WPF hover border effect not being applied - wpf

Question

I am trying (and failing) to change the colour of the circle's border on a radio button when the mouse is hovered over the control, within WPF. My WPF for the Style is as follows:
<Style TargetType="RadioButton"
x:Key="RadioButtonStyling"
BasedOn="{StaticResource {x:Type RadioButton}}">
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="BorderBrush" Value="Red" />
</Trigger>
</Style.Triggers>
</Style
I am then calling this on the radio buttons as follows:
<RadioButton Style="{StaticResource RadioButtonStyling}" ... />
As it stands, no styling is applied to the outline on the circle, and it remains to be the default blue colour (out of the box Windows-esque blue). See the image below

Answer 1

The problem is the template for the radiobutton already has a mouseover trigger in it.
This sets the borderbrush on the border element by name and will therefore over-ride the value your trigger sets the border on the control to.
Here's a modified working version of the win 10 template which sets the circle red on mouse over:
<ControlTemplate x:Key="RadioButtonControlTemplate1" TargetType="{x:Type RadioButton}">
<Grid x:Name="templateRoot" Background="Transparent" SnapsToDevicePixels="True">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Border x:Name="radioButtonBorder" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" CornerRadius="100" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="1,1,2,1" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
<Grid x:Name="markGrid" Margin="2">
<Ellipse x:Name="optionMark" Fill="#FF212121" MinWidth="6" MinHeight="6" Opacity="0"/>
</Grid>
</Border>
<ContentPresenter x:Name="contentPresenter" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" Grid.Column="1" ContentStringFormat="{TemplateBinding ContentStringFormat}" Focusable="False" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="HasContent" Value="True">
<Setter Property="FocusVisualStyle">
<Setter.Value>
<Style>
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate>
<Rectangle Margin="14,0,0,0" SnapsToDevicePixels="True" Stroke="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" StrokeThickness="1" StrokeDashArray="1 2"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Setter.Value>
</Setter>
<Setter Property="Padding" Value="4,-1,0,0"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Background" TargetName="radioButtonBorder" Value="#FFE6E6E6"/>
<Setter Property="BorderBrush" TargetName="radioButtonBorder" Value="#FFBCBCBC"/>
<Setter Property="Fill" TargetName="optionMark" Value="#FF707070"/>
</Trigger>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" TargetName="radioButtonBorder" Value="#FFF3F9FF"/>
<Setter Property="BorderBrush" TargetName="radioButtonBorder" Value="Red"/>
<Setter Property="Fill" TargetName="optionMark" Value="#FF212121"/>
</Trigger>
<Trigger Property="IsPressed" Value="True">
<Setter Property="Background" TargetName="radioButtonBorder" Value="#FFD9ECFF"/>
<Setter Property="BorderBrush" TargetName="radioButtonBorder" Value="#FF3C77DD"/>
<Setter Property="Fill" TargetName="optionMark" Value="#FF212121"/>
</Trigger>
<Trigger Property="IsChecked" Value="True">
<Setter Property="Opacity" TargetName="optionMark" Value="1"/>
</Trigger>
<Trigger Property="IsChecked" Value="{x:Null}">
<Setter Property="Opacity" TargetName="optionMark" Value="0.56"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
The critical part is this trigger:
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" TargetName="radioButtonBorder" Value="#FFF3F9FF"/>
<Setter Property="BorderBrush" TargetName="radioButtonBorder" Value="Red"/>
<Setter Property="Fill" TargetName="optionMark" Value="#FF212121"/>
</Trigger>
Where I've changed the original value to "Red"
I also moved that trigger down in the order of triggers so it's after isenabled.
These are the only changes I made to the default win10 template I extracted.

Answer 2

Each control in WPF has various states like inactive, mouse-over, pressed or disabled. I you want to modify certain states, simple setters on a style will not work, because there are already triggers defined in the control template that will override yours.
Therefore, you need to create a custom control template. You can use tools like Visual Studio or Blend that can automatically extract the default control templates that you can edit. After extraction, you will get one or more styles and a list of brushes like below.
<SolidColorBrush x:Key="RadioButton.Static.Background" Color="#FFFFFFFF"/>
<SolidColorBrush x:Key="RadioButton.Static.Border" Color="#FF707070"/>
<SolidColorBrush x:Key="RadioButton.Static.Glyph" Color="#FF212121"/>
<SolidColorBrush x:Key="RadioButton.MouseOver.Background" Color="#FFF3F9FF"/>
<SolidColorBrush x:Key="RadioButton.MouseOver.Border" Color="#FF5593FF"/>
<SolidColorBrush x:Key="RadioButton.MouseOver.Glyph" Color="#FF212121"/>
<!-- ...and so on. -->
<Style x:Key="OptionMarkFocusVisual">
<!-- ...style used for displaying focus. -->
</Style>
<Style x:Key="RadioButtonStyle" TargetType="{x:Type RadioButton}">
<Setter Property="FocusVisualStyle" Value="{StaticResource FocusVisual}"/>
<Setter Property="Background" Value="{StaticResource RadioButton.Static.Background}"/>
<Setter Property="BorderBrush" Value="{StaticResource RadioButton.Static.Border}"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type RadioButton}">
<!-- ...control template to display the radio button -->
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
In your case you just have to overwrite the border brush for the mouse-over state.
<SolidColorBrush x:Key="RadioButton.MouseOver.Border" Color="Red"/>
Then you apply the changed style to your radio button.
<RadioButton Style="{DynamicResource RadioButtonStyle}"/>