INPC Parent Child Notification

Here is a simple example of a parent listening to change notification from a child and exposing an aggregate value (or “denormalized” value in database terminology).

The Order (parent) exposes the TotalCost, which is a sum of the Cost properties in OrderLines (child).

public class Order : InpcBase
{
    private readonly ObservableCollection<OrderLine> orderLines;

    public Order()
    {
        orderLines = new ObservableCollection<OrderLine>();
    }

    public decimal TotalCost
    {
        get { return orderLines.Sum(ol => ol.Cost); }
    }

    public void AddOrderLine(OrderLine orderLine)
    {
        orderLine.PropertyChanged += orderLine_PropertyChanged;
    }

    private void orderLine_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        if (e.PropertyName == "Cost") OnPropertyChanged("TotalCost");
    }
}

public class OrderLine : InpcBase
{
    private decimal cost;
    public decimal Cost
    {
        get { return cost; }
        set
        {
            if (cost == value) return;
            cost = value;
            OnPropertyChanged("Cost");
        }
    }
}

public abstract class InpcBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    protected void OnPropertyChanged(string name)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler == null) return;
        handler(this, new PropertyChangedEventArgs(name));
    }
}