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.
Related
Trying to figure it out. Someone told me
Dispose is not just a method - it's equivalent to a destructor in other languages.
Ok. Msdn is also very loud about this.
But then
class Test : IDisposable
{
public string Property { get; set; } = "Test";
public void Dispose() => Console.WriteLine("Disposed, very scary");
}
class Program
{
static void Main(string[] args)
{
var test = new Test();
test.Dispose();
test.Property = "123"; // but it's disposed OMG! do not do this!
test.Dispose();
using (var another = new Test())
for (int i = 0; i < 10; i++)
{
another.Dispose();
GC.Collect(); // or what should I call to make it crash?
}
Console.ReadKey();
}
}
And there are no problems.
What I think about Dispose:
it's a normal public method;
IDisposable is useful in conjunction with using to automatically call Dispose, nothing more;
it's totally fine to put into dispose any code an call it internally at any time if object state properly maintained.
Correct me if I am wrong.
P.S: Downvote means "question is bad/not useful/has problem". If you simply disagree with my thoughts - post comment or answer. It will be useful for people who think as I do right now (because I am wrong? then prove it)
Dispose is just a method, you can call it just like any other method. It is always exposed through the IDisposable interface (yes, obviously you can name a method Dispose without implementing IDisposable, don't do that!).
However, calling it manually is sometimes a code smell, smell of code that should probably be using using instead. By "manually" here I mean calling Dispose outside of the implementation of another Dispose.
Calling it twice should also be safe and is documented:
If an object's Dispose method is called more than once, the object must ignore all calls after the first one. The object must not throw an exception if its Dispose method is called multiple times. Instance methods other than Dispose can throw an ObjectDisposedException when resources are already disposed.
(my emphasis)
Should you be calling Dispose twice? No!. That is also a code smell of code that no longer has control of what it has done and what it has left to do and ends up doing things "just to be sure". Don't do that either!
So if you write code correctly, sure you can call Dispose manually.
You're right, Dispose is just another method, belonging to IDisposable. It just has the added benefit of being able to be automatically called when a using() scope ends - it is not equivalent to a destructor and whoever told you that doesn't truly understand what they're saying.
I'd always use a using() block where possible, but if you're careful you can manually call Dispose instead and it will have the same effect.
Visual Studio 2015 offers up (finally) a suggested pattern for implementation of IDisposable which matches the guidelines.
This is the stub that VS creates for you when selecting to implement the disposable pattern. The TODOs guide us through the implementation.
class Disposable : IDisposable
{
#region IDisposable Support
private bool disposedValue = false; // To detect redundant calls
protected virtual void Dispose(bool disposing)
{
if (!disposedValue)
{
if (disposing)
{
// TODO: dispose managed state (managed objects).
}
// TODO: free unmanaged resources (unmanaged objects) and override a finalizer below.
// TODO: set large fields to null.
disposedValue = true;
}
}
// TODO: override a finalizer only if Dispose(bool disposing) above has code to free unmanaged resources.
// ~Disposable() {
// // Do not change this code. Put cleanup code in Dispose(bool disposing) above.
// Dispose(false);
// }
// This code added to correctly implement the disposable pattern.
public void Dispose()
{
// Do not change this code. Put cleanup code in Dispose(bool disposing) above.
Dispose(true);
// TODO: uncomment the following line if the finalizer is overridden above.
// GC.SuppressFinalize(this);
}
#endregion
}
Note the use of a flag to indicate if the Dispose has already been called. This flag can be used to throw ObjectDisposedException instances on method calls and properties across the classes implementation.
The intention of Dispose has always been to clean up unmanaged resources. However, the syntactic sugar of the using keyword (requires an IDisposable) lets you create really tidy 'context' pattern classes for managing resource access, even if there are no unmanaged resources used.
As usual, document the usage and the intentions clearly and you can't go wrong but please avoid methods called 'Dispose' if you don't implement the interface.
