<Window 
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication1"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525" KeyDown="Window_KeyDown">
    <Grid>
        <TextBlock FontFamily="Segoe, Segoe UI, Arial" FontWeight="Bold" FontSize="40pt" Foreground="Chartreuse">
      Hello World!
        </TextBlock>
    </Grid>
</Window>

string xamlFile = @"D:\test.xaml";
            DependencyObject obj;
            using (FileStream fs = new FileStream(xamlFile, FileMode.Open))
            {                               
                obj = (DependencyObject)XamlReader.Load(fs);
            }            

            if (obj != null && obj is Window)            
            {
                Window win = (Window)obj;
                win.Show();                
            }

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication1"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525" KeyDown="Window_KeyDown">
    <Grid>
        <TextBlock FontFamily="Segoe, Segoe UI, Arial" FontWeight="Bold" FontSize="40pt" Foreground="Chartreuse">
      Hello World!
        </TextBlock>
    </Grid>
</Window>

namespace WpfApplication1
{
    public partial class MainWindow : Window
    {        

        public MainWindow()
        {
            
        }

    }
}

            string xamlFile = @"D:\test.xaml";
            DependencyObject obj;

            using (FileStream fs = new FileStream(xamlFile, FileMode.Open))
            {                               
                obj = (DependencyObject)XamlReader.Load(fs);
            }            

            if (obj != null && obj is WpfApplication1.MainWindow)            
            {
                WpfApplication1.MainWindow win = (WpfApplication1.MainWindow)obj;
                win.Show();                
            }