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How to properly dispose locally created object in another method?

So I have a class which implements IDisposable, and I have several methods (in another class) which follow the pattern below:
public void SomeMethod()
{
DisposableObject disposableObject = new DisposableObject();
// Do some stuff with the object
SomeOtherMethod(disposableObject);
disposableObject.Dispose();
}
While all of these methods do different things, they all call SomeOtherMethod at the end, which does a few more things with the disposable object before it's no longer needed.
When I move disposableObject.Dispose(); into SomeOtherMethod, Visual Studio gives me a message saying:
"Use recommended dispose pattern to ensure that object created by 'new DisposableObject()' is disposed on all paths: using statement/declaration or try/finally"
This message appears regardless of whether or not I pass the disposable object to SomeOtherMethod using the ref keyword.
My question is, will that object be disposed as long as SomeOtherMethod calls Dispose() on it? I'm assuming it will, and Visual Studio continues to send the message simply because it isn't "aware" of what's happening to that object in subsequent methods, but I'd love to get some confirmation on that!

It may be disposed or may be not, depends on the fact whether the execution will reach the Dispose invocation or not and that's because an exception can be thrown before the Dispose is called. Using try finally construction explicitly or implicitly by using keyword ensures that it will be called for any scenario and that's why VS gives you the warning.

will that object be disposed
Sorry, but that’s a meaningless question. The CLR does not keep track of whether an object has had its “dispose” method called or not (see Will the Garbage Collector call IDisposable.Dispose for me? )
As a general rule, it is always much nicer (readable/ maintainable / less-bug-prone / etc) that a method that creates an issue should also be the one that cleans up after itself. As you’ve just found, this pattern also allows automated checking by the compiler - and again, it is also a good rule to ensure that your code compiles cleanly without errors OR WARNINGS.
In this case, the warning is giving you a couple of ways to implement this cleanly; personally, I would prefer the “using” clause (so avoiding having to have an explicit call to “dispose”) like :
public void SomeMethod()
{
using (DisposableObject disposableObject = new DisposableObject() )
{
// Do some stuff with the object
SomeOtherMethod(disposableObject);
}
}

No matter where one call the Dispose(), it is called.
Not using the language keyword using for the disposable pattern, therefore moving the dispose in another method, is an anti-pattern, therefore it is a bad practice and a source of potential problems.
You can only remove the warning by adding the warning number in the project build settings.
The method Dispose() doesn't destroy the object.
The dispose pattern is only for freeing unmanaged resources like windows handle and shared memories.
After a call to Dispose() you still have the object and so the reference to the object that remains referenced in the managed memory.
Dispose() is made to be called once time at the end of object usage and no more.
The compiler send you a warning because you break the standard behavior of the pattern usage that is to use the using keyword.
And breaking standards can be source of problems.
The using of disposable objects standard is made to avoid bugs by letting the compiler generates the try { ... } finally { Dispose() } block to be sure that Dispose() is correctly called in the right place to avoid mistakes.
So avoid calling the Dispose() directly.
Unless you are sure of what you do, prefer using:
public void SomeMethod()
{
using ( DisposableObject disposableObject = new DisposableObject() )
{
// Do some stuff with the object
SomeOtherMethod(disposableObject);
}
}
And your code may be robust.

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.

Simple destructor issue (IDisposable interface)

I am a beginner in C#, I don't know why this isn't working, I just want to set this object to null by calling Dispose() method.
Why this is not possible?
class MyClass:IDisposable
{
public void Dispose()
{
this = null;
}
}

The purpose of the Dispose method isn't to clean up that class, but to clean up the disposable dependencies that the class is holding on to so that it can be disposed of normally by the garbage collector.
I'd suggest reading up more on the Dispose pattern and how to implement it in C#.
A bit of pedantry: The Dispose method is not a destructor, nor is it a finalizer.

