Setting NotifyIcon.TooltipText to non-ASCII text renders as ? characters in the tray tooltip #1705

Summary

Setting NotifyIcon.TooltipText to a string containing non-ASCII characters results in those characters being replaced with ? in the tray tooltip on systems whose system ANSI codepage cannot represent them. The same defect affects szInfo (balloon body) and szInfoTitle (balloon title).

Reproduction

Tested on en-US Windows 11 24H2 (build 26200), WPF-UI 4.2.1, .NET 10:

<tray:NotifyIcon TooltipText="80% 已连接(未充电)" Icon="..." />

Hovering the tray icon shows 80% ???(???). The percentage and ASCII parens render correctly; only the CJK characters are lost.

Root cause

src/Wpf.Ui.Tray/Interop/Shell32.cs::NOTIFYICONDATA is declared without an explicit CharSet:

[StructLayout(LayoutKind.Sequential)]
public class NOTIFYICONDATA
{
    ...
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 0x80)]
    public string? szTip;
    ...
}

C# defaults StructLayoutAttribute.CharSet to Ansi. UnmanagedType.ByValTStr follows the enclosing struct's charset, so szTip/szInfo/szInfoTitle are marshaled via Encoding.Default.GetBytes(...), which substitutes ? for code points unrepresentable in the active ANSI codepage. The matching import:

[DllImport(Libraries.Shell32)]
public static extern bool Shell_NotifyIcon([In] NIM dwMessage, [In] NOTIFYICONDATA lpdata);

also has no CharSet, so it resolves to Shell_NotifyIconA and Windows stores the already-corrupted text.

Suggested fix

Two-line change:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public class NOTIFYICONDATA { ... }

[DllImport(Libraries.Shell32, CharSet = CharSet.Unicode)]
public static extern bool Shell_NotifyIcon([In] NIM dwMessage, [In] NOTIFYICONDATA lpdata);

ByValTStr's SizeConst slots are interpreted per the new charset (i.e. 128 wide chars instead of 128 ANSI bytes for szTip), so the struct's binary size grows accordingly. cbSize = Marshal.SizeOf(typeof(NOTIFYICONDATA)) recomputes correctly and routes the call through Shell_NotifyIconW. No call-site changes required.

Workaround for consumers

Subclass NotifyIcon, override OnPropertyChanged, and after base re-send the tooltip via your own Unicode-charset NOTIFYICONDATA mirror + Shell_NotifyIconW(NIM.MODIFY, NIF.TIP). Reflection on the existing internal ShellIconData recovers hWnd and uID. Re-apply on both TooltipTextProperty and IconProperty changes — TrayManager.ModifyIcon re-sends the same ANSI szTip from the cached struct, which would otherwise overwrite the Unicode fix on every icon update.

Environment

• WPF-UI: 4.2.1
• Target framework: net10.0-windows10.0.26100.0 (also reproduces on net8.0-windows)
• OS: Windows 11 24H2 (build 26200)
• System ANSI codepage: 1252 (en-US)
• Tooltip text language: zh-Hans