code

code.xaml

        <Grid.Resources>
            <Style TargetType="TextBox">
                <Setter Property="Padding" Value="2 1"></Setter>
                <Setter Property="BorderBrush" Value="Black"></Setter>
                <Setter Property="BorderThickness" Value="1"></Setter>
                <Setter Property="Validation.ErrorTemplate">
                    <Setter.Value>
                        <ControlTemplate>
                            <AdornedElementPlaceholder></AdornedElementPlaceholder>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="TextBox">
                            <StackPanel>
                                <Border
                                    
                                    Padding="{TemplateBinding Padding}"
                                    BorderBrush="{TemplateBinding BorderBrush}"
                                    BorderThickness="{TemplateBinding BorderThickness}"
                                    CornerRadius="5">
                                    <ScrollViewer x:Name="PART_ContentHost" />
                                </Border>
                                
                                <ItemsControl ItemsSource="{Binding RelativeSource={RelativeSource AncestorType={x:Type TextBox}}, Path=(Validation.Errors)}">
                                    <ItemsControl.ItemTemplate>
                                        <DataTemplate>
                                            <TextBlock Foreground="Red" Text="{Binding ErrorContent}" />
                                        </DataTemplate>
                                    </ItemsControl.ItemTemplate>
                                </ItemsControl>

                            </StackPanel>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
        </Grid.Resources>