fix(windows): show app icon in MessageDialog via MessageBoxIndirectW

v2/internal/frontend/desktop/windows/dialog.go

@@ -9,6 +9,7 @@ import (
 	"syscall"
 
 	"github.com/wailsapp/wails/v2/internal/frontend"
+	"github.com/wailsapp/wails/v2/internal/frontend/desktop/windows/winc"
 	"github.com/wailsapp/wails/v2/internal/frontend/desktop/windows/winc/w32"
 	"github.com/wailsapp/wails/v2/internal/go-common-file-dialog/cfd"
 	"golang.org/x/sys/windows"
@@ -177,6 +178,15 @@ func calculateMessageDialogFlags(options frontend.MessageDialogOptions) uint32 {
 	return flags
 }
 
+// messageDialogUserIconFlags prepares flags for MB_USERICON: the stock icon bits
+// must be stripped (mixing MB_ICON* with MB_USERICON is undefined), while the
+// button configuration is preserved (#4233). MB_SYSTEMMODAL is kept: it controls
+// modality, not the icon.
+func messageDialogUserIconFlags(flags uint32) uint32 {
+	const mbIconMask = windows.MB_ICONHAND | windows.MB_ICONQUESTION | windows.MB_ICONASTERISK | windows.MB_USERICON
+	return (flags &^ mbIconMask) | windows.MB_USERICON | windows.MB_SYSTEMMODAL
+}
+
 // MessageDialog show a message dialog to the user
 func (f *Frontend) MessageDialog(options frontend.MessageDialogOptions) (string, error) {
 
@@ -191,7 +201,16 @@ func (f *Frontend) MessageDialog(options frontend.MessageDialogOptions) (string,
 
 	flags := calculateMessageDialogFlags(options)
 
-	button, _ := windows.MessageBox(windows.HWND(f.getHandleForDialog()), message, title, flags|windows.MB_SYSTEMMODAL)
+	// Show the application icon (the same icon the main window uses) in the
+	// message box; MessageBoxW can only show the four stock icons.
+	hwnd := f.getHandleForDialog()
+	button, err := w32.MessageBoxWithIcon(hwnd, message, title, uint16(winc.AppIconID), messageDialogUserIconFlags(flags))
+	if err != nil {
+		// The icon resource is not embedded in dev builds (`go run`, `wails dev`),
+		// which makes MessageBoxIndirect fail outright (#4233). Fall back to a
+		// stock-icon dialog so the message is still shown.
+		button, _ = windows.MessageBox(windows.HWND(hwnd), message, title, flags|windows.MB_SYSTEMMODAL)
+	}
 	// This maps MessageBox return values to strings
 	responses := []string{"", "Ok", "Cancel", "Abort", "Retry", "Ignore", "Yes", "No", "", "", "Try Again", "Continue"}
 	result := "Error"

v2/internal/frontend/desktop/windows/dialog_test.go

@@ -75,3 +75,64 @@ func Test_calculateMessageDialogFlags(t *testing.T) {
 		})
 	}
 }
+
+func Test_messageDialogUserIconFlags(t *testing.T) {
+	tests := []struct {
+		name    string
+		options frontend.MessageDialogOptions
+		want    uint32
+	}{
+		{
+			name: "Test Info Dialog",
+			options: frontend.MessageDialogOptions{
+				Type: frontend.InfoDialog,
+			},
+			want: windows.MB_OK | windows.MB_USERICON | windows.MB_SYSTEMMODAL,
+		},
+		{
+			name: "Test Error Dialog",
+			options: frontend.MessageDialogOptions{
+				Type: frontend.ErrorDialog,
+			},
+			want: windows.MB_OK | windows.MB_USERICON | windows.MB_SYSTEMMODAL,
+		},
+		{
+			name: "Test Question Dialog",
+			options: frontend.MessageDialogOptions{
+				Type: frontend.QuestionDialog,
+			},
+			want: windows.MB_YESNO | windows.MB_USERICON | windows.MB_SYSTEMMODAL,
+		},
+		{
+			name: "Test Question Dialog with default cancel",
+			options: frontend.MessageDialogOptions{
+				Type:          frontend.QuestionDialog,
+				DefaultButton: "No",
+			},
+			want: windows.MB_YESNO | windows.MB_DEFBUTTON2 | windows.MB_USERICON | windows.MB_SYSTEMMODAL,
+		},
+		{
+			name: "Test Question Dialog with default cancel (lowercase)",
+			options: frontend.MessageDialogOptions{
+				Type:          frontend.QuestionDialog,
+				DefaultButton: "no",
+			},
+			want: windows.MB_YESNO | windows.MB_DEFBUTTON2 | windows.MB_USERICON | windows.MB_SYSTEMMODAL,
+		},
+		{
+			name: "Test Warning Dialog",
+			options: frontend.MessageDialogOptions{
+				Type: frontend.WarningDialog,
+			},
+			want: windows.MB_OK | windows.MB_USERICON | windows.MB_SYSTEMMODAL,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			flags := calculateMessageDialogFlags(tt.options)
+			if got := messageDialogUserIconFlags(flags); got != tt.want {
+				t.Errorf("messageDialogUserIconFlags() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}

