INotifyPropertyChanged and calculated property

Question

Suppose I have simple class Order, that have a TotalPrice calculated property, which can be bound to WPF UI

public class Order : INotifyPropertyChanged
{
  public decimal ItemPrice 
  { 
    get { return this.itemPrice; }
    set 
    {
       this.itemPrice = value;
       this.RaisePropertyChanged("ItemPrice");
       this.RaisePropertyChanged("TotalPrice");
    }
  }

  public int Quantity 
  { 
    get { return this.quantity; }
    set 
    {
       this.quantity= value;
       this.RaisePropertyChanged("Quantity");
       this.RaisePropertyChanged("TotalPrice");
    }
  }

  public decimal TotalPrice
  {
    get { return this.ItemPrice * this.Quantity; }    
  }
}

Is it a good practice to call RaisePropertyChanged("TotalPrice") in the properties that affect to TotalPrice calculation? What is the best way to refresh TotalPrice property? The other version to do this of course is to change property like this

public decimal TotalPrice
{
    get { return this.ItemPrice * this.Quantity; } 
    protected set 
    {
        if(value >= 0) 
            throw ArgumentException("set method can be used for refresh purpose only");

    }
}

and call TotalPrice = -1 instead of this.RaisePropertyChanged("TotalPrice"); in other properties. please suggest solutions better

Thanks a lot

Answer 1

Another solution is the one Robert Rossney proposed in this question:

WPF INotifyPropertyChanged for linked read-only properties

You can create a property dependency map (using his code samples):

private static Dictionary<string, string[]> _DependencyMap = 
new Dictionary<string, string[]>
{
   {"Foo", new[] { "Bar", "Baz" } },
};

and then do this in your OnPropertyChanged:

PropertyChanged(this, new PropertyChangedEventArgs(propertyName))
if (_DependencyMap.ContainsKey(propertyName))
{
   foreach (string p in _DependencyMap[propertyName])
   {
      PropertyChanged(this, new PropertyChangedEventArgs(p))
   }
}

You can even attach an attribute to tie the dependent property to the one it depends on. Something like:

[PropertyChangeDependsOn("Foo")]
public int Bar { get { return Foo * Foo; } }
[PropertyChangeDependsOn("Foo")]
public int Baz { get { return Foo * 2; } }

I haven't implemented the details of the attribute yet. I'd better get to working on that now.

Answer 2

It's fine to check to see if you should raise this event as well from any other member that may change the value, but only do so if you actually change the value.

You could encapsulate this in a method:

private void CheckTotalPrice(decimal oldPrice)
{
    if(this.TotalPrice != oldPrice)
    {
         this.RaisePropertyChanged("TotalPrice");
    }
}

Then you need to call that from your other mutating members:

var oldPrice = this.TotalPrice;
// mutate object here...
this.CheckTotalPrice(oldPrice);