I set the property for INPC and it isn't updating when I add to the list?!

When I first started using ObservableCollection I thought that it surfaced individual change notifications, and if bound to a property, would receive those notifications and update accordingly. Man, was I wrong? Recently I've had this conversation several times, and it usually starts with understanding what INotifyPropertyChanged does. So what does it do?


In the MSFT Documentation

INotifyPropertyChanged is an interface that defines a PropertyChangedEventHandler that notifies a listener that the state of the property has mutated. This is a simple and powerful tool. Many UI kits understand and have support for INotifyProperyChanged, which makes this an ideal way to setup MVVM implementations. Your ViewModel is bound to the View, which has a BindingEngine that listens for properties that are changed. Traditionally the implementation of INotifyPropertyChanged will invoke some PropertyChangedEventHandler when the setter is called on that property, like below.

public event PropertyChangedEventHandler PropertyChanged;

public string MyProperty
    get => _myProperty;
        _myProperty = value;

private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
    if (PropertyChanged != null)
        PropertyChanged(this, new PropertyChangedEventArgs(propertyName));

NotifyPropertyChanged will invoke the PropertyChangedEventHandler and lets listeners know that state has mutated. Based on the code above, we see that this changed notification only happens when the property is set. This means the state has to be set and as a result, we notify listeners of the new state. So why the confusion?


We seem to expect that state changes to the list property should propagate as state changes of the list property. If that were true, we'd expect to see every character change notification for a string as it is just a char[] in disguise! So why do we expect that adding items to a collection property would trigger this change? ObservableCollection. For some reason, we as developers have been saddled with the notion that because the contents of this property changes that the INotifyPropertyChanged would handle this case. I thought so until I looked at what it extends. INotifyCollectionChanged.

ObservableCollection implements INotifyCollectionChanged which will notify us when the collection has a mutated state. This means creates, adds, updates, deletes. Very few of us are explained this difference when we come to MVVM. We are told about INotifyPropertyChanged, and shown techniques to clear lists and to create a new list every time so the property changed argument will fire. In the context of a UI tool kit, this would force say a ListView to potentially redraw every ViewCell on the screen.

The code below surfaces internal changes to the collection and invokes it's corresponding property changed handler. This will send a signal to any property change subscribers that the state has mutated. This coupled to your binding engine through your ViewModel should propagate the changes desired. The code is verbose! It solves most of our concerns but leaves a large footprint in our code. Yes, it uses Reactive Extensions (spoiler, I like Rx), but the amount of work required to get that change notification is a lot. Also, we are not handling individual changes, we are only notifying to the binding engine that something in the list changed. Can we do it better?

ObservableCollection<string> collection = new ObservableCollection<string>(Enumerable<string>.Empty);

var changedEvents =
        .FromEvent<NotifyCollectionChangedEventHandler, NotifyCollectionChangedEventArgs>(eventHandler =>
            void Handler = (object sender, NotifyCollectionChangedEventArgs args) => eventHandler(args);
            return Handler;
        x => collection.CollectionChanged += x,
        x => collection.CollectionChanged -= x);

    .Subscribe(_ =>

Why not use DynamicData?

The below code will surface the changes of the internal list to the bound _items list. This means anytime we manipulate the items in the list, we will raise a property changed notification for the property of the list. DynamicData handles this when it binds to any ObservableCollection derivative. This provides a smooth clear understanding of list contents so that a given binding engine will know to add, remove, or update any item in the list when the state mutates. This is a much cleaner approach to me than most of the variants I have tried.

private ReadOnlyObservableCollection<MyViewItemModel> _items = new ReadOnlyObservableCollection<string>();

 public MyViewModel()
        .Transform(x => new MyViewItemModel(x))
        .Bind(out _items)

public ReadOnlyObservableCollection<MyViewItemModel> Items => _items;

The main take away isn't to use DyamicData, although you should. The whole point of this rant is that when you update a list, the property that list represents doesn't know about what happens inside. So you need to surface the change notifications so the property knows to update your binding. DynamicData just provides a clean, fluent and observable approach to making that possible.


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