TextBox字体系列绑定

private System.Windows.Media.FontFamily fontfamily;
public System.Windows.Media.FontFamily FontFamily
{
    get
    {
        return this.fontfamily;
    }
    set
    {
        this.fontfamily = value;
        this.PropertyChanged(this, new PropertyChangedEventArgs("FontFamily"));
    }
}
<ComboBox Height="20"  SelectedValue="{Binding TopPanelViewModel.FontFamily,UpdateSourceTrigger=PropertyChanged}"
                                  ItemsSource="{Binding Source={x:Static Fonts.SystemFontFamilies}}" Name="Type_Font"   Width="120" Margin="20,0,10,0">
                                <ComboBox.ItemTemplate>
                                    <DataTemplate>
                                        <TextBlock FontFamily="{Binding .}" Text="{Binding Source}" FontSize="15"/>
                                    </DataTemplate>
                                </ComboBox.ItemTemplate>
                                <ComboBox.ToolTip>
                                    <TextBlock Text="{Binding LangiageViewModel.CurrentToolTips[25]}"></TextBlock>
                                </ComboBox.ToolTip>
                            </ComboBox>
  <TextBox Style="{StaticResource HardBox}" Background="Transparent"  TextWrapping="Wrap" x:Name="TextBoxInsertText" 
                                FontFamily="{Binding TopPanelViewModel.FontFamily,UpdateSourceTrigger=PropertyChanged}" 
                             AcceptsReturn="True" AcceptsTab="True" ScrollViewer.VerticalScrollBarVisibility="Disabled"
                             KeyUp="TextBoxInsertText_KeyDown" AllowDrop="True" IsUndoEnabled="True"
                         Visibility="{Binding VisibilityPropertiesViewModel.Instance.InsertTextVisibility,Converter={StaticResource BooleanToVisibilityConverter}}">
                    </TextBox>'

当组合框所选项目发生更改时，文本框族不会发生更改。该属性工作正常，但文本框fontfamily不正常。组合框和文本框位于不同的用户控件中。