C# disposable objects

Question

Are there some advices about how I should deal with the IDisposable object sequences?

For example, I have a method that builds a IEnumerable<System.Drawing.Image> sequence and at some point I would need to dispose that objects manually, because otherwise this might lead to some leaks.

Now, is there a way to bind the Dispose() call to garbage collector actions, because I want these objects disposed right in the moment they are no longer accessible from other code parts?

**Or maybe you could advice me some other approach? **

---

Generally, this seems to be the same problem as it comes, for example, in unmanaged C++ without shared pointers, where you can have a method:

SomeObject* AllocateAndConstruct();

and then you can't be sure when to dispose it, if you don't use code contracts or don't state something in the comments.

I guess the situation with disposable objects is pretty the same, but I hope there is an appropriate solution for this.

Solution 1

You should design your system in a way that you know when the resources are no longer needed. In the worst case, they'll be eventually disposed when the garbage collector gets to it, but the point of IDisposable is that you can release important resources earlier.

This "earlier" is up to you to define, for example, you can release them when the window that's using them closes, or when your unit of work finishes doing whatever operations on them. But at some point, some object should "own" these resources, and therefore should know when they're no longer needed.

Solution 2

(from the question)

Now, is there a way to bind the Dispose() call to garbage collector actions, because I want these objects disposed right in the moment they are no longer accessible from other code parts?

GC doesn't happen immediately when your object goes out of scope / reach; it is non-deterministic. By the time GC sees it, there is no point doing anything else (that isn't already handled by the finalizer), as it is too late.

The trick, then, is to know when you are done with it, and call Dispose() yourself. In many cases using achieves this. For example you could write a class that implements IDisposable and encapsulates a set of Images - and wrap your use of that encapsulating object with using. The Dispose() on the wrapper could Dispose() all the images held.

i.e.

using(var imageWrapper = GetImages()) {
    foreach(var image in imageWrapper) {
         ...
    }
    // etc
} // assume imageWrapper is something you write, which disposes the child items

however, this is a bit trickier if you are displaying the data on the UI. There is no shortcut there; you will have to track when you are done with each image, or accept non-deterministic finalization.

Solution 3

If you want to determiniscally dispose of the objects in the collection, you should call Dispose on each:

myImages.ToList().ForEach(image => image.Dispose());

If you don't do this, and if your objects become unreachable, the GC will eventually run and release them.

Now, if you don't want to manually code the Dispose calls, you can create a wrapper class that implements IDisposable and use it through a using statement:

using (myImages.AsDisposable()) { 
  // ... process the images
}

This is the needed "infrastructure":

public class DisposableCollectionWrapper<D> : IDisposable
where D : IDisposable {

  private readonly IEnumerable<D> _disposables;

  public DisposableCollectionWrapper(IEnumerable<D> disposables) {
    _disposables = disposables;
  }

  public void Dispose() {
    if (_disposables == null) return;
    foreach (var disposable in _disposables) {
      disposable.Dispose();
    }
  }

}

public static class CollectionExtensions {

  public static IDisposable AsDisposable<D>(this IEnumerable<D> self)
  where D : IDisposable {
    return new DisposableCollectionWrapper<D>(self);
  }

}

Also notice that this is not the same as the situation you described with C++. In C++, if you don't delete your object, you have a genuine memory leak. In C#, if you don't dispose of your object, the garbage collector will eventually run and clean it up.

Solution 4

You can use the 'using' block, to make sure, the IDisposable is disposed as soon the block is left. The compiler does encapsulate such blocks into try - finally statements in order to make sure, Dispose is called in any case when leaving the block.

By using a finalizer, one can make the GC call the Dispose method for those objects which where "missed" somehow. However, implementing a finalizer is more expensive and decreases the garbage collection efficiency - and possibly the overall performance of your application. So if any possible, you should try to make sure to dispose your IDisposables on your own; deterministically:

public class Context : IDisposable {

    List<IDisposable> m_disposable = new List<IDisposable>();
    public void AddDisposable(IDisposable disposable) {
        m_disposable.Add(disposable); 
    }

    public void Dispose() {
        foreach (IDisposable disp in m_disposable)
            disp.Dispose(); 
    }

    // the Context class is used that way: 
    static void Main(string[] args) {

        using (Context context = new Context()) {
            // create your images here, add each to the context
            context.AddDisposable(image); 
            // add more objects here 

        } // <- leaving the scope will dispose the context
    }
}

By using some clever design, the process of adding objects to the context may can get even more easier. One might give the context to the creation method or publish it via a static singleton. That way, it would be available for child method scopes as well - without having to pass a reference to the contex around. By utilizing that scheme it is even possible, to simulate an artificial destructor functionality like f.e. known from C++.

Solution 5

The neat method would be to create your own generic collection class that implements IDisposable. When this collection class is Disposed() ask for each element if it implements IDisposed, and if so Dispose it.

