public DefaultBoolean IsExpandButtonVisible { get; set; }

Public Property IsExpandButtonVisible As DefaultBoolean

<dxg:GridControl x:Name="grid">
    <dxg:GridControl.Columns>
        <dxg:GridColumn FieldName="Name"/>
        <dxg:GridColumn FieldName="ItemType"/>
        <dxg:GridColumn FieldName="Size">
            <dxg:GridColumn.EditSettings>
                <dxe:TextEditSettings HorizontalContentAlignment="Right"/>
            </dxg:GridColumn.EditSettings>
        </dxg:GridColumn>
        <dxg:GridColumn FieldName="FullName"/>
    </dxg:GridControl.Columns>
    <dxg:GridControl.View>
        <dxg:TreeListView x:Name="view" 
                          AllowEditing="False"
                          AutoWidth="True"
                          NodeExpanding="OnNodeExpanding"/>
    </dxg:GridControl.View>
</dxg:GridControl>

public partial class MainWindow : Window {
    FileSystemDataProvider Helper { get; set; }
    public MainWindow() {
        InitializeComponent();
        Helper = new FileSystemHelper();
        InitDrives();
    }
    public void InitDrives() {
        grid.BeginDataUpdate();
        try {
            string[] root = Helper.GetLogicalDrives();

            foreach (string s in root) {
                TreeListNode node = new TreeListNode() { Content = new FileSystemItem(s, "Drive", "<Drive>", s) };
                view.Nodes.Add(node);
                node.IsExpandButtonVisible = DefaultBoolean.True;
            }
        }
        catch { }
        grid.EndDataUpdate();
    }

    private void OnNodeExpanding(object sender, DevExpress.Xpf.Grid.TreeList.TreeListNodeAllowEventArgs e) {
        TreeListNode node = e.Node;
        if (node.Tag == null || (bool)node.Tag == false) {
            InitFolder(node);
            node.Tag = true;
        }
    }

    private void InitFolder(TreeListNode treeListNode) {
        grid.BeginDataUpdate();
        InitFolders(treeListNode);
        InitFiles(treeListNode);
        grid.EndDataUpdate();
    }

    private void InitFolders(TreeListNode treeListNode) {
        FileSystemItem item = treeListNode.Content as FileSystemItem;
        if (item == null) return;

        try {
            string[] root = Helper.GetDirectories(item.FullName);
            foreach (string s in root) {
                try {
                    TreeListNode node = new TreeListNode() { Content = new FileSystemItem(Helper.GetDirectoryName(s), "Folder", "<Folder>", s) };
                    treeListNode.Nodes.Add(node);

                    node.IsExpandButtonVisible = HasFiles(s) ? DefaultBoolean.True : DefaultBoolean.False;
                }
                catch { }
            }
        }
        catch { }
    }

    private void InitFiles(TreeListNode treeListNode) {
        FileSystemItem item = treeListNode.Content as FileSystemItem;
        if (item == null) return;
        TreeListNode node;
        try {
            string[] root = Helper.GetFiles(item.FullName);
            foreach (string s in root) {
                node = new TreeListNode() { Content = new FileSystemItem(Helper.GetFileName(s), "File", Helper.GetFileSize(s).ToString(), s) };
                node.IsExpandButtonVisible = DefaultBoolean.False;
                treeListNode.Nodes.Add(node);
            }
        }
        catch { }
    }

    private bool HasFiles(string path) {
        string[] root = Helper.GetFiles(path);
        if (root.Length > 0) return true;
        root = Helper.GetDirectories(path);
        if (root.Length > 0) return true;
        return false;
    }
}

Public Partial Class MainWindow
    Inherits Window

    Private Property Helper As FileSystemDataProvider

    Public Sub New()
        Me.InitializeComponent()
        Helper = New FileSystemHelper()
        InitDrives()
    End Sub

    Public Sub InitDrives()
        Me.grid.BeginDataUpdate()
        Try
            Dim root As String() = Helper.GetLogicalDrives()
            For Each s As String In root
                Dim node As TreeListNode = New TreeListNode() With {.Content = New FileSystemItem(s, "Drive", "<Drive>", s)}
                Me.view.Nodes.Add(node)
                node.IsExpandButtonVisible = DefaultBoolean.True
            Next
        Catch
        End Try

        Me.grid.EndDataUpdate()
    End Sub

    Private Sub OnNodeExpanding(ByVal sender As Object, ByVal e As TreeList.TreeListNodeAllowEventArgs)
        Dim node As TreeListNode = e.Node
        If node.Tag Is Nothing OrElse CBool(node.Tag) = False Then
            InitFolder(node)
            node.Tag = True
        End If
    End Sub

    Private Sub InitFolder(ByVal treeListNode As TreeListNode)
        Me.grid.BeginDataUpdate()
        InitFolders(treeListNode)
        InitFiles(treeListNode)
        Me.grid.EndDataUpdate()
    End Sub

    Private Sub InitFolders(ByVal treeListNode As TreeListNode)
        Dim item As FileSystemItem = TryCast(treeListNode.Content, FileSystemItem)
        If item Is Nothing Then Return
        Try
            Dim root As String() = Helper.GetDirectories(item.FullName)
            For Each s As String In root
                Try
                    Dim node As TreeListNode = New TreeListNode() With {.Content = New FileSystemItem(Helper.GetDirectoryName(s), "Folder", "<Folder>", s)}
                    treeListNode.Nodes.Add(node)
                    node.IsExpandButtonVisible = If(HasFiles(s), DefaultBoolean.True, DefaultBoolean.False)
                Catch
                End Try
            Next
        Catch
        End Try
    End Sub

    Private Sub InitFiles(ByVal treeListNode As TreeListNode)
        Dim item As FileSystemItem = TryCast(treeListNode.Content, FileSystemItem)
        If item Is Nothing Then Return
        Dim node As TreeListNode
        Try
            Dim root As String() = Helper.GetFiles(item.FullName)
            For Each s As String In root
                node = New TreeListNode() With {.Content = New FileSystemItem(Helper.GetFileName(s), "File", Helper.GetFileSize(s).ToString(), s)}
                node.IsExpandButtonVisible = DefaultBoolean.False
                treeListNode.Nodes.Add(node)
            Next
        Catch
        End Try
    End Sub

    Private Function HasFiles(ByVal path As String) As Boolean
        Dim root As String() = Helper.GetFiles(path)
        If root.Length > 0 Then Return True
        root = Helper.GetDirectories(path)
        If root.Length > 0 Then Return True
        Return False
    End Function
End Class

view.Nodes.Add(node);
    node.IsExpandButtonVisible = DefaultBoolean.True;
}

Me.view.Nodes.Add(node)
    node.IsExpandButtonVisible = DefaultBoolean.True
Next