Your question really seems to boild down to 'How do I delete stuff in C#'. The short answer is you can't, that's the job of the Garbage Collector System.GC. The IDisposable inerface is used to ensure that imporant resources that don't belong to .Net (like files, or network/database connections, but not a .Net class), are cleaned up on demand.
The IDisposable interface is important because it allows you to use the using pattern. That is, if your class implements IDisposable it can be used in a using block. Consider for instance:
class MyClass : IDisposable {
public void Dispose() { }
}
this class can now be used like this:
using (MyClass instance = new MyClass())
{
} // instance.Dispose() is called here at the end of the block.
The point of the using block is that when the block ends, it the compiler will throw in a call to Dispose() which you can use to get rid of important resources like files and database connections. For example, all of the Stream classes, like FileStream and what not implement IDisposable because it's a bad idea to leave a file open. Instead, you wrap all of your access in a using block, and then you are guaranteed that FileStream.Dispose will close the file out. Consider:
using (FileStream myFile = File.OpenRead("..."))
{
// Read the content of the file.
} // The file is guaranteed to be closed here. Cool!
This is much neater than doing something like this:
FileStream stream = File.OpenRead(" ... ");
stream.Close(); // Yes, you closed it manually, but it's error prone. What if you forget to do this?
Now what you're thinking of is a term called "Finalization", that is when the class is actually destroyed. This happens when the garbage collector (the System.GC class) actually destroys objects and cleans up their memory. Consider:
public class MyClass {
// This method, the 'Finalizer' will be called when the class is destroyed.
// The 'finalizer' is essentially just the name of the class with a '~' in front.
~MyClass() {
Console.WriteLine("Destroyed!");
}
}
public class Program {
public static void Main() {
MyClass referenceHeld = new MyClass(); // Reference held
new MyClass(); // No reference held on this class
WeakReference sameAsNoReference = new WeakReference(new MyClass()); // Equivalent to no reference.
System.GC.Collect(); // Force the garbage collector to collect
Console.ReadLine();
}
}
In short, the Garbage collector is the part of the runtime that cleans up stuff that isn't being used. What does it mean to not be used? It means that there are no references attached to the object. For example, if you run the program above, you'll notice that the word "Destroyed" gets printed on the screen twice. That's because two of the instances of MyClass created in the Main function are not pointed to by reference (A WeakReference is essentially the same thing as no reference). When we call GC.Collect() the garbage collector runs and cleans up the references.
That said, you should NOT call GC.Collect on your own. You can, for experimentation and education of course, but most people will tell you that the garbage collector does a fine job of keeping things clean on it's own. It doesn't make sense to have a bunch of GC.Collect scattered throughout your code, because that's the whole point of having a Garbage collector - to not have to worry about cleaning things up yourself.
So in short, you really can't destroy objects on your own, unless you call GC.Collect() (which you shouldn't do). The IDisposable interface allows you to work with the using pattern, ensuring that important resources are released (This is not the same thing as destroying the object though! All IDisposable does is ensure that Dispose() is called when the using block exits so you can clean up important stuff, but the object is still alive - an important distinction).

A class cannot make itself null because it has no control of who is referencing it.
For example, if you have a variable with an object in your code, that object cannot make itself null in YOUR code, it can only set it's OWN members to null.
In addition to this, think of a case where multiple classes reference the same object, where would he be null?
This is the kind of things the containing class should do.

The simple answer is that the "this" keyword is read only and can't be set.
A longer more fundamental answer is that you can't set objects themselves to null in c#, however you can set the reference to an object to null. When you set an object's reference to null and nothing else references that object, that is when the object is in a state to be garbage collected. (This is a simplification of what actually occurs.)
For example:
Object oTest = new Object;
oTest = null;
In the example above the object still exists after its reference oTest has been set to null. Its simply waiting for the garbage collector to come around and delete it.
So in your code it looks like your trying to set all of your references to your object equal to null, even if that may not be what your intending. This can't be done from the object itself. You need to make sure that all of the reference to your object are manually set to null or they are guaranteed to leave scope in your program.
null (C# Reference)
this (C# Reference)

You cannot modify your this pointer. In your example, your Dispose method doesn't need to do anything, so could be omitted altogether (along with updating the class to no longer implement IDisposable)

Dispose pattern is only needed when you use non-CLR resources, like graphics contexts or low level io. There are edge cases when you need to free up resources now, but as you say you are a beginner you really should not bother (yet).
Setting this to nil does not help. Consider this
MyClass sample = new MyClass();
sample.Dispose();
// at this point, sample still has a value
When you want to get rid of an object in C#, all you need is to let all the references go out of scope, or set them to nil. (Multiple variables can refer to the same instance). The runtime will free the object (and its subobjects) automatically because nobody is using it anymore.
Roughly speaking, you can think of them as pointers (technically they are not, but we are trying to explain the principle here)

When does the Dispose Method get called? [duplicate]

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.

