public HorizontalExpandMode HorizontalExpand { get; set; }

Public Property HorizontalExpand As HorizontalExpandMode

<Application x:Class="DXExpander_CreatingAndUsing.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">
    <Application.Resources>

    </Application.Resources>
</Application>

Imports System.Windows
Imports DevExpress.Xpf.Core
Imports System.Collections.ObjectModel

Namespace DXExpander_CreatingAndUsing
    Partial Public Class MainWindow
        Inherits Window

        Public Sub New()
            InitializeComponent()
            Me.DataContext = New MyViewModel()
            ApplicationThemeHelper.ApplicationThemeName = "Office2016White"

        End Sub
        Private Sub Button_Click(ByVal sender As Object, ByVal e As RoutedEventArgs)
            expander.IsExpanded = Not expander.IsExpanded
        End Sub
        Private Sub rbAzure_Checked(ByVal sender As Object, ByVal e As RoutedEventArgs)
            If gridControl1 Is Nothing Then
                Return
            End If
            If rbAzure.IsChecked = True Then
                ApplicationThemeHelper.ApplicationThemeName = "Office2016White"
            End If
        End Sub
        Private Sub rbGray_Checked(ByVal sender As Object, ByVal e As RoutedEventArgs)
            If gridControl1 Is Nothing Then
                Return
            End If
            If rbGray.IsChecked = True Then
                ApplicationThemeHelper.ApplicationThemeName = "LightGray"
            End If
        End Sub
        Private Sub rbOfficeBlack_Checked(ByVal sender As Object, ByVal e As RoutedEventArgs)
            If gridControl1 Is Nothing Then
                Return
            End If
            If rbOfficeBlack.IsChecked = True Then
                ApplicationThemeHelper.ApplicationThemeName = "Office2007Black"
            End If
        End Sub
        Private Sub rbOfficeBlue_Checked(ByVal sender As Object, ByVal e As RoutedEventArgs)
            If gridControl1 Is Nothing Then
                Return
            End If
            If rbOfficeBlue.IsChecked = True Then
                ApplicationThemeHelper.ApplicationThemeName = "Office2007Blue"
            End If
        End Sub
        Private Sub rbOfficeSilver_Checked(ByVal sender As Object, ByVal e As RoutedEventArgs)
            If gridControl1 Is Nothing Then
                Return
            End If
            If rbOfficeSilver.IsChecked = True Then
                ApplicationThemeHelper.ApplicationThemeName = "Office2007Silver"
            End If
        End Sub
    End Class

    Partial Public Class Person
        Public Sub New()

        End Sub
        Public Sub New(ByVal i As Integer)
            FirstName = "FirstName" & i
            LastName = "LastName" & i
            Age = i * 10

        End Sub

        Private _firstName As String
        Public Property LastName() As String
        Private _age As Integer

        Public Property FirstName() As String
            Get
                Return _firstName
            End Get
            Set(ByVal value As String)
                _firstName = value
            End Set
        End Property

        Public Property Age() As Integer
            Get
                Return _age
            End Get
            Set(ByVal value As Integer)
                _age = value
            End Set
        End Property
    End Class
    Partial Public Class MyViewModel
        Public Sub New()
            CreateList()
        End Sub

        Public Property ListPerson() As ObservableCollection(Of Person)

        Private Sub CreateList()
            ListPerson = New ObservableCollection(Of Person)()

            For i As Integer = 0 To 9
                Dim p As New Person(i)
                ListPerson.Add(p)
            Next i
        End Sub
    End Class
End Namespace

Imports System
Imports System.Collections.Generic
Imports System.Configuration
Imports System.Data
Imports System.Linq
Imports System.Windows

Namespace DXExpander_CreatingAndUsing
    ''' <summary>
    ''' Interaction logic for App.xaml
    ''' </summary>
    Partial Public Class App
        Inherits Application

        Private Sub OnAppStartup_UpdateThemeName(ByVal sender As Object, ByVal e As StartupEventArgs)

