WeakReference understanding

Question

I want to create the dictionary of all the ViewModels.

   public static Dictionary<string, WeakReference> vmCollection = new Dictionary<string, WeakReference>();

Adding it like this

 vmCollection.Add(name, new WeakReference(viewModel));

And calling the required method like this..

((vmCollection[viewModel].Target) as BaseViewModel).NewMessage(message);

Do I need maintain it as a WeakReference? What could be the consequences if I don't maintain it as a WeakReference.

Answer 1

The only consequence of not using a WeakReference is that the reference in your dictionary will prevent the View Model instances from being garbage collected. A WeakReference allows garbage collection (assuming there are no other solid references elsewhere).

An item becomes eligible for garbage collection when it has no references to it. WeakReference does not create a "countable" reference, thus you can keep a sort-of-reference to it, but still let it be eligible if your WeakReference is the only thing left looking at it.

Whether you need it or not really depends on what sort of life-cycle your View Models have. If they need disposing or otherwise "letting go of", then you may need to use WeakReference or expose a way to remove the reference from the dictionary instead.

As I mention in the comments. I tend to err away from using WeakReference as opposed to handling the life-cycle of the relevant objects explicitly. That said, they are useful when you simply don't have visibility of the life-cycle at the relevant points. I think in your situation, you should have the necessary visibility, as these are all likely in the UI layer, and thus should try to not use them.

Here are some resources on the topic:

• Weak References - When and How to Use Them
• Weak References MSDN article

Guidelines extract from the above MSDN link:

Use long weak references only when necessary as the state of the object is unpredictable after finalization.

Avoid using weak references to small objects because the pointer itself may be as large or larger.

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

I believe the last guideline point applies to your situation.

Answer 2

I took a slightly different approach.

For this example, I only have a single instance, but I'm sure it's fairly easily extendable for multiple instances...

So, on my class I create the following Action (it could be a Func if you need something returned). For my example, I'm just pushing an Exception around:

private  static Action<Exception> StaticAccessorToInstanceMethod { get; set; }

And the instance method I want to call is:

public void HandleExceptionDetails(Exception e)
{
    // Content of the method on the instance
}

I then have this in my constructor:

StaticAccessorToInstanceMethod = this.HandleExceptionDetails;

And the following in the destructor:

StaticAccessorToInstanceMethod = null;

(If you're dealing with multiple instances, then the constructor and destructor code would be a bit different).

Then the static method simply calls the instance method:

public static void HandleGeneralException(Exception ex)
{    
     StaticAccessorToInstanceMethod(result);
}

I've left out defensive logic.