Develop Reference

How to dispose object

How to dispose my object? I am doing it by this. Whats is wrong? The Obj.Dispose() function does not appear to be right.
class MainclassForm : Form, IDisposeable
{
public void createanddispose()
{
A obj = new A();
obj.dowork();
obj.Dispose();//does not appear as a function
}
}

You can better use the using statement. Something like
using (MyIDisposableObject obj = new MyIDisposableObject())
{
// object here
}
A good reference to check on MSDN: Avoiding Problems with the Using Statement
The C# "using" statement results in a call to Dispose(). This is the
same as Close(), which may throw exceptions when a network error
occurs. Because the call to Dispose() happens implicitly at the
closing brace of the "using" block, this source of exceptions is
likely to go unnoticed both by people writing the code and reading the
code. This represents a potential source of application errors.

To call Dispose() on an object, your class must be inherited from IDisposeable interface and have an implementation of it:
class A : IDisposable
{
public void Dispose()
{
GC.Collect();
}
}

There are a couple things wrong here, but I believe the root is that you might confusing IDisposable with a Garbage Collector.
IDisposable is an interface that the type A, in your example, may implement. It that were the case, you could be sure that any instance of A had the method Dispose() you're looking for. This is useful for things like Streams, where they need to be closed, but it's not useful for (just as an example) a ComplexNumber type that has no outstanding resources.
In these cases, your managed C# code will handle all the "disposal" that needs to happen. It will get rid of the memory that object is using. That feature comes for free (sort of), and you don't need (or want) to do anything to explicitly invoke it. That's the main difference between managed (C#, Java, etc.) and unmanaged (C++) code.
Essentially, if an object is using more than just memory (like an open file does), IDisposable will be useful. If it is just memory (like a Form, or DateTime, or a majority of other things), there's no need for it.
As for your specific code, you've applied IDisposable to the type that contains your functions. Just as you couldn't call this.AddMinutes(1) to get at DateTime's method, you can't get at this one.
If you do need the added functionality of implementing IDisposable, you'll have to do it on A. Applying IDisposable, like any other interface, to MainclassForm will tell the compiler that you have a Dispose() method on that class, which is true only because the Windows Forms object has one. If it didn't, this would throw a compiler error, and you'd have to add one.

Collection class and IDisposable interface

I've been reading about IDisposable interface lately (this topic was quite useful Proper use of the IDisposable interface) and also about usage of using statement (topic Uses of "using" in C#). Then I started wondering what if I should somehow free my own collection class from memory.
class User{
private string username {set;get;}
private string password {set;get;}
...
}
Should it implement IDisposable interface?
class User : IDisposable
{
...
public void Dispose()
{
this.Dispose();
}
}
so it could be freed from memory? Or does GC do it automaticly and I shouldn't even bother about it.
As far as I understand it's important to free unmanaged resources like DB connections and such but what about those collection classes. Since I use them quite frequently It really started to bug me.
tl;dr;
should I implement IDisposable on User class?
Kind Regards.
edit: thanks everyone for the replies!

Or does GC do it automaticly and I shouldn't even bother about it.
This. Unless you have unmanaged resources (either directly or by way of a reference to something else which is disposable), you almost certainly shouldn't implement IDisposable.
Your current implementation would just call itself:
public void Dispose()
{
this.Dispose();
}
... so assuming you don't really want to call this.Dispose(), what would you want to do when Dispose() is called? It's not like disposal causes garbage collection - so what action do you want to take? If the answer is "nothing" then you probably shouldn't be implementing IDisposable. (The exception here is if this is designed to be a base class and you're expecting some derived classes to require disposal... that's a more complex scenario.)

All unused resources will be Garbage collected eventually when the program unloads or after separate interval if the resources are no longer used.
Its a better practice to clean/dispose used resources(variables, objects) when you no longer wish to use them to free memory.

As said in the other answers, the only times you have to implement IDisposable is when you deal with unmanaged resources or class members that are IDisposable themselves (in this case you should dispose them in your own Dispose method).
Another scenario you might see is when people want to use the using { } syntactic sugar to automatically call some method at the end of the variable scope without using the try { } finally { } form.
I.e.:
public class MyObject : IDisposable
{
public void Foo()
{
}
public void Dispose()
{
// Something they want to call after the use of an instance of MyObject
}
}
...
using (var myObj = new MyObject())
{
myObj.Foo();
}
instead of
public class MyObject
{
public void Foo()
{
}
public void MethodToCallAfterUse()
{
// Something they want to call after the use of an instance of MyObject
}
}
var myObj = new MyObject();
try
{
myObj.Foo();
}
finally
{
myObj.MethodToCallAfterUse();
}

If an object asks an outside entity to "do something" (perform an action, reserve a resource, etc.) on its behalf until further notice, and that outside entity might continue to exist after the object requesting its services has ceased to be useful, then the object should ensure that the outside entity receives notice when its services are no longer required. The IDisposable resource exists as a standard means by which an object can say "I may be obligated to let outside entities know when I don't need their services; let me know when my services will no longer be needed, so I can satisfy my obligation to tell any outside entities that I no longer need their services."
In .NET, once no references to an object exist anywhere in the universe, the object itself will likewise cease to exist. If the only object which knows of an obligation to do perform some action ceases to exist without having performed the action, the action will not get performed. If an object has no obligations, however, having it simply cease to exist once there are no references is just fine.

Can we use Dispose method without Implementing IDisposable Interface?

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.

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.

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
}