How can I add a WPF overlay over an external Win32 application's window?

Question

I am trying to attach a WPF Window as a child window of an external application such as Notepad to provide an overlay. Having researched all the answers I can find on SO and MSDN, I've got as far as creating a solid overlay over the corner of Notepad when my WPF application runs. However,

as soon as Notepad gains focus, the overlay disappears,
as well as the overlay showing on Notepad, the Overlay is also shown separately as a window
the overlay on Notepad does not receive any MouseMove events (but the separate window does.

Here is the minimal example to demonstrate the issue:

Overlay.xaml

<Window x:Class="WindowControlTest.Overlay"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Height="300" Width="300"
        Opacity="1"
        Background="Azure"
        MouseMove="Window_MouseMove"
        GotFocus="Window_GotFocus"
        Loaded="Window_Loaded"
        Title="Overlay" 
        WindowStyle="None"
        >
</Window>

Overlay.xaml.cs

using System;
using System.Windows;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Interop;

namespace WindowControlTest
{

    public partial class Overlay : Window
    {
        IntPtr m_ParentHwnd;
        HwndSource m_HwndSource;

        public Overlay(IntPtr parentHwnd)
        {
            InitializeComponent();
            m_ParentHwnd = parentHwnd;
        }

        private void Window_MouseMove(object sender, MouseEventArgs e)
        {
            Console.WriteLine("Overlay.Window_MouseMove: " + e.GetPosition(this));
        }

        private void Window_GotFocus(object sender, RoutedEventArgs e)
        {
            Console.WriteLine("Overlay.Window_GotFocus");
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            HwndSourceParameters parameters = new HwndSourceParameters();
            parameters.WindowStyle = (int) (WindowStyles.WS_VISIBLE | WindowStyles.WS_CHILD);
            parameters.SetPosition(0, 0);
            parameters.UsesPerPixelOpacity = true;
            parameters.SetSize((int)Width, (int)Height);
            parameters.ParentWindow = m_ParentHwnd;
            m_HwndSource = new HwndSource(parameters);
            m_HwndSource.CompositionTarget.BackgroundColor = Colors.Aqua;
            m_HwndSource.RootVisual = (Visual)Content;
        }
    }
}

MainWindow.xaml

<Window x:Class="WindowControlTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525"
        Loaded="OnLoaded"
        >

    <StackPanel>
        <Label x:Name="stateLabel">Label</Label>
    </StackPanel>
</Window>

MainWindow.xaml.cs - Finds and handle to Notepad and creates an Overlay

using System;
using System.Text;
using System.Windows;

namespace WindowControlTest
{
    public partial class MainWindow : Window
    {
        private IntPtr m_TargetHwnd;
        private Overlay m_Overlay;

        public MainWindow()
        {
            InitializeComponent();
        }

        private void OnLoaded(object sender, RoutedEventArgs e)
        {
            processWindows();

            if (m_TargetHwnd != IntPtr.Zero)
            {
                m_Overlay = new Overlay(m_TargetHwnd);
                m_Overlay.Show();
            }
        }

        private void processWindows()
        {
            Win32.EnumWindows(delegate(IntPtr wnd, IntPtr param)
            {
                String text = GetWindowText(wnd);
                Console.WriteLine("Window: " + text);
                if (text.Contains("Notepad"))
                {
                    m_TargetHwnd = wnd;
                }
                return true;
            }, IntPtr.Zero);
        }

        public static string GetWindowText(IntPtr hWnd)
        {
            int size = Win32.GetWindowTextLength(hWnd);
            if (size++ > 0)
            {
                var builder = new StringBuilder(size);
                Win32.GetWindowText(hWnd, builder, builder.Capacity);
                return builder.ToString();
            }
            return String.Empty;
        }
    }
}

(Note: A number of SO questions address a similar but different issue, e.g. How to set Win32 window as owner of WPF window? assumes I am in control of the source code for the Win32 window, as do the examples I can find on MSDN.)

score 6 · Accepted Answer · edited Jun 20 '20 at 09:12

6

I think your going the wrong path, instead of making your overlay a child of the notepad window, you should try and make notepad the child of your main window through an HwndHost and then maintain the overlay above the host.

Here's my blog post introducing a library I wrote for layering a WPF adorner over any hwnd hosted by an HwndHost. A simple web browser demo looks like this:

edited Jun 20 '20 at 09:12

Community

1
1

answered Jul 14 '15 at 16:58

Michael Sutton

418
3
9

This is the path we went down in the end. It's not completely satisfactory as it's not always trivial to make your child window look like the original free window. But certainly things start behaving better when keeping WPF as the parent. – Tim MB Jul 16 '15 at 09:27
Also - nice blog post! – Tim MB Jul 16 '15 at 09:27
Nice blog post, and nice demo project. Unfortunately we cannot use your code since it is copyrighted (no license) :/ – Kryptos Nov 04 '15 at 03:36
Now has an MIT license. – ulatekh Feb 08 '16 at 21:45
Blog link in Wayback Machine: https://web.archive.org/web/20191230144649/http://blogs.microsoft.co.il/michaels/2014/11/28/wpf-hwnd-adorner/ – moon Jan 17 '23 at 12:41

How can I add a WPF overlay over an external Win32 application's window?

1 Answers1

Linked