Опрация творога с использованием MVVM в WPF

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невозможно получить значения текстового поля в viewmodel

[edit]добавлен блок кода-OriginalGriff [/edit]

Что я уже пробовал:

код пользовательского интерфейса-->

<Grid Margin="0,0,0,20">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <ListView Name="UserGrid" Grid.Row="1" Margin="4,242,12,-21"  ItemsSource="{Binding Users}" Grid.RowSpan="2"  >
            <ListView.View>
                <GridView x:Name="grdTest">
                    <GridViewColumn Header="UserId" DisplayMemberBinding="{Binding UserId,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"  Width="50"/>
                    <GridViewColumn Header="First Name" DisplayMemberBinding="{Binding FirstName}"  Width="80" />
                    <GridViewColumn Header="Last Name" DisplayMemberBinding="{Binding LastName}" Width="100" />
                    <GridViewColumn Header="City" DisplayMemberBinding="{Binding City}" Width="80" />
                    <GridViewColumn Header="State" DisplayMemberBinding="{Binding State}" Width="80" />
                    <GridViewColumn Header="Country" DisplayMemberBinding="{Binding Country}" Width="100" />
                </GridView>
            </ListView.View>
        </ListView>
        <TextBox Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="80,7,0,0" Name="txtUserId" VerticalAlignment="Top" Width="178" Text="{Binding ElementName=UserGrid,Path=SelectedItem.UserId,Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
        <TextBox Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="80,35,0,0" Name="txtFirstName" VerticalAlignment="Top" Width="178" Text="{Binding ElementName=UserGrid,Path=SelectedItem.FirstName,Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
        <TextBox Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="80,62,0,0" Name="txtLastName" VerticalAlignment="Top" Width="178" Text="{Binding ElementName=UserGrid,Path=SelectedItem.LastName,Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
        <Label Content="UserId" Grid.Row="1" HorizontalAlignment="Left" Margin="12,12,0,274" />
        <Label Content="Last Name" Grid.Row="1" Height="28" HorizontalAlignment="Left" Margin="12,60,0,0"  VerticalAlignment="Top" />
        <Label Content="First Name" Grid.Row="1" Height="28" HorizontalAlignment="Left" Margin="12,35,0,0" VerticalAlignment="Top" />
        <Button Content="Update" Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="184,199,0,0"  VerticalAlignment="Top" Width="95"
                Command="{Binding UpdateCommand,Source={StaticResource VM}}"  />
        <TextBox Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="80,87,0,0"  VerticalAlignment="Top" Width="178" Text="{Binding SelectedItem.City, ElementName=UserGrid,Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
        <Label Content="Country" Grid.Row="1" Height="28" HorizontalAlignment="Left" Margin="12,141,0,0"  VerticalAlignment="Top" />
        <TextBox Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="80,150,0,0"  VerticalAlignment="Top" Width="178" Text="{Binding SelectedItem.Country, ElementName=UserGrid,Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
        <Label Content="City" Grid.Row="1" Height="28" HorizontalAlignment="Left" Margin="12,86,0,0"  VerticalAlignment="Top" />
        <TextBox Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="80,115,0,0" VerticalAlignment="Top" Width="178" Text="{Binding SelectedItem.State, ElementName=UserGrid,Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
        <Label Content="State" Grid.Row="1" Height="28" HorizontalAlignment="Left" Margin="12,113,0,0"  VerticalAlignment="Top" />
        <Button Content="Save" HorizontalAlignment="Left" Margin="80,199,0,0" Grid.Row="1" VerticalAlignment="Top" Width="99"
                Command="{Binding SaveCommand,Source={StaticResource VM}}" />
        <Button Content="Delete" HorizontalAlignment="Left" Margin="297,199,0,0" Grid.Row="1" VerticalAlignment="Top" Width="80" Command="{Binding DeleteCommand,Source={StaticResource VM}}" />
        <Button Content="Canceal" Command="{Binding CancealComand,Source={StaticResource VM}}"
                HorizontalAlignment="Left" Margin="394,199,0,0" Grid.Row="1" VerticalAlignment="Top" Width="74"/>
    </Grid>

класс моделей:-

public class User : INotifyPropertyChanged
    {
        private int userId;
        private string firstName;
        private string lastName;
        private string city;
        private string state;
        private string country;

        public int UserId
        {
            get
            {
                return userId;
            }
            set
            {
                userId = value;
                OnPropertyChanged("UserId");
            }
        }
        public string FirstName
        {
            get
            {
                return firstName;
            }
            set
            {
                firstName = value;
                OnPropertyChanged("FirstName");
            }
        }
        public string LastName
        {
            get
            {
                return lastName;
            }
            set
            {
                lastName = value;
                OnPropertyChanged("LastName");
            }
        }
        public string City
        {
            get
            {
                return city;
            }
            set
            {
                city = value;
                OnPropertyChanged("City");
            }
        }
        public string State
        {
            get
            {
                return state;
            }
            set
            {
                state = value;
                OnPropertyChanged("State");
            }
        }
        public string Country
        {
            get
            {
                return country;
            }
            set
            {
                country = value;
                OnPropertyChanged("Country");
            }
        }


        #region INotifyPropertyChanged Members
        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
        #endregion
    }

ViewModel Class:-
 private ObservableCollection<user> _UsersList;
        public UserViewModel()
        {
            

            getUSerList();
        }
        public ObservableCollection<user> Users
        {
            get { return _UsersList; }
            set { _UsersList = value; }
        }
         #region SaveCommand
        private ICommand saveCommand;
        public ICommand SaveCommand
        {
            get
            {
                if (saveCommand == null)
                {
                    saveCommand = new RelayCommand(SaveExecuted, CanSaveExecute);
                }
                return saveCommand;
            }

        }

        public bool CanSaveExecute(object parameter)
        {
            // for save command 
            User _uuser=new User ();

            int str = _uuser.UserId;

            return true;
        }
        public void SaveExecuted(object parameter)
        {
            string str = userId;



            MessageBox.Show("Save Button clicked");
        }
        #endregion
  #region INotifyPropertyChanged

        public event PropertyChangedEventHandler PropertyChanged;
        public void NotifyPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
        #endregion

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