如果选择了组合框项目，C# WPF 更改网格可见性

Question

我正在尝试显示 Grid combobox 中的项目(任何人)及其内容被选中。如果未选择任何内容，网格将保持隐藏状态。

XAML

    <ComboBox x:Name="cb" HorizontalAlignment="Left" VerticalAlignment="Top" Width="140" Height="25"/>
    <Grid x:Name="gr" Visibility="Hidden">
            <Border BorderThickness="1" HorizontalAlignment="Left" Height="600"  VerticalAlignment="Top" Width="346">
                <Border  BorderThickness="1" RenderTransformOrigin="0.5,0.5">
            </Border>
    </Grid>

我试过这个:

XAML.CS

public void ChangeVisibility(ComboBox cb, Grid gr)
    {
        if (cb.SelectedItem != null)
        {
            gr.Visibility = Visibility.Visible;
        }
        else
        {
            gr.Visibility = Visibility.Hidden;
        }

但这并没有改变任何东西。我尝试过多种方式，即使是 string.IsNullOrEmpty .来源combobox是 List<string> .

编辑

方法在这里调用

public MainWindow()
        {
            InitializeComponent();
            WindowStartupLocation = WindowStartupLocation.CenterScreen;
            ChangeVisibility(cb, gr);
        }

Answer 1

 <ComboBox x:Name="cb" HorizontalAlignment="Left" VerticalAlignment="Top" Width="140" Height="25"/>
    <Grid x:Name="gr">
        <Grid.Style>
            <Style TargetType="Grid">
                <Setter Property="Visibility" Value="Visible"/>
                <Style.Triggers>
                    <DataTrigger Binding="{Binding ElementName=cb, Path=SelectedItem}" Value="{x:Null}">
                        <Setter Property="Visibility" Value="Collapsed"/>
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </Grid.Style>
        <Border BorderThickness="1" HorizontalAlignment="Left" Height="600"  VerticalAlignment="Top" Width="346">
            <Border  BorderThickness="1" RenderTransformOrigin="0.5,0.5">
            </Border>
    </Grid>