to ensure Dispose() was called these resources would hang around after the app was closed. Also understood by the using() syntax, that is using will implement the following block for you:
DisposableType someDisposable = new DisposableType();
try
{
// Do whatever
}
finally
{
((IDisposable)someDisposable).Dispose();
}
This is all implemented in a nice design like so:
using(DisposableType someDisposable = new DisposableType())
{
// Do whatever
}

How to implement IDisposable properly

I've seen so much C# code in my time as a developer that attempt to help the GC along by setting variables to null or calling Dispose() on classes (DataSet for example) within thier own classes Dispose() method that
I've been wondering if there's any need to implement it in a managed environment.
Is this code a waste of time in its design pattern?
class MyClass : IDisposable
{
#region IDisposable Members
public void Dispose()
{
otherVariable = null;
if (dataSet != null)
{
dataSet.Dispose();
}
}
#endregion
}

The GC does not call .Dispose() (It does, however, call the finalize ~MyClass() method, which you can provide a call to the Dispose() method to have resources automatically managed when the GC decides to clean up your class).
You must always provide a way of disposing internal resources such as DataSets to code that uses your classes (and ensure you actually call .Dispose() or wrap the constructor in a using). Using IDisposable on your classes that use internal resources is highly recommended.
From MSDN:
The primary use of this interface is
to release unmanaged resources. The
garbage collector automatically
releases the memory allocated to a
managed object when that object is no
longer used. However, it is not
possible to predict when garbage
collection will occur. Furthermore,
the garbage collector has no knowledge
of unmanaged resources such as window
handles, or open files and streams.
public void Dispose()
{
otherVariable = null;
if (dataSet != null)
{
dataSet.Dispose();
dataSet = null;
}
}

No, Dispose methods are not a waste of time.
The dispose pattern is there to allow a caller to clean up a class as soon as they have finished with it, rather than waiting for the GC to collect it. The delay doesn't matter much for plain heap memory, which is why basic classes like String don't implement it. What Dispose is useful for however is cleaning up unmanaged resources. Somewhere internally, the Dataset class is using an unmanaged resource, so it provides a dispose method to allow you to let it know when that unmanaged resource can be released.
If the pattern has been followed correctly Dataset will also have a finalizer (or some subclass will) which means that if you didn't dispose of it manually, eventually the GC would run, the finalizer would get called and the unmanaged resource would be cleaned up that way. This unmanaged resource might be important though, imagine if it was a file lock, or a database connection, you don't really want to hang around waiting for the GC to run before you can reuse your database connection. Dispose provides a deterministic way of cleaning up resources when they are finished rather than relying on the non-deterministic GC.
As for setting variables to null in a dispose method. It nearly all cases it would be pointless. setting a variable to null removes a reference to that variable, which will make it eligible for garbage collection (if that's the last reference), but as you are disposing of the class anyway, you are likely to be going out of scope for the containing class so the internal class will become eligible for collection anyway.
If you have member variables inside your class that are disposable that you created (not just references you hold), then you should always call dispose on them from your own class's dispose method, but don't bother setting them to null.

Not entirely. If you have member variables which are disposable, then you probably should dispose of it like that. Your object may live longer than the scope of the work it is doing as the garbage collector is not guaranteed to run at any particular time.
Setting managed variables to null is a waste of time though. The object won't get GC'd any faster.

Garbage truck comes to my area every week but it doesn't collect my garbage unless I put my garbage bin in a way that it can collect.
You should simply remove all unwanted event subscriptions, reference and clear unmanaged handlers. Then Garbage Collector will take care of the rest.
Below example show the general best practice to implement IDisposable interface. Reference : https://msdn.microsoft.com/en-us/library/system.idisposable.dispose(v=vs.110).aspx
public class DisposeExample
{
// A base class that implements IDisposable.
// By implementing IDisposable, you are announcing that
// instances of this type allocate scarce resources.
public class MyResource: IDisposable
{
// Pointer to an external unmanaged resource.
private IntPtr handle;
// Other managed resource this class uses.
private Component component = new Component();
// Track whether Dispose has been called.
private bool disposed = false;
// The class constructor.
public MyResource(IntPtr handle)
{
this.handle = handle;
}
// Implement IDisposable.
// Do not make this method virtual.
// A derived class should not be able to override this method.
public void Dispose()
{
Dispose(true);
// This object will be cleaned up by the Dispose method.
// Therefore, you should call GC.SupressFinalize to
// take this object off the finalization queue
// and prevent finalization code for this object
// from executing a second time.
GC.SuppressFinalize(this);
}
// Dispose(bool disposing) executes in two distinct scenarios.
// If disposing equals true, the method has been called directly
// or indirectly by a user's code. Managed and unmanaged resources
// can be disposed.
// If disposing equals false, the method has been called by the
// runtime from inside the finalizer and you should not reference
// other objects. Only unmanaged resources can be 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;
}
}
// Use interop to call the method necessary
// to clean up the unmanaged resource.
[System.Runtime.InteropServices.DllImport("Kernel32")]
private extern static Boolean CloseHandle(IntPtr handle);
// Use C# destructor syntax for finalization code.
// This destructor will run only if the Dispose method
// does not get called.
// It gives your base class the opportunity to finalize.
// Do not provide destructors in types derived from this class.
~MyResource()
{
// Do not re-create Dispose clean-up code here.
// Calling Dispose(false) is optimal in terms of
// readability and maintainability.
Dispose(false);
}
}
public static void Main()
{
// Insert code here to create
// and use the MyResource object.
}
}