首先，将TextBlock替换为Label，它具有用于绑定的ContentStringFormat属性。
然后，定义一个实现INotifyPropertyChanged接口的类。
具体实现如下：

// in Window_Loaded
panel.DataContext = new PanelModel();

class PanelModel : INotifyPropertyChanged
{
    public Dictionary<string, double> DecimalPlaces { get; } = new() {
        { "N0", 1 },
        { "N1", 0.1 },
        { "N2", 0.01 },
        { "N3", 0.001 },
        { "N4", 0.0001 }
    };

    string _ContentFormat = "N0";
    double _SomeNumber = 123.45678;

    public string ContentFormat {
        get => _ContentFormat;
        set
        {
            if (value != _ContentFormat)
            {
                _ContentFormat = value;
                OnPropertyChanged(nameof(ContentFormat));
                OnPropertyChanged(nameof(SomeNumber));
            }
        }
    }

    public double SomeNumber
    {
        get => _SomeNumber;
        set
        {
            if (value != _SomeNumber)
            {
                _SomeNumber = value;
                OnPropertyChanged(nameof(SomeNumber));
            }
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

我尝试将comboBox1.SelectedValue绑定到ContentStringFormat，但不起作用。
因此，我使用了INotifyPropertyChanged接口，并在ContentFormat更改时触发了SomeNumberChanged 通知。

<StackPanel x:Name="panel" Orientation="Horizontal">

    <Label Content="Precision: " />

    <ComboBox x:Name="comboBox1"
                ItemsSource="{Binding Path=DecimalPlaces}"
                DisplayMemberPath="Value"
                SelectedValuePath="Key"
                SelectedValue="{Binding Path=ContentFormat}"
                VerticalContentAlignment="Center"
                Width="120"/>

    <TextBox Text="{Binding Path=SomeNumber, UpdateSourceTrigger=PropertyChanged}"
             Width="120"
             Margin="15,0"/>

    <Label x:Name="labelSomeNumber"
            Content="{Binding Path=SomeNumber}"
            ContentStringFormat="{Binding Path=ContentFormat}" />

</StackPanel>

根据克莱门斯的评论，我创建了一个转换器。完整的解决方案：

• MainWindow.xaml：*

<Window x:Class="WpfApp1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    xmlns:local="clr-namespace:WpfApp1"
    DataContext="{Binding RelativeSource={RelativeSource Self}}"
    Width="400"  Height="100">

<StackPanel Orientation="Horizontal" Height="30">

    <TextBlock  Margin="5" >Precision:</TextBlock>
    
    <ComboBox x:Name="cbPrecision" 
                Margin="5" 
                MinWidth="80"
                ItemsSource="{Binding Path=DecimalPlaces}" 
                DisplayMemberPath="Value" 
                SelectedValuePath="Key"/>

    <TextBlock Margin="5" >
        <TextBlock.Text>
            <MultiBinding>
                <MultiBinding.Converter>
                    <local:DecimalPlacesConverter />
                </MultiBinding.Converter>
                <Binding Path="SomeNumber"/>
                <Binding Path="SelectedValue" ElementName="cbPrecision"/>
            </MultiBinding>
        </TextBlock.Text>
    </TextBlock> 
</StackPanel>

• MainWindow.xaml.cs：*

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }
    public Dictionary<int, double> DecimalPlaces { get; } = new() {
        { 0, 1 },
        { 1, 0.1 },
        { 2, 0.01 },
        { 3, 0.001 }
    };
    public double SomeNumber { get; set; } = 123.45678;
}

• DecimalPlacesConverter.cs：*

public class DecimalPlacesConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        try
        {
            return Math.Round((double)values[0], (int)values[1]).ToString("N" + values[1]);
        }
        catch (Exception)
        {
            return double.NaN;
        }
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}