Imports System.ComponentModel
Imports C1.WPF.FlexGrid

Class MainWindow
    Sub New()
        InitializeComponent()
        Dim rnd = New Random()
        flexGrid.ItemsSource = Enumerable.Range(1, 100).Select(Function(x) New User With {.Id = rnd.Next(1, 100), .Name = $"Name {rnd.Next(1, 100)}", .Age = rnd.Next(18, 45)}).ToList()
        C1FlexGridMultiColumnSortBehavior.Apply(flexGrid)
    End Sub
End Class

Public Class C1FlexGridMultiColumnSortBehavior
    Shared _applied As Boolean = False
    Shared _flexGrid As C1FlexGrid

    Public Shared Sub Apply(flexGrid As C1FlexGrid)
        If _applied Then
            Return
        End If

        _flexGrid = flexGrid
        AddHandler _flexGrid.PreviewMouseLeftButtonUp, AddressOf OnFlexGridPreviewMouseLeftButtonUp

        _applied = True
    End Sub

    Public Shared Sub Remove(flexGrid As C1FlexGrid)
        If _applied = True And flexGrid.Equals(_flexGrid) Then
            RemoveHandler flexGrid.PreviewMouseLeftButtonUp, AddressOf OnFlexGridPreviewMouseLeftButtonUp
            _applied = False
        End If
    End Sub

    Private Shared Sub OnFlexGridPreviewMouseLeftButtonUp(sender As Object, e As MouseButtonEventArgs)
        Dim hitTest = _flexGrid.HitTest(e)

        If Keyboard.Modifiers = ModifierKeys.Control Then
            If hitTest.CellType = CellType.ColumnHeader Then
                Dim column = _flexGrid.Columns(hitTest.Column)
                Dim sortDescriptions = _flexGrid.CollectionView.SortDescriptions

                If _flexGrid.ActiveEditor IsNot Nothing Then
                    CType(_flexGrid.CollectionView, ListCollectionView).CommitEdit()
                End If

                If sortDescriptions.Any(Function(x) x.PropertyName = column.BoundPropertyName) Then
                    Dim sortDescription = sortDescriptions.FirstOrDefault(Function(x) x.PropertyName = column.BoundPropertyName)
                    Console.WriteLine(sortDescription.Direction)
                    If sortDescription.Direction = ListSortDirection.Ascending Then
                        sortDescriptions.Remove(sortDescription)
                        sortDescriptions.Add(New SortDescription(column.BoundPropertyName, ListSortDirection.Descending))
                    Else
                        sortDescriptions.Remove(sortDescription)
                        sortDescriptions.Add(New SortDescription(column.BoundPropertyName, ListSortDirection.Ascending))
                    End If
                Else
                    sortDescriptions.Add(New SortDescription(column.BoundPropertyName, ListSortDirection.Ascending))
                End If

                e.Handled = True
            End If
        End If
    End Sub
End Class

Public Class User
    Public Property Id As Integer
    Public Property Name As String
    Public Property Age As Integer
End Class

using System;
using System.Linq;
using System.Windows;
using System.Windows.Data;
using System.Windows.Input;
using System.ComponentModel;
using C1.WPF.FlexGrid;

namespace WpfCsFlexGridMultiColumnSort
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            var rnd = new Random();
            this.flexGrid.ItemsSource = Enumerable.Range(1, 100).Select(x => new User() { Id = rnd.Next(1, 100), Name = $"Name {rnd.Next(1, 100)}", Age = rnd.Next(18, 45) }).ToList();
            C1FlexGridMultiColumnSortBehavior.Apply(this.flexGrid);
        }
    }
}

public class C1FlexGridMultiColumnSortBehavior
{
    private static bool _applied = false;
    private static C1FlexGrid _flexGrid;

    public static void Apply(C1FlexGrid flexGrid)
    {
        if (_applied)
        {
            return;
        }

        _flexGrid = flexGrid;
        _flexGrid.PreviewMouseLeftButtonUp += OnFlexGridPreviewMouseLeftButtonUp;

        _applied = true;
    }

    public static void Remove(C1FlexGrid flexGrid)
    {
        if (_applied == true & flexGrid.Equals(_flexGrid))
        {
            flexGrid.PreviewMouseLeftButtonUp -= OnFlexGridPreviewMouseLeftButtonUp;
            _applied = false;
        }
    }

    private static void OnFlexGridPreviewMouseLeftButtonUp(object sender, MouseButtonEventArgs e)
    {
        var hitTest = _flexGrid.HitTest(e);

        if (Keyboard.Modifiers == ModifierKeys.Control)
        {
            if (hitTest.CellType == CellType.ColumnHeader)
            {
                var column = _flexGrid.Columns[hitTest.Column];
                var sortDescriptions = _flexGrid.CollectionView.SortDescriptions;

                if (_flexGrid.ActiveEditor != null)
                {
                    ((ListCollectionView)_flexGrid.CollectionView).CommitEdit();
                }

                if (sortDescriptions.Any(x => (x.PropertyName ?? "") == (column.BoundPropertyName ?? "")))
                {
                    var sortDescription = sortDescriptions.FirstOrDefault(x => (x.PropertyName ?? "") == (column.BoundPropertyName ?? ""));
                    Console.WriteLine((int)sortDescription.Direction);
                    if (sortDescription.Direction == ListSortDirection.Ascending)
                    {
                        sortDescriptions.Remove(sortDescription);
                        sortDescriptions.Add(new SortDescription(column.BoundPropertyName, ListSortDirection.Descending));
                    }
                    else
                    {
                        sortDescriptions.Remove(sortDescription);
                        sortDescriptions.Add(new SortDescription(column.BoundPropertyName, ListSortDirection.Ascending));
                    }
                }
                else
                {
                    sortDescriptions.Add(new SortDescription(column.BoundPropertyName, ListSortDirection.Ascending));
                }

                e.Handled = true;
            }
        }
    }
}

public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }
}