v2/internal/frontend/desktop/windows/winc/w32/typedef.go

@@ -279,6 +279,20 @@ type MINMAXINFO struct {
 	PtMaxTrackSize POINT
 }
 
+// https://docs.microsoft.com/en-us/windows/win32/api/winuser/ns-winuser-msgboxparamsw
+type MSGBOXPARAMS struct {
+	Size           uint32
+	Owner          HWND
+	Instance       HINSTANCE
+	Text           *uint16
+	Caption        *uint16
+	Style          uint32
+	Icon           *uint16
+	ContextHelpID  uintptr
+	MsgBoxCallback uintptr
+	LanguageID     uint32
+}
+
 // http://msdn.microsoft.com/en-us/library/windows/desktop/dd145037.aspx
 type LOGFONT struct {
 	Height         int32

v2/internal/frontend/desktop/windows/winc/w32/user32.go

@@ -66,6 +66,7 @@ var (
 	procReleaseCapture                = moduser32.NewProc("ReleaseCapture")
 	procGetWindowThreadProcessId      = moduser32.NewProc("GetWindowThreadProcessId")
 	procMessageBox                    = moduser32.NewProc("MessageBoxW")
+	procMessageBoxIndirect            = moduser32.NewProc("MessageBoxIndirectW")
 	procGetSystemMetrics              = moduser32.NewProc("GetSystemMetrics")
 	procPostThreadMessageW            = moduser32.NewProc("PostThreadMessageW")
 	procRegisterWindowMessageA        = moduser32.NewProc("RegisterWindowMessageA")
@@ -623,6 +624,29 @@ func MessageBox(hwnd HWND, title, caption string, flags uint) int {
 	return int(ret)
 }
 
+// MessageBoxWithIcon shows a modal message box that displays the icon stored in
+// resource iconID of the given module. MessageBoxW cannot show an application
+// icon, only the four stock icons. Returns an error if the icon resource is
+// missing, which happens in dev builds (`go run`, `wails dev`) where no icon
+// resource is embedded - see #4233.
+func MessageBoxWithIcon(hwnd HWND, text, caption *uint16, iconID uint16, flags uint32) (int32, error) {
+	params := MSGBOXPARAMS{
+		Size:     uint32(unsafe.Sizeof(MSGBOXPARAMS{})),
+		Owner:    hwnd,
+		Instance: GetModuleHandle(""),
+		Text:     text,
+		Caption:  caption,
+		Style:    flags,
+		Icon:     (*uint16)(unsafe.Pointer(uintptr(iconID))), // MAKEINTRESOURCE
+	}
+
+	ret, _, err := procMessageBoxIndirect.Call(uintptr(unsafe.Pointer(&params)))
+	if ret == 0 {
+		return 0, err
+	}
+	return int32(ret), nil
+}
+
 func GetSystemMetrics(index int) int {
 	ret, _, _ := procGetSystemMetrics.Call(
 		uintptr(index))

website/src/pages/changelog.mdx

@@ -14,6 +14,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed `MessageDialog` not showing the application icon on Windows [#3684](https://github.com/wailsapp/wails/issues/3684) by @DangTAnh
+
 ## v2.16.0 - 2026-09-14
 
 ### Fixed