Principal.cs

using System;
using System.ComponentModel;
using System.Windows.Forms;

namespace R721WPFEnWindowsForms
{
    public partial class Principal : Form
    {
        private VentanaWPF ventanaWpfNoModal;

        private CancelEventHandler ventanaNoModalHandlerCierre;

        public Principal()
        {
            InitializeComponent();

            ventanaNoModalHandlerCierre = new CancelEventHandler(Window_Closing);
        }

        private void btnAbrirVentanaWPFNoModal_Click(object sender, EventArgs e)
        {
            if (ventanaWpfNoModal == null)
            {
                ventanaWpfNoModal = new VentanaWPF();

                ventanaWpfNoModal.Closing += ventanaNoModalHandlerCierre;
                btnAbrirVentanaWPFNoModal.Text = "Cerrar Ventana WPF No-Modal";

                ventanaWpfNoModal.Show();
            }
            else
            {
                ventanaWpfNoModal.Close();
            }
        }

        private void btnVentanaWFPModal_Click(object sender, EventArgs e)
        {
            VentanaWPF ventanaWpfModal = new VentanaWPF();
            ventanaWpfModal.ShowDialog();
        }

        private void Window_Closing(object sender, CancelEventArgs e)
        {
            ventanaWpfNoModal.Closing -= ventanaNoModalHandlerCierre;
            ventanaWpfNoModal = null;

            btnAbrirVentanaWPFNoModal.Text = "Abrir Ventana WPF No-Modal";
        }
    }
}