Implementing An IsChanged Flag

A common scenario around INotifyPropertyChanged is the use of an “IsChanged” flag. This could be used to help decide if an object needs to be saved back to a database. This can be done in the following way.

A model class with an IsChanged property

public class Person : INotifyPropertyChanged
{
    public string Name { get; set; }
    public bool IsChanged { get; set; }

    public event PropertyChangedEventHandler PropertyChanged;
}

What gets compiled

public class Person : INotifyPropertyChanged
{
    string name;
    bool isChanged;

    public string Name
    {
        get => name;
        set
        {
            if (name != value)
            {
                name = value;
                IsChanged = true;
                OnPropertyChanged(InternalEventArgsCache.Name);
            }
        }
    }

    public bool IsChanged
    {
        get => isChanged;
        set
        {
            if (isChanged != value)
            {
                isChanged = value;
                OnPropertyChanged(InternalEventArgsCache.IsChanged);
            }
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(PropertyChangedEventArgs eventArgs)
    {
        PropertyChanged?.Invoke(this, eventArgs);
    }
}

You will notice that IsChanged is set to true inside the setter of Name.

Note that after "saving/persisting" the object it is your responsibility to set the IsChanged property back to false.