Develop Reference

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.

Is it safe to create and use a disposable object inline?

I have seen so many times developers using a disposable object inline, here for instance. By inline I mean:
var result = new DataTable().Compute("1 + 4 * 7", null);
I know that the Dispose method won't be called, but since no reference is held to the object, how will the garbage collector handle it? Is it safe to use a disposable object like that?
I used a DataTable in my example because it is the only concrete example I found, but my question applies to disposable objects in general. I do not personally use them like that, I just wanted to know if they are handled diffently by the GC if they are used that way.

The key problem here is the timing when Dispose is called, which is just unknown in your example (providing a good implementation of IDisposable — see below). I'd consider using an IDisposable instance without a using statement a code smell. The class is implementing IDisposable for a reason, and thus you as a user should obey its contract.
However, note that in a correct implementation of IDisposable the class'es finalizer handles the disposal of an un-disposed object. Hence even if not instantiated within a using, the disposal shall be performed, but in unknown time and on a different (the GC's) thread.
I just wanted to know if they are handled diffently by the GC if they are used that way.
No, the GC treats all object alike and doesn't treat IDisposable implementations anyhow differently. However, in a correct implemenetation of IDisposable the Finalize method (invoked by the GC on every object unless suppressed on a per-object basis) call would lead to invoking the Dispose.

If the object that is being initialized inline has a destructor implemented, that destructor could be calling Dispose() after the object has gone out of scope.
However, the cleaner and correct way is to use a using statement because this could potentially lead to just instances of objects hanging around with no purpose.

It is not safe to use disposable objects in this way. Yes, no reference wont be held but the purpose of Disposable pattern is to dispose all unmanaged resources(like OS handles) which cannot be garbage collected.

Implementing IDisposable in a class exposing Begin/End methods

Say I have a class that exposes BeginLongOperation() and EndLongOperation() methods with the standard Begin/End pattern, and implements IDisposable.
Is it my class's responsibility to handle a call to Dispose() between the calls to BeginLongOperation() and EndLongOperation()?
If so, what is the proper way to do this?

Is it my class's responsibility to handle a call to Dispose() between
the calls to BeginLongOperation() and EndLongOperation()?
No, it's the responsibility of the caller of your class to properly dispose it. This could be done in the EndLongOperation method. You cannot wrap the instance in a using statement because the BeginLongOperation method will return immediately.
Example with a WebClient:
var client = new WebClient();
client.DownloadStringCompleted += (sender, e) =>
{
try
{
if (e.Error == null)
{
Console.WriteLine(e.Result);
}
}
finally
{
((WebClient)sender).Dispose();
}
};
client.DownloadStringAsync(new Uri("http://www.google.com"));

I assume that Begin/End methods are asynchronous. Or, that your objects can be manipulated from another thread.
First, you need to determine if you really need to dispose the instances of your class.
If so, you need to design your Dispose() method (your class should implement IDisposable interface) in such way that it won't interfere with the long operation being executed. Well, it depends what policy you want to implement: do you want to wait for long operation to finish, or a call to Dispose() method should interrupt the long operation?
Normally you never call Dispose() method from your class' inside code, but indeed you need to protect the Dispose() call from unappropriate usage.
So I think that is YOUR responsability to protect your code against any possible scenarios, even against a (undesired) call between Begin and End.
LATER EDIT: of course, as other guys here told you, the responsability of the user of your class is to properly use it, BUT I would NOT rely on this. As you know, when the last reference to your object is gone, your object is subject to be garbage collected. This bad usage pattern could indeed determine a call to Dispose() between Start/End even in a non-async / single thread design.

You should not declare object of such class inside a using block but try to dispose it manually.
You can call dispose of such class inside EndLongOperation handler in the class which uses this object.

Idea behind the IDisposable is below and I think that answers your question also.
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.
Use the Dispose method of this interface to explicitly release unmanaged resources in conjunction with the garbage collector. The consumer of an object can call this method when the object is no longer needed.
So its duty of developer who is using your class to call Dispose().

Can I detect whether an object has called GC.SuppressFinalize?

Is there a way to detect whether or not an object has called GC.SuppressFinalize?
I have an object that looks something like this (full-blown Dispose pattern elided for clarity):
public class ResourceWrapper {
private readonly bool _ownsResource;
private readonly UnmanagedResource _resource;
public ResourceWrapper(UnmanagedResource resource, bool ownsResource) {
_resource = resource;
_ownsResource = ownsResource;
if (!ownsResource)
GC.SuppressFinalize(this);
}
~ResourceWrapper() {
if (_ownsResource)
// clean up the unmanaged resource
}
}
If the ownsResource constructor parameter is false, then the finalizer will have nothing to do -- so it seems reasonable (if a bit quirky) to call GC.SuppressFinalize right from the constructor. However, because this behavior is quirky, I'm very tempted to note it in an XML doc comment... and if I'm tempted to comment it, then I ought to write a unit test for it.
But while System.GC has methods to set an object's finalizability (SuppressFinalize, ReRegisterForFinalize), I don't see any methods to get an object's finalizability. Is there any way to query whether GC.SuppressFinalize has been called on a given instance, short of buying Typemock or writing my own CLR host?

