Custom control with text box and text block - xaml

Question

I would like to build a validation text box, which would be a normal UWP TextBox wrapped within a StackPanel, which also contains a TextBlock. The intent is that a validation message can be shown beneath the text box when there is a validation error.
I know I can do this by creating a custom control, but this would require me to implement all the properties I need and create a bunch of dependency properties, etc.
I'm hoping there is an easier way, where I can just completely derive the text box, but override the display template for it and include a label beneath it.

Answer 1

You can get most of the way there in XAML using the built-in IDataErrorInfo-based validation machinery and defining a control template for the TextBox's Validation.ErrorTemplate. There's a pretty good article at this link:
The XAML from the article at the link above follows, also check out this discussion on WPF's built-in validation here.
<Style TargetType="{x:Type Label}">
<Setter Property="Margin" Value="5,0,5,0" />
<Setter Property="HorizontalAlignment" Value="Left" />
</Style>
<Style TargetType="{x:Type TextBox}">
<Setter Property="VerticalAlignment" Value="Center" />
<Setter Property="Margin" Value="0,2,40,2" />
<Setter Property="Validation.ErrorTemplate">
<Setter.Value>
<ControlTemplate>
<DockPanel LastChildFill="true">
<Border Background="OrangeRed" DockPanel.Dock="right" Margin="5,0,0,0"
Width="20" Height="20" CornerRadius="5"
ToolTip="{Binding ElementName=customAdorner,
Path=AdornedElement.(Validation.Errors)[0].ErrorContent}">
<TextBlock Text="!" VerticalAlignment="center" HorizontalAlignment="center"
FontWeight="Bold" Foreground="white" />
</Border>
<AdornedElementPlaceholder Name="customAdorner" VerticalAlignment="Center" >
<Border BorderBrush="red" BorderThickness="1" />
</AdornedElementPlaceholder>
</DockPanel>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>