MainWindow.xaml.cs

using DevExpress.Xpf.Docking;
using DevExpress.Xpf.Layout.Core;
using DevExpress.Xpf.Ribbon;
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;

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : DXRibbonWindow
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void biMDI_ItemClick(object sender, DevExpress.Xpf.Bars.ItemClickEventArgs e)
        {
            ChildDocumentGroup.MDIStyle = MDIStyle.MDI;
        }

        private void biTabbed_ItemClick(object sender, DevExpress.Xpf.Bars.ItemClickEventArgs e)
        {
            ChildDocumentGroup.MDIStyle = MDIStyle.Tabbed;
        }
    }
}