WPF Custom TextBox ErrorTemplate - wpf

Question

I have custom AutoCompleteTextBox class which inherits from TextBox. Everything works just fine, except showing validation error, i.e. red border around the control.
Here is it's style from my Generic.xaml.
<Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.WindowBrushKey}}" />
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" />
<Setter Property="BorderBrush" Value="{DynamicResource {x:Static SystemColors.ControlDarkBrushKey}}" />
<Setter Property="BorderThickness" Value="1" />
<Setter Property="HorizontalContentAlignment" Value="Left" />
<Setter Property="Padding" Value="2" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type local:AutoCompleteTextBox}">
<Grid>
<Border x:Name="PART_Border" SnapsToDevicePixels="true" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}">
<ScrollViewer x:Name="PART_ContentHost" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
</Border>
<Popup x:Name="PART_Popup" StaysOpen="False">
<ListBox x:Name="PART_ListBox" HorizontalContentAlignment="Stretch" />
</Popup>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Background" TargetName="PART_Border" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}" />
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}" />
</Trigger>
<Trigger Property="IsReadOnly" Value="true">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
I've checked with Snoop utility and found no red border. I've reviewed WPF's Generic.xaml Aero.Normal.xaml and, to be honest, have no idea what draws that red validation border around invalid textbox.
I know similar questions were assed before, but I reviwed all answers and non of them helped.

Answer 1

You will find the default ErrorTemplate in validation.cs source code:
public static class Validation
{
/// <summary>
/// Template used to generate validation error feedback on the AdornerLayer. Default
/// Template is:
/// <code>
/// <border borderthickness="1" borderbrush="Red">
/// <adornedelementplaceholder>
/// </adornedelementplaceholder></border>
/// </code>
/// </summary>
public static readonly DependencyProperty ErrorTemplateProperty =
DependencyProperty.RegisterAttached("ErrorTemplate",
typeof(ControlTemplate), typeof(Validation),
new FrameworkPropertyMetadata(
CreateDefaultErrorTemplate(),
FrameworkPropertyMetadataOptions.NotDataBindable,
new PropertyChangedCallback(OnErrorTemplateChanged)));
private static ControlTemplate CreateDefaultErrorTemplate()
{
ControlTemplate defaultTemplate = new ControlTemplate(typeof(Control));
//<border borderthickness="1" borderbrush="Red">
// <adornedelementplaceholder>
//</adornedelementplaceholder></border>
FrameworkElementFactory border = new FrameworkElementFactory(typeof(Border), "Border");
border.SetValue(Border.BorderBrushProperty, Brushes.Red);
border.SetValue(Border.BorderThicknessProperty, new Thickness(1));
FrameworkElementFactory adornedElementPlaceHolder = new FrameworkElementFactory(typeof(AdornedElementPlaceholder), "Placeholder");
border.AppendChild(adornedElementPlaceHolder);
defaultTemplate.VisualTree = border;
defaultTemplate.Seal();
return defaultTemplate;
}
...
}
If you want to remove it just set Validation.ErrorTemplate Attached Property to Null.
<TextBox Validation.ErrorTemplate="{x:Null}" />