WPF textbox style issue - wpf

Question

Can anyone explain why the TextBox resource style of CornerRadius works just fine, but BorderThickness and BorderBrush have zero effect?
<TextBox Text="TextBox with CornerRadius but no thickness and color"
HorizontalContentAlignment="Center"
VerticalContentAlignment="Center"
VerticalAlignment="Center" Width="500" Height="100">
<TextBox.Resources>
<Style TargetType="{x:Type Border}">
<Setter Property="CornerRadius" Value="30" />
<Setter Property="BorderThickness" Value="30" />
<Setter Property="BorderBrush" Value="Red" />
</Style>
</TextBox.Resources>
</TextBox>

Answer 1

The BorderThickness and BorderBrush of the Border element in the Template of a TextBox are bound to the respectice properties of the templated control, i.e. the TextBox:
<ControlTemplate TargetType="TextBox">
<Border BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}" ...>
...
</Border>
</ControlTemplate>
These Bindings override the values from any Border Style Setters.
You should set the values in a TextBox Style:
<TextBox Text="TextBox with CornerRadius but no thickness and color"
HorizontalContentAlignment="Center"
VerticalContentAlignment="Center"
VerticalAlignment="Center" Width="500" Height="100">
<TextBox.Style>
<Style TargetType="TextBox">
<Style.Resources>
<Style TargetType="Border">
<Setter Property="CornerRadius" Value="30"/>
</Style>
</Style.Resources>
<Setter Property="BorderThickness" Value="30"/>
<Setter Property="BorderBrush" Value="Red"/>
</Style>
</TextBox.Style>
</TextBox>

Answer 2

The reason is that in the standard TextBoxBase's template the Border control has its properties BorderThickness and BorderBrush bound to the same properties of the TextBox itself.
Here the standard ControlTemplate used in the style:
<ControlTemplate TargetType="{x:Type TextBoxBase}">
<cbd:ClassicBorderDecorator x:Name="Bd" BorderStyle="Sunken" Background="{TemplateBinding Control.Background}" BorderThickness="{TemplateBinding Control.BorderThickness}" BorderBrush="{TemplateBinding Control.BorderBrush}">
<ScrollViewer Name="PART_ContentHost" />
</cbd:ClassicBorderDecorator>
<ControlTemplate.Triggers>
<Trigger Property="UIElement.IsEnabled" Value="False">
<Setter TargetName="Bd" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}" Property="theme:ClassicBorderDecorator.Background" />
<Setter Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}" Property="Control.Foreground" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
So to reach your target those properties can be set in this way:
<TextBox Text="TextBox with CornerRadius but no thickness and color"
HorizontalContentAlignment="Center" BorderThickness="30" BorderBrush="Red"
VerticalContentAlignment="Center"
VerticalAlignment="Center" Width="500" Height="100">
<TextBox.Resources>
<Style TargetType="{x:Type Border}">
<Setter Property="CornerRadius" Value="30" />
</Style>
</TextBox.Resources>
</TextBox>
I hope it can help you.