从文本框填充Datagrid并再次选择Datagrid行相同的值应填充到文本框

Question

问题：将userId文本框值添加到Datagrid但是当在datagrid中选择行时，它不会填充文本框。我想创建应用程序，因此文本框值填充到datagrid，datagrid行选择填充到同一应用程序的文本框中。但是我一次能够进行一次操作。

Xmal代码：

<Window x:Class="MvvMWithWPF.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"

        xmlns:local="clr-namespace:MvvMWithWPF"
        mc:Ignorable="d"

    Title="MainWindow" Height="485" Width="525" 
        d:DataContext="{d:DesignInstance Type=local:UserViewModel}">

<Grid >
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"></RowDefinition>
        <RowDefinition Height="Auto"  ></RowDefinition>
        <RowDefinition Height="Auto"  ></RowDefinition>
    </Grid.RowDefinitions>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*"></ColumnDefinition>
        <ColumnDefinition Width="*"></ColumnDefinition>
    </Grid.ColumnDefinitions>
    <DataGrid Grid.Row="0" Height="300" Width="450" Grid.ColumnSpan="2" ItemsSource="{Binding Users}"  SelectedItem="{Binding User}"  Name="EmpDataGrid"   AutoGenerateColumns="False">
        <DataGrid.Columns>
            <DataGridTextColumn x:Name="empId" Header="empIdcolumn" Width="85"  Binding="{Binding UserId}"></DataGridTextColumn>
        </DataGrid.Columns>
    </DataGrid>
    <TextBox Grid.Row="2" Grid.Column="1" Height="35" Width="200" HorizontalAlignment="Center"  VerticalAlignment="Bottom" Text="{Binding User.UserId, Mode=TwoWay ,UpdateSourceTrigger=PropertyChanged}" />
    <Button Content="Update" Grid.Row="2"  Height="35" HorizontalAlignment="Right"  Name="btnUpdate" 
            VerticalAlignment="Bottom" Width="85"
            Command="{Binding Path= UpdateCommand}" Grid.Column="2"  />
</Grid>

ViewModel代码：

public class ViewModelBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    public void OnPropertyChanged(string name)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(name));
    }
}
public class UserViewModel : ViewModelBase
{
    private ObservableCollection<User> _UsersList;
    private User _user;
    public UserViewModel()
    {
        _user = new User();
        _UsersList = new ObservableCollection<User>();

    }



    public ObservableCollection<User> Users
    {
        get { return _UsersList; }
        set
        {
            _UsersList = value;
            OnPropertyChanged("Users");

        }
    }
    public User User
    {
        get
        {
            return _user;
        }
        set
        {
            _user = value;
            OnPropertyChanged("User");
        }
    }


    private ICommand mUpdater;
    public ICommand UpdateCommand
    {
        get
        {
            if (mUpdater == null)
                mUpdater = new Updater(this);
            return mUpdater;
        }
        //set
        //{
        //    mUpdater = value;
        //}
    }


    private void Submit()
    {

        Users.Add(User);
        User = new User();

    }

    private class Updater : ICommand
    {
        #region ICommand Members
        UserViewModel _obj;
        public Updater(UserViewModel obj)
        {
            _obj = obj;
        }
        public bool CanExecute(object parameter)
        {
            return true;
        }


        public event EventHandler CanExecuteChanged;


        public void Execute(object parameter)
        {
            _obj.Submit();
        }
    }
    #endregion



}

型号代码：

  public class User : ViewModelBase
{
    private int userId;

    public int UserId
    {
        get
        {
            return userId;
        }
        set
        {
            userId = value;
            OnPropertyChanged("UserId");
        }
    }



}