WPF:如何在对话单元中指定单位？

        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"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Grid.ColumnDefinitions>
           <ColumnDefinition Width="Auto" />
           <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>
        <TextBlock Grid.Column="0" Grid.Row="0" Margin="5">单位：</TextBlock>
       <ComboBox Grid.Column="1" Grid.Row="0" Margin="5" SelectedIndex="0">
           <ComboBoxItem Content="厘米" />
           <ComboBoxItem Content="毫米" />
           <ComboBoxItem Content="英寸" />
        </ComboBox>
        <TextBlock Grid.Column="0" Grid.Row="1" Margin="5">对话单元：</TextBlock>
       <TextBox Grid.Column="1" Grid.Row="1" Margin="5" />
    </Grid>
</Window>