Is Idisposable.Dispose() Called Automatically

Whether dispose is called automatically by GC when not explitly called?

Is in this case Dispose method will be called automatically by GC?

No. It will not call Dispose you have to call it yourself or use using block which will ensure its disposal even in case of an exception.

using(var disposable = new DisposableObject())
{
  //your code
}

IDisposable is used to ensure that unmanaged resources held by the application are released prior to garbage collection. Since GC can't release resources held by unmanaged resources like file handler.

When does the Dispose Method get called?

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).

Do IDisposable objects get disposed of if the program is shut down unexpectedly?

If the cause is an exception and thrown from within a using block or a try catch finally block, it will be disposed as it should. If it is not catched by a using block it is not disposed automatically (like it doesn't do when the application closes properly).

A sample:

IDisposable d1 = new X();

using (IDisposable d2 = new X())
{
    throw new NotImplementedException();
}

d1.Dispose();

d1 is not disposed, d2 usually is. Some types of exceptions may prevent handling of using blocks and also some program crashes. If the cause is a power failure or system crash, there is nothing you can do either of course.

Will the Garbage Collector call IDisposable.Dispose for me?

The .Net Garbage Collector calls the Object.Finalize method of an object on garbage collection. By default this does nothing and must be overidden if you want to free additional resources.

Dispose is NOT automatically called and must be explicity called if resources are to be released, such as within a 'using' or 'try finally' block

see http://msdn.microsoft.com/en-us/library/system.object.finalize.aspx for more information

Does C# app exit automatically dispose managed resources?

It's important to distinguish between objects implementing IDisposable and objects with finalizers. In most cases (probably preferably all), objects with finalizers also implement IDisposable but they are in fact two distinct things, most often used together.

A finalizer is a mechanism to say to the .NET Runtime that before it can collect the object, it has to execute the finalizer. This happens when the .NET Runtime detects that an object is eligible for garbage collection. Normally, if the object does not have a finalizer, it will be collected during this collection. If it has a finalizer, it will instead be placed onto a list, the "freachable queue", and there is a background thread that monitors this thread. Sometimes after the collection has placed the object onto this queue, the finalizer thread will process the object from this queue and call the finalizer method.

Once this has happened, the object is again eligible for collection, but it has also been marked as finalized, which means that when the garbage collector finds the object in a future collection cycle, it no longer places it on this queue but collects it normally.

Note that in the above paragraphs of text, IDisposable is not mentioned once, and there is a good reason for that. None of the above relies on IDisposable at all.

Now, objects implementing IDisposable may or may not have a finalizer. The general rule is that if the object itself owns unmanaged resources it probably should and if it doesn't it probably shouldn't. (I'm hesitant to say always and never here since there always seems to be someone that is able to find a cornercase where it makes sense one way or another but breaks the "typically" rule)

A TL;DR summary of the above could be that a finalizer is a way to get a (semi-)guaranteed cleanup of the object when it is collected, but exactly when that happens is not directly under the programmers control, whereas implementing IDisposable is a way to control this cleanup directly from code.

Anyway, with all that under our belt, let's tackle your specific questions:

When a C# .NET 4.5 (or higher) application closes, what happens to the IDisposables which were not properly disposed?

Answer: Nothing. If they have a finalizer, the finalizer thread will try to pick them up, since when the program terminates, all objects become eligible for collection. The finalizer thread is not allowed to run "forever" to do this, however, so it may also run out of time. If, on the other hand, the object implementing IDisposable does not have a finalizer it will simply be collected normally (again, IDisposable has no bearing at all on garbage collection).

But let's say I have a console app, with a static Stream variable. Is it disposed when I close the console app?

Answer: No, it will not be disposed. Stream by itself is a base class, so depending on the concrete derived class it may or may not have a finalizer. It follows the same rule as above, however, so if it doesn't have a finalizer it will simply be collected. Examples, MemoryStream does not have a finalizer, whereas FileStream does.

What about a HttpClient? And how do you know in which situations it does and in which is does not

Answer: The reference source for HttpClient seems to indicate that HttpClient does not have a finalizer. It will thus simply be collected.

Alright, now some actual background info. I often store certain IDisposables as fields, forcing my class to implement IDisposable. The end user should use using. But what if that does not happen?

Answer: If you forget/don't call IDisposable.Dispose() on objects implementing IDisposable, everything I've stated here regarding finalizers will still happen, once the object is eligible for collection. Other than that, nothing special will happen. Whether the object implements IDisposable or not have no bearing on the garbage collection process, only the presence of a finalizer has.

Is it merely unnecessary memory until GC? Or do you suddenly have a memory leak

Answer: Undetermined from this simple information. It depends on what the Dispose method would do. For instance, if the object has registered itself somewhere so that there is a reference to it, somewhere, for some code to stop using the object may not actually make the object eligible for collection. The Dispose method might be responsible for unregistering it, removing the last reference(s) to it. So this depends on the object. Merely the fact that the object implements IDisposable does not create a memory leak. If the last reference to the object is removed, the object becomes eligible for collection and will be collected during a future collection cycle.

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Remarks:

• Note that the above text is also probably simplified. A full collection cycle to actually "collect memory" is probably not done on application termination as there is no point. The operating system will free the memory allocated by the process when it terminates anyway. When an application terminates, .NET Framework makes every reasonable effort to call finalizers for objects that haven't yet been garbage collected, unless such cleanup has been suppressed (by a call to the library method GC.SuppressFinalize, for example). .NET 5 (including .NET Core) and later versions don't call finalizers as part of application termination.¹ (I have no additional knowledge one way or another what kind of optimizations is done here)

• The more important part here is that you need to distinguish between memory (or other) leaks during program execution and after program execution

  • When the process terminates, the operating system will reclaim all memory allocated to it, it will close all handles (which may keep sockets, files, etc. open), all threads will be terminated. In short, the program is completely removed from memory
  • The process may have left tidbits of itself around though, which are not cleaned up unless the process took care to do this beforehand. An open file is closed, as stated above, but it may not have been completely written and thus may be corrupt in some way.
  • During program execution, leaks may make the program grow in terms of allocated memory, it may allocate too many handles because it fail to close the ones it no longer needs, etc. and this is important in terms of handling IDisposable and finalizers correctly, but when the process terminates, this is no longer a problem.

Dispose, when is it called?

I want to write a class that is
straightforward and very easy to use,
to make sure that every possible
resource is cleaned up. I don't want
to put that responsibilty to the user
of my class.

You can't do that. The memory management is simply not built to accomodate resources that are not memory specifically.

The IDisposable pattern is intended for developers as a way of telling an object when they are done with it, instead of having the memory management trying to figure that out by using things like reference counting.

You can use the Finalizer as a fallback for users who fail to dispose objects properly, but it doesn't work well as the primary method for cleaning up objects. To work smoothly objects should be disposed properly, so that the more costly Finalizer doesn't ever need to be called.

Are disposable fields of disposable classes disposed of automatically

IDisposable is an interface and a pattern. It's not really part of the C# language, outside of explicit support in using and foreach. Nothing happens "automatically".

Consider, contrariwise, if this disposal did happen automatically. And consider further that it's perfectly valid for a class to hold a reference to a shared resource of some kind that may be disposable. You would have to invent new syntax to then be able to "opt out" of this automatic disposal that you were looking for.

C# is a simpler language by not having such a feature. It's always your responsibility to clean up IDisposable objects at a time when you, the programmer, know that they're no longer being used. Often, for a class such as the one here, that will be when your own Dispose method is being called.

Does foreach automatically call Dispose?

Yes, foreach will call Dispose() on the enumerator if it implements IDisposable.

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