There is a critical difference between Dispose in .NET versus destructors in other languages: if one has a pointer to an object in another language and the object is deleted, one will have an invalid pointer and there is no way to do anything with it, including determine whether it is still valid, without invoking Undefined Behavior. By contrast, in .NET if one has a reference to an object which gets Disposed, the reference will continue to be a valid reference to the object in question. The object will likely refuse to do much because it's been disposed, but such refusal is affirmatively generated by the object itself. No Undefined Behavior involved.
On the flip side, when a destructor is invoked in other languages like C++ an object can be certain that no valid pointers exist to it anymore, but that is not true of Dispose. Consequently, code should avoid pooling public-facing objects and instead have every request for a new object return a new object (which may be a wrapper to a pooled object). If no reference to the pooled object exists outside the wrapper, and Dispose invalidates that reference before returning the object to the pool, then a future object request can be satisfied by encapsulating the pooled object in a new wrapper which will again hold the only reference to it.
Can we use Dispose method without Implementing IDisposable Interface ?
You can name a method as Dispose and use it as an ordinary method without any restrictions:
public class MyClass {
public void Dispose() {
...
}
}
...
// using() emulation
MyClass m = null;
try {
m = new MyClass();
...
}
finally {
if (m != null)
m.Dispose();
}
but if you want using() syntax you have to implement IDisposable:
public class MyNextClass: IDisposable {
protected virtual void Dispose(Boolean disposing) {
...
}
public void Dispose() {
Dispose(true);
GC.SuppressFinalize(this);
}
}
...
// compare this with the using() emulation in the code above
using (MyNextClass n = new MyNextClass()) {
...
}
Anyway, even it's possible to name a method Dispose it's not a good practice to surprize (and finally irritate) developers (including you) who read your texts; so either implement IDisposable or find some other name for the method.
Nobody will stop you from doing that but it is very poor design because everybody knows about the idisposable interface. If you give a different meaning to this method, you are obfuscating your design to whoever takes a look at your code later. Maybe even yourself in a few years where you don't remember the reason why you did this .
Managed Objects are disposed automatically even if you do not implement IDisposable, IDisposable let you dispose which runtime won't dispose like files, open handles unmanaged code components.
Implementing a Dispose Method
The pattern for disposing an object, referred to as a dispose pattern,
imposes order on the lifetime of an object. The dispose pattern is
used only for objects that access unmanaged resources, such as file
and pipe handles, registry handles, wait handles, or pointers to
blocks of unmanaged memory. This is because the garbage collector is
very efficient at reclaiming unused managed objects, but it is unable
to reclaim unmanaged objects.
If you do not want to use the IDisposable pattern and want to have your own then I believe that is not recommended way as it one would have to discover that could be obvious using IDisposable.
Please consider the following class:
public class Level : IDisposable
{
public Level() { }
public void Dispose() { }
}
My question is, if I call the Dispose method, will the class actually be disposed (garbage collected)?
ex:
Level level = new Level();
level.Dispose();
Thanks.
My question is, if I call the Dispose method, will the class actually be disposed?
If by disposed you mean garbage collected, then no, this won't happen. What will happen when you call the Dispose method is, well, the Dispose method be called and its body executed.
Also it is recommended to wrap disposable resources in a using statement to ensure that the Dispose method will always be called even in the event of an exception. So instead of manually calling it you could:
using (Level level = new Level())
{
// ... do something with the level
}
Normally the Dispose method is used when the object holds pointers to some unmanaged resources and provides a mechanism to deterministically release those resources.
I assume that what you are after is a way to know that Dispose() was called?
