带有绑定的Label属性ContentStringFormat不会刷新

Question

我希望能够在运行时更改只读数据的显示格式。我发现Label有ContentStringFormat属性。我将ContentStringFormat绑定到DependencyProperty，但是当我更改它时，Label的内容不会改变。

<Window x:Class="TestApp.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525" x:Name="Window">
<Grid>
    <Grid.RowDefinitions>
       <RowDefinition Height="Auto"/>
       <RowDefinition Height="Auto"/>
     <RowDefinition Height="Auto"/>
  </Grid.RowDefinitions>
  <Label Grid.Row="0" x:Name="Label" ContentStringFormat="{Binding ElementName=Window,
                  Path=StringFormat, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
  <StackPanel Orientation="Horizontal" Grid.Row="2">
     <Button Content="Add accuracy" Click="ButtonAdd_OnClick"/>
     <Button Content="Reduce accuracy" Click="ButtonReduce_OnClick"/>
  </StackPanel>

using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Controls;
using TestApp.Annotations;
namespace TestApp
{
  public partial class MainWindow : Window, INotifyPropertyChanged
  {
     public static readonly DependencyProperty StringFormatProperty =    DependencyProperty.Register("StringFormat", typeof(string), typeof(Label),
        new PropertyMetadata(null));

     public string StringFormat
     {
        get { return (string)GetValue(StringFormatProperty); }
        set { SetValue(StringFormatProperty, value); }
     }

  public MainWindow()
  {

     InitializeComponent();
     Label.Content = 10m;
     StringFormat = "f2";
     OnPropertyChanged("StringFormat");
  }

  public event PropertyChangedEventHandler PropertyChanged;

  [NotifyPropertyChangedInvocator]
  protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
  {
     var handler = PropertyChanged;
     if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
  }

  private void ButtonAdd_OnClick(object sender, RoutedEventArgs e)
  {
     var num = int.Parse(StringFormat.Substring(1));
     StringFormat = "f" + (num + 1);
  }

  private void ButtonReduce_OnClick(object sender, RoutedEventArgs e)
  {
     var num = int.Parse(StringFormat.Substring(1));
     if(num<=0)
        return;
     StringFormat = "f" + (num - 1);
  }
  }
}