MainWindow.xaml

<dx:ThemedWindow x:Class="TreeViewGettingStarted.MainWindow" Title="MainWindow" Height="350" Width="525" 
    xmlns:dx="http://schemas.devexpress.com/winfx/2008/xaml/core" 
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    xmlns:dxg="http://schemas.devexpress.com/winfx/2008/xaml/grid" 
    xmlns:local="clr-namespace:TreeViewGettingStarted">
    <dx:ThemedWindow.DataContext>
        <local:MainWindowViewModel />
    </dx:ThemedWindow.DataContext>
    <Grid>
        <dxg:TreeViewControl ItemsSource="{Binding EmployeeDepartments}" 
                             ChildNodesPath="Employees" 
                             TreeViewFieldName="Name"/>
  </Grid>
</dx:ThemedWindow>