You can do that either in the consuming code by setting the instance to null after disposing:
Level level = new Level();
//do stuff with the instance..
level.Dispose();
level = null;
//in other place:
if (level != null)
{
//still available
}
Or in the class itself, add boolean flag and in every method check for it:
public class Level : IDisposable
{
private bool disposing = false;
public Level() { }
public void Foo()
{
if (disposing)
return;
MessageBox.Show("foo");
}
public void Dispose()
{
if (disposing)
return;
disposing = true;
}
}
Each class that implements IDisposable defines what it means to be disposed. By that line of reasoning, yes, your class will be as disposed as it wants to be.
No, the instance won't be garbage collected due to calling Dispose. The Dispose method is where you can release any resources held by the instance, it isn't about disposing the instance itself.
If you call Dispose() it will be disposed, which doesn't absolutely mean that it will be garbage collected,seems to me that is your question, if not please clarify.
In .NET Disposing and garbage collection are two different things:
The point of disposing is to release any resources that are either not managed by .NET (like manually allocated memory) or that have interests in being released as soon es they aren't needed anymore (like file handles or network connections).
The purpose of garbage collection is to free memory that is managed by .NET (like normal objects).
So, as others pointed out: your object will not necessarily be garbage collected when it gets disposed.
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
IDisposable Question
I have written a class and implemented the IDisposable interface.
I implemetned the Dispose method and put a Code Break in the method.
My assumption is it would have been called when the Class went out of scope due to C# Garabage collection.
I want the dispose method to close an unmanaged resource. I thought it would be more elegant than just calling the method LogOff() instead getting it called whenever the method went out of scope?
But it doesn't seem to get called or stop at the code break.
You need to explicitly call Dispose on any objects that implement IDisposable. If you use the using() {} code construct the compiler will automatically call Dispose at the end of the using block.
A good pattern is to also track via a private boolean field whether dispose has been called or not, and if not call it from the objects finalizer (and also call GC.SuppressFinalize() from your Dispose method assuming that you handle all finalization tasks from there also).
You should consider wrapping your interaction with your IDisposable class in a using statement. Doing so will allow you to specify when your object goes out of scope, and ensures the Dispose() method gets called.
For the correct syntax, see the example from the referenced MSDN article:
using System;
class C : IDisposable
{
public void UseLimitedResource()
{
Console.WriteLine("Using limited resource...");
}
void IDisposable.Dispose()
{
Console.WriteLine("Disposing limited resource.");
}
}
class Program
{
static void Main()
{
using (C c = new C())
{
c.UseLimitedResource();
}
Console.WriteLine("Now outside using statement.");
Console.ReadLine();
}
}
Short answer: Dispose() is called when you call it.
Long answer: take a look at using block. This is a syntax sugar that meant to be used together with IDisposable interface for pretty and safely disposing code, and is roughly equivalent to
Foo foo = new Foo();
try
{
// your code that uses foo
}
finally
{
foo.Dispose();
}
In other words foo is guaranteed to be disposed upon leaving using() scope.
Garbage collection does not happen immediately after a variable goes out of scope. The GC runs periodically and .NET has different "levels" of Garbage collection. Different levels get collected more frequently. If you want your object's dispose method to be called immediately, you should use a using statement
using (MyClass object = new MyClass())
{
//ensures Dipose is called, even if exceptions are thrown
}
My assumption is it would have been called when the Class went out of scope due to C# Garabage collection.
That's now how garbage collection works. There's two things going on here:
Garbage collection - this cleans up objects at some time after there's no longer any references left to them. This implies that the objects have gone out of scope (if they are locals), but notably GC doesn't say when this cleanup happens - it usually happens lazily when the system decides it needs to run a collection to free up more memory. The method that is called to clean up resources in this case is the finalizer, which in C# has the form ~Classname().
IDisposable: the problem with GC is that you have no control over when the finalizer is called, so IDisposable was introduced as a pattern to be used when you need resources to be cleaned up at a specific time and don't want to wait for a collect to happen. It's up to the caller code to call Dispose() as appropriate, there's no GC support. C# does have the using(){} syntax, which simplifies this, and calls Dispose() automatically at the end of the using block.
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.
}
}