WPF Expander Validation

Does anyone know of a way to change the style of an expander if a IDataError validation occurs in a control held within the expander. E.g.

<Expander Header="Details">
    <TextBox Text="{Binding Brand.DESCRIPTION,
         开发者_Python百科                   UpdateSourceTrigger=LostFocus,
                            ValidatesOnDataErrors=True}"/>
</Expander>

So if the textbox has an error the style of my expander will change (go red maybe). I'm looking to make this as generic as possible so without binding to each control within the expander manually if possible.

You could make use of the Attached Event Validation.Error (which is raised everytime a validation error is added or removed) through an Attached Behavior. To make this work you need to add NotifyOnValidationError=True to the bindings.

This Attached Behavior, ChildValidation, subscribes to the Validation.Error event for the Expander which is bubbled up if NotifyOnValidationError is set to True on the bindings. Since several Controls may be located within the Expander it also need to keep track of the count of Validation Errors that's currently active to determine if a Red Border should be displayed or not. It could look like this

Xaml

<Expander Header="Details"
            behaviors:ChildValidationBehavior.ChildValidation="True">
    <TextBox Text="{Binding Brand.DESCRIPTION,
                            UpdateSourceTrigger=LostFocus,
                            ValidatesOnDataErrors=True,
                            NotifyOnValidationError=True}"/>
</Expander>

ChildValidationBehavior

public static class ChildValidationBehavior 
{
    private static readonly DependencyProperty ErrorCountProperty =
        DependencyProperty.RegisterAttached("ErrorCount",
                                            typeof(int),
                                            typeof(ChildValidationBehavior));
    private static void SetErrorCount(DependencyObject element, int value)
    {
        element.SetValue(ErrorCountProperty, value);
    }
    private static int GetErrorCount(DependencyObject element)
    {
        return (int)element.GetValue(ErrorCountProperty);
    }

    public static readonly DependencyProperty ChildValidationProperty = 
        DependencyProperty.RegisterAttached("ChildValidation", 
                                            typeof(bool),
                                            typeof(ChildValidationBehavior),
                                            new UIPropertyMetadata(false, OnChildValidationPropertyChanged));
    public static bool GetChildValidation(DependencyObject obj) 
    {
        return (bool)obj.GetValue(ChildValidationProperty); 
    }
    public static void SetChildValidation(DependencyObject obj, bool value) 
    {
        obj.SetValue(ChildValidationProperty, value); 
    }
    private static void OnChildValidationPropertyChanged(DependencyObject dpo, 
                                                         DependencyPropertyChangedEventArgs e)
    {
        Control control = dpo as Control;
        if (control != null)
        { 
            if ((bool)e.NewValue == true) 
            {
                SetErrorCount(control, 0);
                Validation.AddErrorHandler(control, Validation_Error);
            } 
            else 
            {
                Validation.RemoveErrorHandler(control, Validation_Error);
            } 
        } 
    }
    private static void Validation_Error(object sender, ValidationErrorEventArgs e)
    {
        Control control = sender as Control;
        if (e.Action == ValidationErrorEventAction.Added)
        {
            SetErrorCount(control, GetErrorCount(control)+1);
        }
        else
        {
            SetErrorCount(control, GetErrorCount(control)-1);
        }
        int errorCount = GetErrorCount(control);
        if (errorCount > 0)
        {
            control.BorderBrush = Brushes.Red;
        }
        else
        {
            control.ClearValue(Control.BorderBrushProperty);
        }
    }
}