DataBinding to Calculated Field

Question

I'm running into a small problem where I'm trying to bind a DataTextColumn of a DataGrid to a Calculated Field.

WPF

<DataGrid ItemsSource="{Binding Path=CurrentRoster, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" 
          AutoGenerateColumns="False" 
          AlternatingRowBackground="Gainsboro"  
          AlternationCount="2">
    <DataGrid.Columns>
    <DataGridComboBoxColumn Header="Student Enrolled"
                            ItemsSource="{Binding Source={StaticResource AvailableStudents}}"
                            SelectedItemBinding="{Binding Path=Student}">
    </DataGridComboBoxColumn>
    <DataGridTextColumn Header="Registration" Binding="{Binding Path=RegistrationCosts, StringFormat='{}{0:C}'}"/>
    <DataGridTextColumn Header="Lodging" Binding="{Binding Path=LodgingCosts, StringFormat='{}{0:C}'}"/>
    <DataGridTextColumn Header="Travel" Binding="{Binding Path=TravelCosts, StringFormat='{}{0:C}'}"/>
    <DataGridTextColumn Header="Dining" Binding="{Binding Path=DiningCosts, StringFormat='{}{0:C}'}"/>
    <DataGridTextColumn Header="Total Costs" IsReadOnly="True" Binding="{Binding Path=TotalCosts, StringFormat='{}{0:C}'}"/>
</DataGrid.Columns>

Where Student is a Entity object with one small addition. TotalCosts isn't a field on the db tables, so I created a partial class for this.

public partial class Student
{
    public  Decimal TotalCosts
    {
        get { return (LodgingCosts + RegistrationCosts + TravelCosts + DiningCosts); }
    }
}

The problem I'm experiencing is that TotalCosts is not automatically updating when you fill in any of the other fields. My guess it is because it is not listed as a dependency property. How do I resolve this for a property where there is no set ?

Answer 1

I'm assuming Student implements INotifyPropertyChanged. what you have to do is to register to the PropertyChanged Event for LodgingCosts + RegistrationCosts + TravelCosts + DiningCosts, and raise the PropertyChanged event for TotalCosts.

public partial class Student
{
    public Decimal TotalCosts
    {
        get { return (LodgingCosts + RegistrationCosts + TravelCosts + DiningCosts); }
    }

    public Student()
    {
        this.PropertyChanged += new PropertyChangedEventHandler(Student_PropertyChanged);
    }

    void Student_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        if (e.PropertyName == "LodgingCosts" ||
            e.PropertyName == "RegistrationCosts" ||
            e.PropertyName == "TravelCosts" ||
            e.PropertyName == "DiningCosts")
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs("TotalCosts"));
        }
    }

}

Answer 2

You can call OnPropertyChanged("TotalCosts") in the setters of each property that TotalCosts depends on, it will refresh the binding