WPF CustomControl Icon Font Inheritance Issue Resolution

Problem Scenario

When using Segoe Fluent Icons font in WPF CustomControl, TextBlocks within the same ControlTemplate inherit the icon font, causing text to render incorrectly.

Symptoms

• Button text displays as square boxes (□) or strange symbols
• Icons display correctly but regular text doesn't render

Cause

WPF's FontFamily is inherited to child elements following the Visual Tree. When FontFamily="Segoe Fluent Icons" is set on a TextBlock for icons within a ControlTemplate, other TextBlocks in the same container may inherit this font.

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Solution

Explicitly Specify FontFamily on Text-Displaying Elements

<!-- IconButton ControlTemplate Example -->
<ControlTemplate TargetType="{x:Type local:IconButton}">
    <Border Background="{TemplateBinding Background}"
            BorderBrush="{TemplateBinding BorderBrush}"
            BorderThickness="{TemplateBinding BorderThickness}">
        <StackPanel Orientation="{TemplateBinding Orientation}"
                    HorizontalAlignment="Center"
                    VerticalAlignment="Center">

            <!-- Icon: Use Segoe Fluent Icons -->
            <TextBlock x:Name="PART_Icon"
                       Text="{TemplateBinding Icon}"
                       FontFamily="Segoe Fluent Icons, Segoe MDL2 Assets"
                       FontSize="{TemplateBinding IconSize}"
                       Foreground="{TemplateBinding Foreground}" />

            <!-- Text: Explicitly specify Segoe UI (Important!) -->
            <TextBlock x:Name="PART_Text"
                       Text="{TemplateBinding Text}"
                       FontFamily="Segoe UI"
                       FontSize="12"
                       Foreground="{TemplateBinding Foreground}"
                       VerticalAlignment="Center" />
        </StackPanel>
    </Border>
</ControlTemplate>

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Key Points

1. Apply icon font only to specific element: Specify Segoe Fluent Icons only on PART_Icon
2. Explicitly specify FontFamily on text elements: FontFamily="Segoe UI" is required on PART_Text
3. Don't set FontFamily on parent containers: Setting FontFamily on Border or StackPanel inherits to all children

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Applicable Controls

• IconButton (icon + text button)
• IconToggleButton (toggleable icon button)
• NavigationButton (navigation menu item)
• Any other CustomControl using icons and text together

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Related Icon Fonts

Font Name	Windows Version	Purpose
Segoe Fluent Icons	Windows 11+	Latest Fluent Design icons
Segoe MDL2 Assets	Windows 10+	Default system icons

Fallback Pattern

FontFamily="Segoe Fluent Icons, Segoe MDL2 Assets"

Specify fallback font for Windows 10 compatibility