WPF Can't Remove Red Border from UserControl - wpf

Question

I've built a simple login page and when invalid input is detected, a red border is drawn around the usercontrol.
Here is my layout code:
<UserControl x:Class="WPFTest.UI.Views.EmptyLayout"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:WPFTest.UI.Views"
xmlns:vm="clr-namespace:WPFTest.UI.ViewModels"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="800"
d:DataContext="{d:DesignInstance Type=vm:EmptyLayoutViewModel}"
Background="White"
>
<Grid Margin="20">
<Grid.RowDefinitions>
<RowDefinition Height="50"></RowDefinition>
<RowDefinition></RowDefinition>
</Grid.RowDefinitions>
<TextBlock Grid.Row="0" FontSize="20" FontWeight="Bold">Test Sales Estimator</TextBlock>
<ContentControl Content="{Binding Layout}" Grid.Row="1">
<ContentControl.Resources>
<DataTemplate DataType="{x:Type vm:LoginViewModel}">
<local:LoginView/>
</DataTemplate>
</ContentControl.Resources>
</ContentControl>
</Grid>
</UserControl>
And here is the login view:
<UserControl
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:WPFTest.UI.Views" xmlns:viewmodels="clr-namespace:WPFTest.UI.ViewModels"
xmlns:mah="http://metro.mahapps.com/winfx/xaml/controls" x:Class="WPFTest.UI.Views.LoginView"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="800"
Background="White"
d:DataContext="{d:DesignInstance Type={x:Type viewmodels:LoginViewModel}}"
Margin="20 0 0 0"
Padding="10"
>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="150" />
</Grid.ColumnDefinitions>
<StackPanel Grid.Row="2" Margin="0 50 0 0">
<TextBlock Text="Welcome" FontSize="17" Grid.Row="1" Margin="0 0 0 20" Height="50"/>
<StackPanel>
<TextBlock Text="Username" />
<TextBox Text="{Binding Username}"/>
</StackPanel>
<StackPanel Grid.Row="2" Margin="0 10 0 0">
<TextBlock Text="Password" />
<PasswordBox x:Name="Password" PasswordChanged="PasswordBox_PasswordChanged" >
</PasswordBox>
</StackPanel>
<Button
Grid.Row="2"
Margin="0 20 0 0"
Padding="5 2"
HorizontalAlignment="Left"
Command="{Binding HandleLoginCommand}"
IsEnabled="{Binding CanLogin}"
Content="Login">
</Button>
</StackPanel>
</Grid>
</UserControl>
I have tried to override the border with the following template:
<Style TargetType="views:LoginView">
<Style.Triggers>
<Trigger Property="Validation.HasError" Value="True">
<Setter Property="BorderBrush" Value="Blue"/>
<Setter Property="BorderThickness" Value="1"/>
</Trigger>
</Style.Triggers>
</Style>
But the border remains red. I've also tried changing the target on the template to things like UserControl, and ContentControl. I've also tried setting the Validation.ErrorTemplate attribute to {x:Null} on both the UserControl and on the element inside the layout usercontrol.
For the LoginViewModel, I am using the CommunityToolkit.Mvvm's ObservableValidator as my base class, so it handles the validation logic and here is my Username property.
private string _username;
[Required]
[MinLength(4)]
public string Username
{
get { return _username; }
set {
SetProperty(ref _username, value, true);
OnPropertyChanged(nameof(HandleLoginCommand));
}
}

Answer 1

The border is from your contentcontrol.
Set Validation.ErrorTemplate="{x:Null}" on that.
You can just use a contentpresenter.
<ContentPresenter Content="{Binding Layout}" Grid.Row="1"
Validation.ErrorTemplate="{x:Null}">
<ContentPresenter.Resources>
<DataTemplate DataType="{x:Type local:LoginViewModel}">
<local:LoginView/>
</DataTemplate>
</ContentPresenter.Resources>
</ContentPresenter>
I have a simplified layout to explore this, but the outer red border does not appear when I make that change.

Answer 2

The validation decoration does not use the control's Border(Thcikness/Brush). It is instead a separate Visual that is drawn in a separate AdornerLayer. To modify its looks, you should pass a dedicated template for the ErrorTemplate like this: (example for TextBox)
<Style TargetType="TextBox">
<Setter Property="Validation.ErrorTemplate">
<Setter.Value>
<ControlTemplate>
<Grid>
<Border BorderBrush="Yellow" BorderThickness="3">
<AdornedElementPlaceholder x:Name="AdornedElementPlaceholder" />
</Border>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
To actually remove the border, try setting Validation.ErrorTemplate to {x:Null}.