Will object with reference kept be disposed?

May I know if the "objectToBeDisposed" will be disposed if its reference are kept by the "useDisposableObject"?
using(IDisposable objectToBeDisposed = new ObjectImplementedIDisposable ()){
ChildObject useDisposableObject = new ChildObject(objectToBeDisposed);
.... doing stuff with useDisposableObject;
}

Yes, in your example it will be disposed, because the using block explicitly calls the Dispose() method when leaving that block.
As an aside the referencing object is within the using block scope and will go out of scope at the end of the using block, which will avoid the potential issue of accessing a disposed object if the referencing object tried to use it once it was disposed.
Update thanks to #dlev

Disposal, finalization and memory deallocation are 3 totally different things in the .NET world.
Disposal is only an "application logic" thing, it is implemented through the disposable pattern.
If you implement the interface IDisposable, your object can be used with the using keyword.
The using keyword will translate in a simple way:
using (IDisposable xxx = new MyObject())
{
....
}
means
{
IDisposable xxx = new MyObject();
try
{
....
}
finally
{
if (xxx != null)
xxx.Dispose();
}
}
There is no other magic in this. That's all.
Object finalization is instead called by garbage collector when the objects should be freed.
The time when it happens and the order between destructors is unpredictable.
class myclass { ~myclass() { this code will be called by garbage collector, i cannot call it } }
The syntax is very similar to C++ destructors but the meaning is a lot different.
And then memory deallocation.
Memory deallocation is not connected to object disposal!
You can keep forever a reference to a disposed object. Disposal is just something related to application logic.
Destructors can also be cancelled or called more than once by garbage collector (see GC.ReRegisterForFinalize and GC.SuppressFinalize).
The presence of a destructor in a class slow down a lot the garbage collector, and inside destructor, you can do almost nothing except releasing unmanaged resources.
Disposal instead as I said is an application thing, is just a method, and is not related to garbage collector.

"Disposing" an object has nothing to do with which references are kept around. IDisposable itself is just an interface, nothing more.
When a class implements that interface, a user of the class can call a public void Dispose() method on the object. When the user of the class places an instance of the class in a using block, the Dispose() method will automatically get called. When an Enumerator class implements IDisposable and is used in a foreach loop, it will also have Dispose() called on it.
This has absolutely nothing to do with which references are kept around, or the automatic garbage collection system.
However, IDisposable also has a design "contract" that goes with it. This contract is a pattern that classes that implement IDisposable must follow to have correct program behavior.
The contract is that once you've called Dispose(), you should never call any property or method on the object again, otherwise you should expect an ObjectDisposedException to be thrown. The only thing that is allowed to be called on that object after that point is Dispose() again, or the object finalizer (via garbage collection).
Because of this, people will sometimes set an reference to an IDisposable class to null after they have disposed it, so they don't accidentally call methods on the already-disposed object. If you placed the reference in a using block, you don't have to worry about this; The compiler won't even let you touch the reference again. If you try, you will get an error and your program will fail to compile.
This contract is not built into the language or CLR itself - it must manually be adhered to by the implementer of a class.

Yes, it will be. I might be wrong, but I don't think #John Weldon's explanation is not accurate though. You can only call using on an IDisposable implementing class, and when the using exits, it will call Dispose() on the object.
The part where I don't agree is the 'because the referencing object is within the using block scope and will go out of scope at the end of the using block' part. The objects declared in scope will not always be disposed of when the using exits. If you take this example
JustAClass myInstance = new JustAClass();
using ( MyDisposableClass theUsingInstance = new MyDisposableClass() )
{
AnotherClass innerInstance = new AnotherClass();
myInstance.ItsOtherClass = innerInstance;
myInstance.ItsWhatever = theUsingInstance;
}
At the end of this code, myInstance.ItsOtherClass is not null. innerInstance has been created in the using section and won't be garbage collected since there is still a referenced. However, the object refered by myInstance.ItsWhatever has been disposed of, since it's the using target.