This is not possible, the GC just doesn't provide this information. Good reason for that, it isn't just two states that the object can be in. It also might already be on the finalization queue or it might already have been finalized.
A custom CLR host isn't going to help you with that, the hosting interface don't provide any hooks into the gc. You can check if SuppressFinalize has been called when it should have simply by checking this in the finalizer. Log it (quickly). You can't prove the opposite.
Fwiw, the .NET frame classes don't do this, they just let the finalizer run anyway.

If you want to confirm that finalization has been suppressed if your object doesn't own the resource, perhaps you could have the finalizer assert that it owns the resource? The test would have to do GC.Collect and GC.WaitForPendingFinalizers, but production code wouldn't have anything extra except the assert (which could be omitted from the production build). One slight caveat with the assert: if the thread that creates the object dies between creating the object and setting the ownership status, the finalizer may run inappropriately.
That having been said, I wonder whether it would be better to have an abstract ResourceWrapper type, with separate subtypes OwnedResourceWrapper and SharedResourceWrapper, that own or do not own the resource in question. Then the subtype that doesn't own resources wouldn't need to have a finalizer in the first place. Note that it may be useful for SharedResourceWrapper to implement IDisposable as a no-op.

This may help (reductio absurdum). A trick would be do some log (this could be a static state) in finalizers and if somebody is absent, he has called the suppress finalize but still you cannot be sure when.
This is working if you are the author of the type.

Using specific patterns to implement interfaces

I have been very intriuged by design patterns lately, and specifically following correct design patterns in my classes that implement one or several interfaces.
Let's take an example. When a class implement IDisposable you should follow a specific pattern to make sure that your resources are properly cleaned up, by creating a private Dispose(bool disposing) method that differentiates between if it's called by the finalizer, or if it's called from the public Dispose method. Also, a finalizer should be implemented in this case, and you might also need a private bool variable called isDisposed that is set by the Dispose method, so that any method called after th object is Disposed will call a Exception making it clear that this object is already disposed, instead of the code inside the method crashing because some of the required resouces are disposed, and thereby no longer available.
There are also a lot of other interfaces I routinely implement, but it's not all of them I am sure if the way I implement them is the preferred way of doing it, and I might find out later on that it causes a subtle bug that is hard to find, that would probably have been non-existant if I had followed the proper pattern in the first place.
some examples of interfaces I would like to know the best way of implementing are ISerializable, IComparable, IComparable<>, ICloneable, IEnumerable<>, and so on. All interfaces from the Framework are interesting here, so it should not be limited to those I have listed above.
What I'm after is for different interfaces, the preferred way and hopefully also a link to resource on the internet that explains how and also why a specific pattern should be followed.
I hope to get a good collection of these patterns, as I know that they can greatly improve my code and make it more correct, and follow best-practices
It would be great if there are several patterns for the same interface so we can discuss which one is preferred. That might also cause some of you to maybe move over to a new pattern, or make modifications to your existing patterns to improve your code even further, and that would be great!
Edit
After reading Grzenios comment, I would also urge everyone to give the context the pattern should be applied for. For example the IDIsposable pattern should only be followed if you have some unmanaged resources inside your class that you need to dispose, and not if all the objects you need to dispose implements IDisposable themselves.
Edit 2
I should probably start this myself, since I put the question out here. So I'll describe one pattern I know well, and that is the IDisposable pattern.
This pattern should only be used if your class contain one or more unmanaged resources inside your class, and you hav eto make sure that they get Disposed. in this case in addition to the Dispose method we will need a finalizer in case the user of your class forget to dispose it.
So first thing first. Your class should implement the IDisposable interface, and you will have to define the public Dispose method as goverend by the interface. This method should look like this:
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
This will call the protected Dispose(bool) method that takes care of the actual cleanup.
Also, include a vaiable in your class to indicate if the class is disposed or not:
private bool alreadyDisposed = false;
GC.SuppressFinalize tells the garbage collector that this item does not need to be finalized even if it has a finalizer.
Then you need the protected Dispose method. Make it protected instead of private in case any derived class needs to override it:
protected virtual void Dispose(bool isDisposing)
{
if (alreadyDisposed)
{
return;
}
if (isDisposing)
{
// free all managed resources here
}
// free all unmanaged resources here.
alreadyDisposed = true;
}
The finalizer should also call Dispose(bool) if the user forgets to clean up:
~SomeClass(){
Dispose(false);
}
If some method require a disposed resource to function, make the function like this:
public void SomeMethod()
{
if (alreadyDisposed)
throw new ObjectDisposedException("SomeClass",
"Called SomeMethod on Disposed object");
// Method body goes here
}
That's it. This will make sure that the resources gets cleaned up. Preferable by the user of your class calling Dispose, but by adding a Finalizer as a fallback method.

