模板部件作为Visual Child不可用。在默认情况下合并自定义控件可见性时

<Style TargetType="local:MyCustomControl">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="local:MyCustomControl">
                <Border x:Name="PART_Border">
                    <!-- 其他内容 -->
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

public override void OnApplyTemplate()
{
    base.OnApplyTemplate();
    PART_Border = GetTemplateChild("PART_Border") as Border;
}

<Style TargetType="local:MyCustomControl">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="local:MyCustomControl">
                <Border x:Name="PART_Border" Background="{TemplateBinding Background}">
                    <!-- 其他内容 -->
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>