In my VSTO Excel Addin application I wish to show a Systems.Window.MessageBox as TopMost window when the user attempt to close any opened workbook – just like Excel does if you try to close a workbook while the “Move or Copy…” small window is displayed. It would be simpler to use Systems.Window.Forms.MessageBox however I wish to keep the “WPF style” of the Systems.Window.MessageBox.

I found this question on stackoverflow /questions/16105097/why-isnt-messagebox-topmost so I further extended this code – as per https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-messagebox. I have not included the modality of the dialog box and the additional options. It also has an issue: if working on a multi-screen setup, the MessageBox will be displayed as top most on the main display, even if Excel is shown on the other display. Here’s the helper class.

using System;
using System.Runtime.InteropServices;
using System.Windows;
using System.Windows.Forms;
using System.Windows.Interop;


namespace MyAddIn
{
    public class MessageBoxHelperClass
    {
        [DllImport("user32.dll", EntryPoint = "MessageBox", CharSet = CharSet.Auto)]
        public static extern int MessageBoxUser32(IntPtr hWnd, string text, string caption, uint type);

        private const uint MB_TOPMOST = 0x00040000;

        private const uint MB_OK = 0x00000000;
        private const uint MB_OKCANCEL = 0x00000001;
        private const uint MB_YESNOCANCEL = 0x00000003;
        private const uint MB_YESNO = 0x00000004;

        private const uint MB_ICONERROR = 0x00000010;
        private const uint MB_ICONQUESTION = 0x00000020;
        private const uint MB_ICONEXCLAMATION = 0x00000030;
        private const uint MB_ICONINFORMATION = 0x00000040;

        private const uint MB_DEFBUTTON1 = 0x00000000;
        private const uint MB_DEFBUTTON2 = 0x00000100;
        private const uint MB_DEFBUTTON3 = 0x00000200;

        public static MessageBoxResult ShowMessageBox(string message, bool topMost = true, string title = null, MessageBoxButton buttons = MessageBoxButton.OK, MessageBoxImage icon = MessageBoxImage.None, MessageBoxResult defaultResult = MessageBoxResult.None)
        {
            uint type = topMost ? MB_TOPMOST : 0;

            // Add button flags
            switch (buttons)
            {
                case MessageBoxButton.OK:
                    type |= MB_OK;
                    break;
                case MessageBoxButton.OKCancel:
                    type |= MB_OKCANCEL;
                    break;
                case MessageBoxButton.YesNoCancel:
                    type |= MB_YESNOCANCEL;
                    break;
                case MessageBoxButton.YesNo:
                    type |= MB_YESNO;
                    break;
            }

            // Add icon flags
            switch (icon)
            {
                case MessageBoxImage.Error:
                    type |= MB_ICONERROR;
                    break;
                case MessageBoxImage.Question:
                    type |= MB_ICONQUESTION;
                    break;
                case MessageBoxImage.Exclamation:
                    type |= MB_ICONEXCLAMATION;
                    break;
                case MessageBoxImage.Information:
                    type |= MB_ICONINFORMATION;
                    break;
            }

            // Add default button flags
            switch (defaultResult)
            {
                case MessageBoxResult.OK:
                    type |= MB_DEFBUTTON1;
                    break;
                case MessageBoxResult.Cancel:
                    if (buttons == MessageBoxButton.YesNoCancel)
                    {
                        type |= MB_DEFBUTTON2;
                    }
                    else
                    {
                        type |= MB_DEFBUTTON1;
                    }
                    break;
                case MessageBoxResult.Yes:
                    type |= MB_DEFBUTTON1;
                    break;
                case MessageBoxResult.No:
                    type |= MB_DEFBUTTON2;
                    break;
            }

            IntPtr hwnd = IntPtr.Zero;

            if (System.Windows.Application.Current != null && System.Windows.Application.Current.MainWindow != null)
            {
                WindowInteropHelper wih = new WindowInteropHelper(System.Windows.Application.Current.MainWindow);
                hwnd = wih.Handle;
            }

            int result = 0;

            if (topMost)
            {
                result = MessageBoxUser32(hwnd, message, title ?? string.Empty, type);
            }
            else
            {
                System.Windows.MessageBoxResult wpfResult = System.Windows.MessageBox.Show(message, title ?? string.Empty, buttons, icon);
                return ConvertWpfToCustomMessageBoxResult(wpfResult);
            }

            return ConvertWin32ToCustomMessageBoxResult(result, buttons);
        }

        private static MessageBoxResult ConvertWin32ToCustomMessageBoxResult(int win32Result, MessageBoxButton buttons)
        {
            switch (win32Result)
            {
                case 1: // IDOK
                    return MessageBoxResult.OK;
                case 2: // IDCANCEL
                    return MessageBoxResult.Cancel;
                case 6: // IDYES
                    return MessageBoxResult.Yes;
                case 7: // IDNO
                    return MessageBoxResult.No;
                default:
                    return MessageBoxResult.None;
            }
        }

        private static MessageBoxResult ConvertWpfToCustomMessageBoxResult(MessageBoxResult wpfResult)
        {
            switch (wpfResult)
            {
                case System.Windows.MessageBoxResult.OK:
                    return MessageBoxResult.OK;
                case System.Windows.MessageBoxResult.Cancel:
                    return MessageBoxResult.Cancel;
                case System.Windows.MessageBoxResult.Yes:
                    return MessageBoxResult.Yes;
                case System.Windows.MessageBoxResult.No:
                    return MessageBoxResult.No;
                default:
                    return MessageBoxResult.None;
            }
        }
    }
}

I have this solution that is mostly working – just need to figure out how to display the MessageBox on the correct display. However, I was wondering whether there was a simpler way to achieve that.

Many thanks for your support/ideas.