Weak reference benefits

Question

Can someone explain the main benefits of different types of references in C#?

• Weak references
• Soft references
• Phantom references
• Strong references.

We have an application that is consuming a lot of memory and we are trying to determine if this is an area to focus on.

Solution 1

Soft and phantom references come from Java, I believe. A long weak reference (pass true to C#'s WeakReference constructor) might be considered similar to Java's PhantomReference. If there is an analog to SoftReference in C#, I don't know what it is.

Weak references do not extend the lifespan of an object, thus allowing it to be garbage collected once all strong references have gone out of scope. They can be useful for holding on to large objects that are expensive to initialize, but should be available for garbage collection if they are not actively in use.

Whether or not this will be useful in reducing the memory consumption of your application will depend completely on the specifics of the application. For example, if you have a moderate number of cached objects hanging around that may or may not be reused in the future, weak references could help improve the memory consumption of the caches. However, if the app is working with a very large number of small objects, weak references will make the problem worse since the reference objects will take up as much or more memory.

Solution 2

MSDN has a good explanation of weak references. The key quote is at the bottom where it says:

Avoid using weak references as an automatic solution to memory management problems. Instead, develop an effective caching policy for handling your application's objects.

Every time I've seen a WeakReference in the wild, it's been used as an automatic solution to memory management problems. There are likely better solutions to your application's problems.

Solution 3

Brilliant real example with WeakReference is explained in Android development tutorial.

There is an image (Bitmap) and image container on the view (ImageView). If image will be loaded not from memory (but e.g. from disk, net) then it can lock UI thread and the screen. To avoid it an async task can be used.

The problem arises when async task finishes. Image container can be not useful at all at that time (screen is changed or Android unloads invisible view part after scrolling). WeakReference can help here and ImageView will be garbage collected.

class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> {
    private final WeakReference<ImageView> imageViewReference;

    public BitmapWorkerTask(ImageView imageView) {
        imageViewReference = new WeakReference<ImageView>(imageView);
    }
    // Method for getting bitmap is removed for code clearness

    // Once complete, see if ImageView is still around and set bitmap.
    @Override
    protected void onPostExecute(Bitmap bitmap) {
        if (imageViewReference != null && bitmap != null) {
            final ImageView imageView = imageViewReference.get();
            if (imageView != null) {
                imageView.setImageBitmap(bitmap);
            }
        }
    }
}

P.S. the example is in Java, but can be understood by C# developers.
Source: http://developersdev.blogspot.ru/2014/01/weakreference-example.html

Comments

[George Duckett]

See also: Weak References (MSDN)

[Carl G]

I found that quote confusing, because it seems as though every use of weak references could be handled by the application. In the example the documentation uses, of a TreeView, the application could monitor whether the user has used the TreeView for a period of time, and, if not, set the TreeView to null, allowing the garbage collector to have at it. That would accomplish the same task, but from the application.

[MusiGenesis]

@DGGenuine: personally, I see no use at all for weak references. I've encountered them a few times in projects I've inherited from other developers, and in each case I had to remove them entirely, since the original developers did not understand what they are and what they're meant for.

[paercebal]

@MusiGenesis : I see no use at all for weak references : Until you have a memory leak in your GC-enabled runtime because somewhere in the code there is a reference to a Tree Node which keeps all the Tree referenced, and thus, not garbage collectable despite the fact it is not used anymore. The WeakReference is used as a weak reference (pun intented) : "If the object is still in use, then I want to be able to do something with it, but if it is not anymore, then by no means I want to be the one to keep it alive." It is code hygiene, determining ownership or non-ownership of an object.

[MusiGenesis]

@paercebal: you seem to be saying that a valid use for weak references is as a hack/workaround for a bug somewhere else in your code. I suppose you could make that argument.

[BlueRaja - Danny Pflughoeft]

Events should always be weak references... but unfortunately they're not by default. MS has a class that handles events using weak references, but unfortunately its syntax is atrocious. But thankfully there's an event library that gives a simple syntax for weak events, along with a few other improvements to events (eg. simple to have multiple publishes to the same event, removes unnecessary coupling, etc)