ValidateWithoutUpdate doesn't ever execute configured ValidationRules? - wpf

Question

I'm trying to understand why calls to BindingExpression.ValidateWithoutUpdate() doesn't actually do what it advertises.
I've got some cranky validation (I've removed the boring details from my sample code; suffice it to say it has to do with WF4 ModelItem limitations) that I have to add to a binding via an attached property (e.g., from code and not in xaml).
// d is DependencyObject and prop is DependencyProperty
var binding = BindingOperations.GetBinding(d, prop);
binding.ValidationRules.Add(new MyDerpyValidatonRule());
Nothing crazy here. But the problem is that validation isn't run the first time the control is shown, so validation errors are not exhibited in the UI.
<TextBox
Grid.Column="1"
x:Name="derp"
Text="{Binding Derp, NotifyOnValidationError=True,
ValidatesOnDataErrors=True}"
t:MyDerpyValidator.TargetProperty="{x:Static TextBox.TextProperty}" />
Binding looks good, works after the value is changed, but when first shown, I get a frownyface instead of the expected red Border:
Initially, I tried calling ValidateWithoutUpdate after I added the ValidationRule to the Binding. This didn't seem to work. Later, I used the Dispatcher to try and put this call off until the application was all warm and cozy (maybe it didn't validate because the tea hadn't finished brewing yet, hell I dunno)
var exp = BindingOperations.GetBindingExpression(d, prop);
Dispatcher.CurrentDispatcher.BeginInvoke(
(Action<BindingExpression>)(x =>
{
x.ValidateWithoutUpdate();
}),
DispatcherPriority.ApplicationIdle,
exp);
I'm pretty sure this worked once. Once. Never worked again. Might have been an incorrect observation on my part.
Later, I tried all kinds of things to get ValidateWithoutUpdate to actually do something. I even tried calling it from within an event handler that would happen way down the road
public DerpyControl()
{
InitializeComponent();
derp.MouseEnter += DERPDAMNYOU;
}
void DERPDAMNYOU(object sender, MouseEventArgs e)
{
derp.GetBindingExpression(TextBox.TextProperty).ValidateWithoutUpdate();
}
It never friggen works.
What do I need to do to get BindingExpression.ValidateWithoutUpdate() to friggen validate?!?!

Answer 1

Wow, that's some weird behaviour. Looking at some of the validation code with ILSpy it has some checks in the background that determine if validation is required or not, and I was too scared to follow it all the way through, so, I too, just tried some random stuff.
This is the first thing I tried that worked:
In your attached property changed handler, after
binding.ValidationRules.Add(new MyDerpyValidatonRule());
add
BindingOperations.ClearBinding(d, prop);
BindingOperations.SetBinding(d, prop, binding);
This must somehow set the internal 'validation requred' flag which forces it to validate.

Answer 2

The solution for my project was:
var binding = BindingOperations.GetBindingExpression(textbox, TextBox.TextProperty);
binding.UpdateSource();
binding.ValidateWithoutUpdate();