validation not fired unless you type - wpf

Question

I have three textbox in a WPF window with UpdateSourceTrigger="LostFocus".
I have also a validation class (:ValidationRule) that return false or true based on my condition, and to keep it so simple: the condition is to check if the string is empty or not.
<TextBox x:Name="TestBox">
<TextBox.Text>
<Binding ElementName="This" Path="test"
UpdateSourceTrigger="LostFocus">
<Binding.ValidationRules>
<local:IPv4ValidationRule />
</Binding.ValidationRules>
</Binding>
</TextBox.Text>
</TextBox>
<TextBlock Margin="2" Foreground="Red" FontWeight="Bold"
Text="{Binding ElementName=TestBox,
Path=(Validation.Errors),
Converter={StaticResource eToMConverter}}" />
the problem is:
if you run the application, and you go through textboxs, no error will be shown on lost focus. I put a button to fire the validation in code, and no error is shown.
ONLY if you type in the textbox, and then clear it, the validation will work.
how can I solve this? because in this case, I cannot confirm if someone leave a textbox empty unless he type in and then delete.

Answer 1

Reading on MSDN, you can find that:
Validation usually occurs when the value of a target is transferred to
the binding source property.
So your validation rule will not be evaluated without typing a key, unless you update the source.
You can do it in you code behind. Supposing that MainWindow is your window, you need to add a Loaded event handler:
public MainWindow()
{
InitializeComponent();
Loaded += new RoutedEventHandler(MainWindow_Loaded);
}
private void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
BindingExpression bindingExpression = TestBox.GetBindingExpression(TextBox.TextProperty);
bindingExpression.UpdateSource();
}
As you can see, the handler's code updates the source, therefore the ValidationRule is evaluated.