Example (look elsewhere if you don't know about the IDisposable pattern)

public class MyDisposableList<T> : List<T> : IDisposable
{
    private bool disposed = false;

    ~MyDisposableList()
    {
        Dispose(false);
    }

    public void Dispose()
    {
        Dispose(true);
    }

    protected void Dispose(bool disposing)
    {
        if (!disposed)
        {
            foreach (T myT in this)
            {
                IDisposable myDisposableT = myT as IDisposable;
                if (myDisposableT != null)
                {
                    myDisposableT.Dispose();
                }
                myT = null;
            }
            this.Clear();
            this.TrimExcess();
            disposed = true;
        }
    }
    ...
}

usage:

using (MyDisposableList<System.Drawing.Bitmap> myList = new ...)
{
    // add bitmaps to the list (bitmaps are IDisposable)
    // use the elements in the list
}

The end of the using statement automatically Disposes myList, and thus all bitMaps in myList By the way: if you loaded the bitmap from a file and you forgot to Dispose() the bitmap you don't know when you can delete that file.

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Comments

[Steven]

This answer is plain wrong. 1. Calling GC.Collect() is bad practice and MS discourages this. 2. GC.Collect does not call Dispose, it calls Finalize, but certainly not during that call to GC.Collect. 3. While GC will eventually clean up the memory, this is often much to late when dealing with large block of unmanaged memory, as is with images. This could easily result in out of memory exceptions.

[user492238]

@Steven the last part of your comment is plain wrong ;). Calling GC manually will in no way delay any garbage collections. Besides that OOM do not have to do with calling GC, it will certainly not increase the risk of getting OOM.

[Steven]

@user492238: I didn’t mean that calling GC.Collect increases the risk of getting OOM, but that not calling Dispose() on an object does and that GC.Collect doesn't make this much better. Okay, you got me.. in fact it does make things ‘better’ ;-), because the finalizer of such object will typically run sooner, and with this it lowers the chance of having an OOM. Still, remember that the GC will not collect objects that implement a finalizer and didn't get disposed right away. It takes a collect of a higher generation or another GC.Collect call to really clean those objects.

[Steven]

@the_drow: Your update is better, but is still a bit misleading. You shouldn't advice the OP to call GC.Collect but just let him call Dispose or use the using keyword.

[the_drow]

@Steven: Why won't GC.Collect() call the Dispose() method? It doesn't make any sense.

[Steven]

@the_drow: GC doesn't recognize the IDisposable interface. IDisposable is designed for deterministic clean-up and what the GC does is per definition not deterministic; we don’t know when it’s going to happen. Test it yourself. GC will never call your Dispose method.

[the_drow]

@Steven: So what you're saying is that if I am not 'using' on an unmanaged resource it will never be released unless I overload the Finalize method as well?

[Steven]

@the_drow: That's correct. It is for this reason that things like System.Drawing.Image and System.Drawing.Bitmap override the Finalize method.

[the_drow]

@Steven: So calling GC.Collect() will collect the memory used by the image. Why will it be too late? They are already explicitly marked for collection?

[Steven]

@the_drow: Well, it depends on what you call 'too late', but I corrected that statement a bit in one of my comments. The finalizer will be called, 'not long' after the call to GC.Collect has finished (but never during that call). So native memory will be reclaimed still rather quickly after calling GC.Collect. However, you can't call GC.Collect after each finalizable object gets out of scope, because this would make your application very slow. Therefore, you will probably be calling GC.Collect once in a while and this will lead to native memory being held on for a longr period of time.

[Steven]

... Because of this, your application will hold on to that memory longer than needed, and this increases the chance of getting out of memory exceptions. You can solve this in 2 ways: calling GC.Collect more often (but this will lead to a terrible performing application) or ensure that objects that implement IDisposable get disposed. I vote for option 2 ;-)

[the_drow]

@Steven: So my assumption was still right. They will be released soon enough. I wasn't suggesting to use GC.Collect() all over the place. I was under the impression that this is a special case where many images are held in memory at once and the OP wants to make sure they are all released without writing a new collection that implements IDisposeable. The quickest solution is to use GC.Collect(). Abusing GC.Collect() is a bad idea but there are valid use cases to it, otherwise it would not exist. Therefor, I have to disagree with the downvoters.

[Steven]

@the_drow: I don't agree with "They will be released soon enough.", but that's of course the reason for my down vote. I feel using GC.Collect is bad advice, so my downvote remains. I do find it unfortunate that the other downvoter didn't respond.

[supercat]

If an application finds itself unable to proceed because some resource is unavailable, and it's possible that such a reference to such a resource has been abandoned, calling GC.Collect and GC.WaitForPendingFinalizers to see if they free up the needed resource may be better than crashing outright, but a properly-written program generally shouldn't get into a state where doing those things would be necessary.