// exhibit "A"
class Sale
{
    public string Name { get; set; }
    public Decimal Price { get; set; }
    public Decimal Quantity { get; set; }
    public Decimal Total { get { return Price * Quantity; } }
}

// exhibit "B"
class Sale : INotifyPropertyChanged
{
    private string name;
    public string Name
    {
        get { return name; }
        set
        {
            if (name != value)
            {
                name = value;
                NotifyPropertyChanged("Name");
            }
        }
    }

    private Decimal price;
    public Decimal Price
    {
        get { return price; }
        set
        {
            if (price != value)
            {
                price = value;
                NotifyPropertyChanged("Price");
                NotifyPropertyChanged("Total");
            }
        }
    }

    private Decimal quantity;
    public Decimal Quantity
    {
        get { return quantity; }
        set
        {
            if (quantity != value)
            {
                quantity = value;
                NotifyPropertyChanged("Quantity");
                NotifyPropertyChanged("Total");
            }
        }
    }

    public Decimal Total { get { return Price * Quantity; } }

    public event PropertyChangedEventHandler PropertyChanged;

    public void NotifyPropertyChanged(String propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (null != handler)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

Everybody can see what the problem with “B” is. Not only the code is ten times longer but it does not mean or do anything particularly fancy. The code that I can live with looks something like this:

// exhibit "C"
class Sale : BaseClass
{
    [NotifyPropertyChanged]
    public string Name { get; set; }

    [NotifyPropertyChanged("Price", "Total")]
    public Decimal Price { get; set; }

    [NotifyPropertyChanged("Quantity", "Total")]
    public Decimal Quantity { get; set; }

    public Decimal Total { get { return Price * Quanitity; } }
}

As of this moment I do not know how to make “C” happen. My guess would be that we either need some (pre-) compiler magic or something like an aspect (as in AOP). I am willing to bet $20 that something like this will either happen in one of the MVVM frameworks very soon (and that will be my framework of choice) or Microsoft will implement it in the next version of C#.