问 WPF MVVM自定义排序问题

EN

<DataGrid HorizontalAlignment="Stretch" Grid.Row="1" Grid.Column="0"   SelectedItem="{Binding SelectedPerson}"   ItemsSource="{Binding CollectionView}"  GridLinesVisibility="All"  AutoGenerateColumns="False" SelectionUnit="FullRow" BorderThickness="1"  IsReadOnly="True"  >
        <DataGrid.ContextMenu>
            <ContextMenu>
                <MenuItem Header="ADD" Command="{Binding AddNewPersonCommand}">
                    <MenuItem.Icon>
                        <Image Source="../Icons/Add.png" Height="16" Width="16" Margin="0"   />
                    </MenuItem.Icon>
                </MenuItem>
                <MenuItem Header="EDIT" Command="{Binding EditPersonCommand}">
                    <MenuItem.Icon>
                        <Image Source="../Icons/edit.png" Height="16" Width="16" Margin="0"    />
                    </MenuItem.Icon>
                </MenuItem>
                <MenuItem Header="DELETE">
                    <MenuItem.Icon>
                        <Image Source="../Icons/delete.png" Height="16" Width="16" Margin="0"   />
                    </MenuItem.Icon>
                </MenuItem>
            </ContextMenu>
        </DataGrid.ContextMenu>
        <i:Interaction.Triggers>
            <i:EventTrigger EventName="MouseDoubleClick" >
                <i:InvokeCommandAction Command="{Binding EditPersonCommand}"  />
            </i:EventTrigger>
            <i:EventTrigger EventName="Sorting">
                <command:EventToCommand Command="{Binding PersonGridSortingCommand}" PassEventArgsToCommand="True"  />
            </i:EventTrigger>
        </i:Interaction.Triggers>
        <DataGrid.Columns>
            <DataGridTextColumn Header="Record ID" SortMemberPath="RecordID" Binding="{Binding RecordID}" Width="*" />
            <DataGridTextColumn Header="Last Name" Binding= "{Binding LAST_NAME}" Width="*"  />
            <DataGridTextColumn Header="2nd Last Name" Binding= "{Binding SecondLastName}" Width="*"  />
            <DataGridTextColumn Header="First Name" Binding= "{Binding FIRST_NAME}" Width="*" />
            <DataGridTextColumn Header="Date Of Birth" Binding= "{Binding Date_Of_Birth}"  Width="*"   />
            <!--<DataGridTextColumn Header="Town" Binding= "{Binding PrimaryAddress.City}" Width="*"   />-->
        </DataGrid.Columns>
    </DataGrid>
    private ICollectionView _collectionView;
    public ICollectionView CollectionView
            return _collectionView;
            _collectionView = value;
            OnPropertyChanged("CollectionView");
    public async Task UpdateData(string sortCoulmnName = null, string sortDirection = null)
        var lstPersons = await PersonDAO.SelectAll(PagerViewModel.CurrentPage, PagerViewModel.PageSize, sortCoulmnName, sortDirection);
        if (lstPersons.DataSource != null) { }
        Persons = new ObservableCollection<PersonDTO>((PersonListDTO)lstPersons.DataSource);
        CollectionView = CollectionViewSource.GetDefaultView(Persons);                    
    public async void Sorting(DataGridSortingEventArgs e)
        e.Handled = true;
        ListSortDirection direction = (e.Column.SortDirection != ListSortDirection.Ascending) ? ListSortDirection.Ascending : ListSortDirection.Descending;
        e.Column.SortDirection = direction;
        string dir = direction.ToString() == "Ascending" ? "ASC" : "DESC";
        await UpdateData(e.Column.SortMemberPath, dir);
        CollectionView.SortDescriptions.Clear();
        CollectionView.SortDescriptions.Add(new SortDescription(e.Column.SortMemberPath, direction));
    }