While you're learning about design patterns, you should also look at some common anti-patterns, you get an idea where the pattern comes from. IDisposable is somewhat of an anti-pattern, a minor version of sequential coupling, in that it requires a user to call dispose, and if he forgets, you're in the shit. One of the main reasons of the "disposable pattern" is to fix this issue.
A preferred technique (of mine anyway), wherever possible, is not to expose an IDisposable object to a user, but expose a single method (call it Using(...) for example), which takes a delegate which would execute the code normally contained in your using(...) { } block. This method can perform your construction, execute the delegate, then dispose the resources it consumed, and omits at least 3 problems with IDisposable: That of the user forgetting to call it, the user calling it more than once, and the user calling it too early - you don't need to bother with that boilerplate "disposable pattern" when there's no IDisposable exposed.
An example, lets say I have a File IO requirement where I need to open the same files regularly (and thus, I can't be waiting on the garbage collector to call Finalize in the event that a user forgets to call Dispose).
class MyFileStream {
FileStream fs;
private MyFileStream(string filename, FileMode mode) {
fs = new FileStream(filename, FileMode.Open);
}
private void Dispose() {
fs.Dispose();
}
public static void Using(string filename, FileMode mode, Action<MyFileStream> use) {
MyFileStream mfs = new MyFileStream(filename, mode);
use(mfs);
mfs.Dispose();
}
public void Read(...) { ... }
}
A caller can then say
var x = default(...);
MyFileStream.Using("filename.txt", FileMode.Open, (stream) => {
x = stream.Read(...);
});
Console.WriteLine(x);
Note that this is rather similar to the using() { } language syntax anyway, only this time you are forced to use it, and it imposes further restrictions. One cannot forget to clear up the resources this way.
Still, this pattern isn't always suitable because you sometimes need the resources to last longer than just a method call or so, but if you find the opportunity to use it, do so.
I've completely gone off topic anyway, so back to what you were asking.
Do not use ICloneable - it says nothing about how an object is being cloned (deep or shallow), if you require such thing, create your own IDeepCloneable or IShallowCloneable interfaces instead.
For IEnumerable<>, it's a rare event that you need to create your own, because there's a rather large collection of existing classes in the framework already, and you can usually get additional features much more simply by implementing extension methods (Such as those in LINQ, or your own, making use of the powerful yield keyword, which will create an IEnumerable<> for you in the background).
I wouldn't say there's any specific pattern for the rest, they're pretty self explanatory.

For example the IDIsposable pattern should only be followed if you have some unmanaged resources inside your class that you need to dispose, and not if all the objects you need to dispose implements IDisposable themselves.
I disagee with the above. I never recommend allowing the GC to do clean up and be implicit about object and resource disposal. It's kind of like waiting for the maid to come around and pick up your wet towels at a hotel; it will happen eventually, but the right thing to do is pick them up and hang them up yourself.
Deterministic disposal of objects and keeping scope of those resources as minimal as possible will make for the leanest and most efficient applications, not to mention that for developers reading the code, it is much more explicit to dispose of resources and reads better. Like a beginning and an end to a story that one can see all in one place. I instantiate the object as late as possible and dispose of it as soon as possible after use minimizing scope. Either explicitly calling .Dispose on the object or utilizing a Using block that calls the .Dispose method automatically are (2) good ways of cleaning up.
There are also a lot of other interfaces I routinely implement, but it's not all of them I am sure if the way I implement them is the preferred way of doing it, and I might find out later on that it causes a subtle bug that is hard to find, that would probably have been non-existant if I had followed the proper pattern in the first place.
The entire purpose of an Interface is to create a guidline for the methods, properties, events, etc. that a class is to implement, but not provide the details on how to do it. That is up to you. There is no "pattern" per se of how to implement specefic Interfaces. Don't get thrown off by the IDisposable Interface. Upon creating the Interface in VS.NET, the plumming code is created for you which is not typical for an Interface, and in reality could be completely changed to what you need.
I think where you might be getting confused is about Implementing Interfaces that are in the .NET Framework, but you need to look at the concept of an Interface and not just the ones available in the Framework. If you really want to see examples of how the Interfaces in the .NET Framework are implemented, look at either the MSDN or decompile other Framework objects that implement those Interfaces to see how Microsoft implements them.
As you grow in OOD, you will begin to see the power behind interfaces and begin to create them for your classes. For example lets say you have an interface named IMachine with a method called .StartEngine(). We just want the implementer of the Interface to define all of the details themself; there is no 'pattern' to follow and we the Interface designer don't need to know or care how it is implemented. For a car maybe the methods implementation involves "Get the keys, place them in the ignition, put in park, press brake pedal, turn ignition..." However for a lawnmower the same methods implementation is "Put gass in the mower, prime the carborateor, pull the clutch, pull the cord..."
So you see you can't just look at Interfaces by the ones in the .NET Framework and how to apply a certain pattern to thier implementation. The implementation details, patterns, etc. are up to the implementing class, and as long as the details of the Interface are sufficed, then thats what matters.
Hope this helps a bit...