WPF converter to update in real time background colour of textbox on text change

Question

I have two textboxes for the firstname and second name of a user and I have created a converter to change the background colour of the textbox when the text equals a specific string. The problem I am having is that the textbox will only update at run time and doesn't update when I change the text is the textbox.

XAML:

<TextBox x:Name="forenameTextBox" Grid.Column="1" HorizontalAlignment="Left" Height="23" Margin="3" Grid.Row="1" 
                 Background="{Binding Staff,Converter ={StaticResource StaffNameToBackgroundColourConverter1}}"  
                 Text="{Binding Staff.Forename, Mode=TwoWay, NotifyOnValidationError=true, ValidatesOnExceptions=true}" VerticalAlignment="Center" Width="120"/>
<Label  Content="Surname:" Grid.Column="0" HorizontalAlignment="Left" Margin="3" Grid.Row="2" VerticalAlignment="Center"/>
<TextBox x:Name="surnameTextBox" Grid.Column="1" HorizontalAlignment="Left" Height="23" Margin="3" Grid.Row="2"
                 Background="{Binding Staff,Converter={StaticResource StaffNameToBackgroundColourConverter1}}"  
                 Text="{Binding Staff.Surname, Mode=TwoWay, NotifyOnValidationError=true, ValidatesOnExceptions=true}" VerticalAlignment="Center" Width="120"/>

Converter code:

public class StaffNameToBackgroundColourConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var staff  = (Staff) value;
        if (staff.Forename == "Donald" && staff.Surname == "Duck")
        {
            return "Yellow";
        }
        else
        {
            return "White";
        }
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return value;
    }
}

Correct text input:

Wrong text input - no change:

Answer 1

You need to add UpdateSourceTrigger=PropertyChanged to your Binding:

<TextBox x:Name="forenameTextBox" Grid.Column="1" HorizontalAlignment="Left" 
    Height="23" Margin="3" Grid.Row="1" Background="{Binding Staff, 
    UpdateSourceTrigger=PropertyChanged, Converter ={StaticResource 
    StaffNameToBackgroundColourConverter1}}" Text="{Binding Staff.Forename, 
    Mode=TwoWay, NotifyOnValidationError=true, ValidatesOnExceptions=true}" 
    VerticalAlignment="Center" Width="120"/>

<TextBox x:Name="surnameTextBox" Grid.Column="1" HorizontalAlignment="Left" Height="23" 
    Margin="3" Grid.Row="2" Background="{Binding Staff, 
    UpdateSourceTrigger=PropertyChanged, Converter={StaticResource 
    StaffNameToBackgroundColourConverter1}}" Text="{Binding Staff.Surname, Mode=TwoWay, 
    NotifyOnValidationError=true, ValidatesOnExceptions=true}" 
    VerticalAlignment="Center" Width="120"/>

This will update the binding source as the user types each letter. You can find out more from the Binding.UpdateSourceTrigger Property page at MSDN.