WPF/XAML: ExceptionValidationRule different when applying in code vs markup? - wpf

Question

I've run across the need to apply the ExceptionValidationRule to many textboxes on a form in WPF. I can do this with markup and I get the desired result (the textbox gets a red outline when an invalid value is entered) but only when I supply the rule in markup:
<TextBox x:Name="Name" Width="150" >
<TextBox.Text>
<Binding Path="Name" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged" NotifyOnValidationError="True">
<Binding.ValidationRules>
<ExceptionValidationRule />
</Binding.ValidationRules>
</Binding>
</TextBox.Text>
</TextBox>
But when I apply the value using code:
Name.GetBindingExpression(TextBox.TextProperty).ParentBinding.ValidationRules.Add(new ExceptionValidationRule());
I don't get the desired results. This code is applied in a userControl's constructor after the InitalizeComponent() call. The user control has the textbox "Name" as a child control.
I've gone through and I can see, when using both, that two validation rules are put in the ValidationRules collection but when I am using just the code version I don't get the desired result of a red outline around the textbox when an invalid value is entered.
Am I just misunderstanding a fundamental rule to WPF?
Or, is there a way I can apply this validation rule using a Style? I'd prefer that, to tell you the truth.
Thanks,
M

Answer 1

You can't change a Binding after it has been used, and apperently that goes for the ValidationRules as well. You can create a new Binding in code but that's probably not what you're after.
Binding binding = new Binding("Name");
binding.Mode = BindingMode.TwoWay;
binding.UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged;
binding.NotifyOnValidationError = true;
binding.ValidationRules.Add(new ExceptionValidationRule());
nameTextBox.SetBinding(TextBox.TextProperty, binding);
A Style won't work either since a Binding or ValidationRule doesn't derive from FrameworkElement. What I would do in your situation is a subclassed Binding where you add all the things you need. Something like this
<TextBox x:Name="Name" Width="150" >
<TextBox.Text>
<local:ExBinding Path="Name"
Mode="TwoWay"
UpdateSourceTrigger="PropertyChanged"/>
</TextBox.Text>
</TextBox>
The ExBinding, adding ValidationRule etc.
public class ExBinding : Binding
{
public ExBinding()
: base()
{
NotifyOnValidationError = true;
ValidationRules.Add(new ExceptionValidationRule());
}
}