ObservableCollection如何 <T> 实现INotifyPropertyChanged为受保护的吗？

Question

This is the definition for INotifyPropertyChanged

public interface INotifyPropertyChanged
{
    event PropertyChangedEventHandler PropertyChanged;
}

ObservableCollection<T> implements this...

public class ObservableCollection<T> : Collection<T>, INotifyCollectionChanged, INotifyPropertyChanged
{
    protected virtual event PropertyChangedEventHandler PropertyChanged;
}

When I tested this...

public class Test : INotifyPropertyChanged
{
    protected virtual event PropertyChangedEventHandler PropertyChanged;
}

I got the following error:

Test does not implement interface member 'System.ComponentModel.INotifyPropertyChanged.PropertyChanged'. 'Test.PropertyChanged' cannot implement an interface member because it is not public.

How is this possible?

Answer 1

The interface itself is implemented explicitly, with a protected event for classes to override

public class Test : INotifyPropertyChanged
{
   // explicit interface implementation
    event PropertyChangedEventHandler INotifyPropertyChanged.PropertyChanged
    {
        add
        {
            PropertyChanged += value;
        }
        remove
        {
            PropertyChanged -= value;
        }
    }

    // protected virtual (for derived classes to override)   
    protected virtual event PropertyChangedEventHandler PropertyChanged;
}

Answer 2

In addition to the protected PropertyChanged event , ObservableCollection<T> provides an explicit interface implementation for the INotifyPropertyChanged.PropertyChanged event , which is lacking in your class (and not shown in your code snippet from ObservableCollection<T> ).

You can read about explicit interface implementation on MSDN .