        End Sub
    End Class
End Namespace

<Window x:Class="DXExpander_CreatingAndUsing.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:dx="clr-namespace:DevExpress.Xpf.Core;assembly=DevExpress.Xpf.Core.v16.1"
        xmlns:dxg="http://schemas.devexpress.com/winfx/2008/xaml/grid"
        xmlns:dxe="http://schemas.devexpress.com/winfx/2008/xaml/editors"
        Title="Using DXExpander" Height="400" Width="600">
    <Grid x:Name="LayoutRoot" Background="White">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="20" />
            <ColumnDefinition Width="Auto" />
        </Grid.ColumnDefinitions>
        <Button Grid.Column="1" Content="..." Click="Button_Click" />
        <dx:DXExpander x:Name="expander" Grid.Column="2" FlowDirection="LeftToRight"
                       VerticalExpand="None" HorizontalExpand="FromLeftToRight"
                       IsExpanded="False">
                <GroupBox Header="Application Theme:" VerticalAlignment="Center"
                          Margin="5,0,5,0">
                    <StackPanel>
                    <RadioButton x:Name="rbAzure" Content="Office2016White"
                                     Checked="rbAzure_Checked" Margin="0,5,0,2" IsChecked="True" />
                        <RadioButton x:Name="rbGray" Content="Light Gray"
                                     Checked="rbGray_Checked" Margin="0,2,0,2" />
                        <RadioButton x:Name="rbOfficeBlack" Content="Office 2007 Black"
                                     Checked="rbOfficeBlack_Checked" Margin="0,2,0,2" />
                        <RadioButton x:Name="rbOfficeBlue" Content="Office 2007 Blue"
                                     Checked="rbOfficeBlue_Checked" 
                                     Margin="0,2,0,2" />
                        <RadioButton x:Name="rbOfficeSilver" Content="Office 2007 Silver"
                                     Checked="rbOfficeSilver_Checked" Margin="0,2,0,0" />
                    </StackPanel>
                </GroupBox>
        </dx:DXExpander>
        <dxg:GridControl x:Name="gridControl1" AutoGenerateColumns="AddNew" ItemsSource="{Binding ListPerson}">
            <dxg:GridControl.View>
                <dxg:TableView />
            </dxg:GridControl.View>
        </dxg:GridControl>
    </Grid>
</Window>

using System.Windows;
using DevExpress.Xpf.Core;
using System.Collections.ObjectModel;

namespace DXExpander_CreatingAndUsing {
    public partial class MainWindow : Window {
        public MainWindow() {
            InitializeComponent();
            this.DataContext = new MyViewModel();
            ApplicationThemeHelper.ApplicationThemeName = "Office2016White";

        }
        private void Button_Click(object sender, RoutedEventArgs e) {
            expander.IsExpanded = !expander.IsExpanded;
        }
        private void rbAzure_Checked(object sender, RoutedEventArgs e) {
            if (gridControl1 == null)
                return;
            if (rbAzure.IsChecked == true)
                ApplicationThemeHelper.ApplicationThemeName = "Office2016White";
        }
        private void rbGray_Checked(object sender, RoutedEventArgs e) {
            if (gridControl1 == null)
                return;
            if (rbGray.IsChecked == true)
                ApplicationThemeHelper.ApplicationThemeName = "LightGray";
        }
        private void rbOfficeBlack_Checked(object sender, RoutedEventArgs e) {
            if (gridControl1 == null)
                return;
            if (rbOfficeBlack.IsChecked == true)
                ApplicationThemeHelper.ApplicationThemeName = "Office2007Black";
        }
        private void rbOfficeBlue_Checked(object sender, RoutedEventArgs e) {
            if (gridControl1 == null)
                return;
            if (rbOfficeBlue.IsChecked == true)
                ApplicationThemeHelper.ApplicationThemeName = "Office2007Blue";
        }
        private void rbOfficeSilver_Checked(object sender, RoutedEventArgs e) {
            if (gridControl1 == null)
                return;
            if (rbOfficeSilver.IsChecked == true)
                ApplicationThemeHelper.ApplicationThemeName = "Office2007Silver";
        }
    }

    public partial class Person {
        public Person() {

        }
        public Person(int i) {
            FirstName = "FirstName" + i;
            LastName = "LastName" + i;
            Age = i * 10;

        }

        string _firstName;
        public string LastName { get; set; }
        int _age;

        public string FirstName {
            get { return _firstName; }
            set {
                _firstName = value;
            }
        }

        public int Age {
            get { return _age; }
            set { _age = value; }
        }
    }
    public partial class MyViewModel {
        public MyViewModel() {
            CreateList();
        }

        public ObservableCollection<Person> ListPerson { get; set; }

        void CreateList() {
            ListPerson = new ObservableCollection<Person>();

            for (int i = 0; i < 10; i++) {
                Person p = new Person(i);
                ListPerson.Add(p);
            }
        }
    }
}