20180804_partial_corner_border_usage.xaml

<ControlTemplate TargetType="{x:Type Button}">
    <Grid>
        <Grid x:Name="HiddenGrid" Visibility="Hidden">
            <Grid.RowDefinitions>
                <RowDefinition/>
                <RowDefinition/>
                <RowDefinition/>
                <RowDefinition/>
            </Grid.RowDefinitions>
            <!-- ボタンの高さの１／４の値を取得するためだけに使うGrid -->
            <Grid x:Name="CornerRadiusSizeGrid"/>
        </Grid>
        <!-- 左上と左下の角だけ丸いBorder（の派生クラス）-->
        <local:PartialCornerBorder
            x:Name="border"
            BorderBrush="{TemplateBinding BorderBrush}"
            BorderThickness="{TemplateBinding BorderThickness}"
            Background="{TemplateBinding Background}"
            SnapsToDevicePixels="true"
            TopLeftCornerRadius="{Binding ActualHeight, ElementName=CornerRadiusSizeGrid}"
            BottomLeftCornerRadius="{Binding ActualHeight, ElementName=CornerRadiusSizeGrid}">
            <ContentPresenter x:Name="contentPresenter" Focusable="False" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
        </local:PartialCornerBorder>
    </Grid>
    <ControlTemplate.Triggers>
        <!-- 略 -->
    </ControlTemplate.Triggers>
</ControlTemplate>