My name
is
Jon Skeet

Why does ObservableCollection<T> implement INotifyPropertyChanged?

In .NET 4.0, there isn't a single property defined by ObservableCollection<T> nor does it override any property of its parent or interfaces. So why does ObservableCollection<T> implement INotifyPropertyChanged?

One reason I can think of is that it makes it easier for subclasses to define their own properties and use the OnPropertyChanged method implemented by ObservableCollection<T>. But is this the main reason?

Both Count and Item[] changes are notified. Here's a sample (using C# 6 just for the string interpolation):

using System;
using System.ComponentModel;
using System.Collections.ObjectModel;

class Test
{
    static void Main(string[] args)
    {
        var collection = new ObservableCollection<string>();
        ((INotifyPropertyChanged)collection).PropertyChanged += (sender, e) =>
        {
            Console.WriteLine($"  {e.PropertyName} changed");
        };

        Console.WriteLine("Adding");
        collection.Add("Item");
        Console.WriteLine("Adding");
        collection.Add("Other item");
        Console.WriteLine("Removing");
        collection.RemoveAt(0);
        Console.WriteLine("Changing");
        collection[0] = "Different";
    }
}

Output:

Adding
  Count changed
  Item[] changed
Adding
  Count changed
  Item[] changed
Removing
  Count changed
  Item[] changed
Changing
  Item[] changed

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