PasswordBox does not apply style - wpf

Question

I have the following style definitions:
<!-- Border -->
<Style x:Key="MyControlBorder" TargetType="{x:Type Border}">
<Setter Property="BorderBrush" Value="DarkKhaki" />
<Setter Property="Background" Value="White" />
<Setter Property="BorderThickness" Value="1" />
<Setter Property="CornerRadius" Value="10" />
</Style>
<!-- TextBox -->
<Style x:Key="MyTextBox" TargetType="{x:Type TextBox}">
<Setter Property="Height" Value="30" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TextBoxBase}">
<Border Name="TextBoxBorder" Style="{StaticResource MyControlBorder}">
<ScrollViewer x:Name="PART_ContentHost"/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<!-- PasswordBox -->
<Style x:Key="MyPasswordBox" TargetType="{x:Type PasswordBox}">
<Setter Property="Height" Value="30" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Control}">
<Border Name="Border" Style="{StaticResource MyControlBorder}">
<ScrollViewer x:Name="PART_ContentHost" />
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
And the following XAML code:
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="*" />
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<TextBox Grid.Row="0" Style="{StaticResource MyTextBox}" />
<PasswordBox Grid.Row="1" Style="{StaticResource MyPasswordBox}" />
</Grid>
Now I got this result:
The TextBox applies the style correctly, but why does the PasswordBox not apply the style?

Answer 1

If you wrap the Border in another Border everything just works as expected (I don't know why).
As a bonus, you can now have PasswordBoxes and TextBoxes "inherit" from the same Style, keeping things nice and DRY.
<!-- Border Style Definition -->
<Style x:Key="MyControlBorder" TargetType="Border">
<Setter Property="BorderBrush" Value="DarkKhaki" />
<Setter Property="Background" Value="White" />
<Setter Property="BorderThickness" Value="1" />
<Setter Property="CornerRadius" Value="10" />
</Style>
<!-- TextBox and PasswordBox Style -->
<Style x:Key="MyControlInputBox" TargetType="Control">
<Setter Property="Height" Value="30" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Control}">
<Border>
<Border Name="Border" Style="{StaticResource MyControlBorder}">
<ScrollViewer x:Name="PART_ContentHost" />
</Border>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<!-- TextBox -->
<Style x:Key="MyTextBox" TargetType="{x:Type TextBox}" BasedOn="{StaticResource MyControlInputBox}" />
<!-- PasswordBox -->
<Style x:Key="MyPasswordBox" TargetType="{x:Type PasswordBox}" BasedOn="{StaticResource MyControlInputBox}" />

Answer 2

Somehow Border within ControlTemplate of PasswordBox does not take MyControlBorder style.
When you modify MyPasswordBox style like this... then it will work.
<Style x:Key="MyPasswordBox" TargetType="{x:Type PasswordBox}">
<Setter Property="Height" Value="30" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Control}">
<Border Name="Border" BorderBrush="DarkKhaki" Background="White" BorderThickness="1" CornerRadius="10">
<ScrollViewer x:Name="PART_ContentHost" />
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
I know it's not the best solution... but I can't figure out why MyControlBorder is not applied. It doesn't even work when you get rid of MyTextBox style. Then you are left only with MyControlBorder and MyPasswordBox ...it does not work either.

Answer 3

The PasswordBox is strange in this regard, as it is explicitly implemented to show this behavior.
The default control template contains a Border, as you can see here:
<ControlTemplate TargetType="{x:Type PasswordBox}">
<Border x:Name="border" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" SnapsToDevicePixels="True">
<ScrollViewer x:Name="PART_ContentHost" Focusable="false" HorizontalScrollBarVisibility="Hidden" VerticalScrollBarVisibility="Hidden"/>
</Border>
<!-- ...other markup, triggers. -->
</ControlTemplate>
The code for PasswordBox (taken from reference source) does the following:
OnApplyTemplate on line 473 is called when the control template is applied.
AttachToVisualTree on line 476 is called which is defined on line 1176.
SetRenderScopeToContentHost on line 1181 is called which is defined on line 956.
From line 1014 on it searches a Border up the visual tree beginning from the password content host. Note that it searches until it does not find a Border anymore, which means it stores the last found Border within its _border field.
In line 1200, if a Border was found, its Style property is set to null.
Consequently, whenever a control template is applied, the Style of the outermost Border is reset. This is still the same in .NET Core and >.NET , as you can see from the code on GitHub. This is the reason why #David Murdoch's answer with a redundant nested Border works, when the style is applied to the inner Border.
Now, what can do? You can either insert a dummy Border as a workaround or you have to set the CornerRadius directly to the Border inside your custom control template.
<ControlTemplate TargetType="{x:Type Control}">
<Border Name="Border" CornerRadius="10" Style="{StaticResource MyControlBorder}">
<ScrollViewer x:Name="PART_ContentHost" />
</Border>
</ControlTemplate>