WPF DataContext order and filter via CollectionViewSource.GetDefaultView(DataContext)

//xaml
<Window x:Class="WpfApp146.MainWindow"
        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:WpfApp146"
        mc:Ignorable="d" WindowState="Maximized"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <local:SortFieldList x:Key="sortFields">
            <local:SortField DisplayName="(Unsorted)"/>
            <local:SortField DisplayName="Process Name" PropertyName="ProcessName"/>
            <local:SortField DisplayName="Process ID" PropertyName="Id"/>
            <local:SortField DisplayName="# Threads" PropertyName="Threads.Count"/>
        </local:SortFieldList>
    </Window.Resources>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <ListBox HorizontalContentAlignment="Stretch" ItemsSource="{Binding}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <Border Margin="2" Padding="4" BorderThickness="3" BorderBrush="Blue">
                        <StackPanel Orientation="Horizontal" TextBlock.FontSize="20">
                            <TextBlock Foreground="Black" FontWeight="Bold"
                                       Text="{Binding ProcessName,StringFormat='Name:{0}'}" />
                            <TextBlock Margin="10,0,0,0" Foreground="Blue"
                                       Text="{Binding Id,StringFormat='Id:{0}'}"/>
                            <TextBlock Margin="20,0,0,0" Foreground="Red"
                                       Text="{Binding Threads.Count,StringFormat='Threads Count:{0}'}"/>
                        </StackPanel>
                    </Border>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
        <StackPanel Grid.Row="1" Orientation="Horizontal" Margin="4" TextBlock.FontSize="14">
            <TextBlock Text="Sort by" VerticalAlignment="Center"/>
            <ComboBox Width="150" Margin="6,0,0,0" x:Name="_sortField" SelectedIndex="0"
                      ItemsSource="{StaticResource sortFields}"
                      DisplayMemberPath="DisplayName"
                      SelectedValuePath="PropertyName">
            </ComboBox>
            <CheckBox Margin="10,0,0,0" Content="Ascending" VerticalAlignment="Center"
                      IsChecked="True" x:Name="_ascending"/>
            <Button Margin="4,0,0,0" Content="Sort" x:Name="sortBtn" Click="sortBtn_Click"/>
            <TextBox Margin="10,0,0,0" x:Name="filterText" Width="120"/>
            <Button Margin="4,0,0,0" Content="Filter" x:Name="filerBtn" Click="filerBtnClick" />
        </StackPanel>
    </Grid>
</Window>



//cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Diagnostics;
using System.ComponentModel;

namespace WpfApp146
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = Process.GetProcesses();
        }

        private void sortBtn_Click(object sender, RoutedEventArgs e)
        {
            var view = CollectionViewSource.GetDefaultView(DataContext);
            view.SortDescriptions.Clear();
            if(_sortField.SelectedValue!=null)
            {
                view.SortDescriptions.Add(new SortDescription((string)_sortField.SelectedValue,
                    _ascending.IsChecked == true ? 
                    ListSortDirection.Ascending :
                    ListSortDirection.Descending));
            }
        }

        private void filerBtnClick(object sender, RoutedEventArgs e)
        {
            var view=CollectionViewSource.GetDefaultView(DataContext);
            if(string.IsNullOrWhiteSpace(filterText.Text))
            {
                view.Filter = null;
            }
            else
            {
                view.Filter = obj => ((Process)obj).ProcessName.IndexOf(filterText.Text, StringComparison.InvariantCultureIgnoreCase) > -1;
            }
        }
    }

    class SortFieldList:List<SortField>
    {

    }

    class SortField
    {
        public string DisplayName { get; set; }
        public string PropertyName { get; set; }
    }
}