R29/WpfApp29/AppHelper.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;

namespace WpfApp29
{
    public class AppHelper
    {
        public static T? GetWindowAtType<T>() where T: Window
        {
            foreach (var unknownWindoew in App.Current.Windows)
            {
                if (unknownWindoew is T window)
                {
                    return window;
                }
            }
            return null;
        }
    }
}