我正在尝试将组合框绑定到DataGrid中的字段，但无法更新该字段。
下面是来自xaml的ComboBox：

<ComboBox Grid.Row="6" Grid.Column="1" x:Name="cmbBedArea" HorizontalAlignment="Left" VerticalAlignment="Center" SelectedValue="{Binding ElementName=gridUsers, Path=SelectedValue.BedArea, Mode=OneTime}" Width="200"/>

下面是完整的xaml文件：

<Window x:Class="WinPharm.Net.frmDbUsers"
        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:WinPharm.Net"
        mc:Ignorable="d"
        WindowStartupLocation="CenterScreen"
        Title="WinPharm User Management" Height="450" Width="800">

    <Window.Resources>
        <Style x:Key="spacerRow" TargetType="RowDefinition">
            <Setter Property="Height" Value="10"/>
        </Style>
    </Window.Resources>

    <Grid>
        <!--Users Grid-->
        <DataGrid
            x:Name="gridUsers" 
            VerticalAlignment="Top" 
            Height="150" 
            Margin="10,10,10,0" 
            IsReadOnly="True" 
            AutoGenerateColumns="False" 
            GridLinesVisibility="All" 
            AlternatingRowBackground="LightGray"
            SelectionMode="Single"
            SelectionUnit="FullRow" 
            Loaded="gridUsers_Loaded" 
            SelectionChanged="gridUsers_SelectionChanged"            
            >
            <DataGrid.Columns>
                <DataGridTextColumn Header="" Width="20"/>
                <DataGridTextColumn x:Name="LoginId" Header="Id" Width="150" Binding="{Binding LoginID}"/>
                <DataGridTextColumn x:Name="Name" Header="Name" Width="200" Binding="{Binding Name}"/>
                <DataGridTextColumn x:Name="BedArea" Header="Bed Area" Width="200" Binding="{Binding BedArea}"  />
            </DataGrid.Columns>
        </DataGrid>
        <!--Form Fields-->
        <Grid Margin="10,175,0,0" VerticalAlignment="Top" HorizontalAlignment="Left" Height="Auto" Width="400">
            <Grid.RowDefinitions>
                <RowDefinition></RowDefinition>
                <RowDefinition Style="{StaticResource spacerRow}"></RowDefinition>
                <RowDefinition></RowDefinition>
                <RowDefinition Style="{StaticResource spacerRow}"></RowDefinition>
                <RowDefinition></RowDefinition>
                <RowDefinition Style="{StaticResource spacerRow}"></RowDefinition>
                <RowDefinition></RowDefinition>
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="80"></ColumnDefinition>
                <ColumnDefinition Width="280"></ColumnDefinition>
            </Grid.ColumnDefinitions>

            <Label Grid.Row="0" Grid.Column="0" Content="Login Id:" HorizontalAlignment="Right" VerticalAlignment="Top"/>
            <TextBox IsEnabled="False" Grid.Row="0" Grid.Column="1" x:Name="txtLoginId" HorizontalAlignment="Left" VerticalAlignment="Center" Text="{Binding ElementName=gridUsers, Path=SelectedItem.LoginID}" Width="200" MaxLength="50"/>

            <Label Grid.Row="2" Grid.Column="0" Content="Name:" HorizontalAlignment="Right" VerticalAlignment="Top"/>
            <TextBox Grid.Row="2" Grid.Column="1" x:Name="txtName" HorizontalAlignment="Left" VerticalAlignment="Center" Text="{Binding ElementName=gridUsers, Path=SelectedItem.Name, Mode=OneWay}" Width="200" MaxLength="50"/>

            <Label Grid.Row="4" Grid.Column="0" Content="External Id:" HorizontalAlignment="Right" VerticalAlignment="Top"/>
            <TextBox Grid.Row="4" Grid.Column="1" x:Name="txtExternalId" HorizontalAlignment="Left" VerticalAlignment="Center" Text="{Binding ElementName=gridUsers, Path=SelectedItem.ID_External, Mode=OneWay}" Width="200" MaxLength="50"/>

            <Label Grid.Row="6" Grid.Column="0" Content="Bed Area:" HorizontalAlignment="Right" VerticalAlignment="Top"/>
            <ComboBox Grid.Row="6" Grid.Column="1" x:Name="cmbBedArea" HorizontalAlignment="Left" VerticalAlignment="Center" SelectedValue="{Binding ElementName=gridUsers, Path=SelectedValue.BedArea, Mode=OneTime}" Width="200"/>
        </Grid>
        <!--Form Buttons-->
        <Grid HorizontalAlignment="Center" VerticalAlignment="Bottom">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="120"></ColumnDefinition>
                <ColumnDefinition Width="120"></ColumnDefinition>
                <ColumnDefinition Width="120"></ColumnDefinition>
            </Grid.ColumnDefinitions>
            <Button x:Name="btnAdd" Grid.Column="0" Content="Add" Height="40" Width="100" Margin="10,0,0,10" Click="btnAdd_Click"></Button>
            <Button x:Name="btnDelete" Grid.Column="1" Content="Delete" Height="40" Width="100" Margin="10,0,0,10" Click="btnDelete_Click"></Button>
            <Button x:Name="btnUpdate" Grid.Column="2" Content="Update" Height="40" Width="100" Margin="10,0,0,10" Click="btnUpdate_Click"></Button>
        </Grid>
    </Grid>
</Window>

你知道为什么ComboBox没有更新吗？
我把这个放到后面的代码中，并能够让它更新，但我想在没有这个代码的情况下这样做。

private void gridUsers_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            var drv = ((DataRowView)((DataGrid)sender).SelectedItem).Row;
            if (drv != null)
            {
                cmbBedArea.SelectedValue = drv["BedArea"];
            }
        }