NotImplementedException in Avalonia.Win32.WindowImpl.LoadIcon

[Bondarenko1990]

Describe the bug

I encountered a NotImplementedException in the LoadIcon method of Avalonia.Win32.WindowImpl. This issue occurs when running the application, and it seems related to window icon handling.

The issue was reported by a user in Sentry, and the AppDomain.UnhandledException handler captured the error.
Error Details:
System.NotImplementedException: The method or operation is not implemented. at Avalonia.Win32.WindowImpl.LoadIcon(UnmanagedMethods.Icons, UInt32) at Avalonia.Win32.WindowImpl.AppWndProc(IntPtr, UInt32, IntPtr, IntPtr) at Avalonia.Win32.WindowImpl.WndProc(IntPtr, UInt32, IntPtr, IntPtr) at Avalonia.Win32.WindowImpl.WndProcMessageHandler(IntPtr hWnd, UInt32 msg, IntPtr wParam, IntPtr lParam)

To Reproduce

1. Run the application using Avalonia.
2. The exception is thrown during the initialization of the window.

Expected behavior

The application should initialize without throwing a NotImplementedException.

Avalonia version

11.2.7

OS

Windows

Additional context

No response

[timunie]

please provide a minimum sample for us to investigate further

[marpisco]

@timunie I'm currently facing this issue too, while working on a tray-icon implementation for a private app.

Here is the reproduction that was missing. It is deterministic and does not
depend on any third-party software being installed.

Cause

WM_GETICON is handled in WindowImpl.AppWndProc.cs by casting wParam
straight to the Icons enum, with no check that the value is one of the three
documented ones:

case WindowsMessage.WM_GETICON:
    if (_iconImpl == null)
    {
        break;
    }

    var requestIcon = (Icons)wParam;
    var requestDpi = (uint)lParam;
    ...
    return LoadIcon(requestIcon, requestDpi)?.Handle ?? default;

LoadIcon then throws on anything it does not recognise
(src/Windows/Avalonia.Win32/WindowImpl.cs):

_iconCache[iconKey] = icon = type switch
{
    Icons.ICON_SMALL => _iconImpl.LoadSmallIcon(scale),
    Icons.ICON_BIG => _iconImpl.LoadBigIcon(scale),
    _ => throw new NotImplementedException(),
};

WM_GETICON is a public message. Any process holding the HWND can send it,
and Windows does not require wParam to be one of ICON_SMALL, ICON_BIG or
ICON_SMALL2. Because the throw happens inside the native window procedure,
it cannot be caught from managed code — neither a try/catch around the
message pump nor Dispatcher.UIThread.UnhandledException with
Handled = true prevents the process from dying.

Steps

1. Build and run the attached project (Program.cs is ~50 lines; a window with
   an icon and nothing else).
2. From a separate process, send WM_GETICON with a wParam outside
   {0, 1, 2} — poke.ps1 does exactly this.
3. The target process terminates.

Sending it from the same process does not reproduce it: the exception unwinds
back through the SendMessage call and can be caught there. It has to come
from outside, which is also how it happens in the wild.

An icon must be set on the window. Without one, _iconImpl is null and both
the handler and LoadIcon return before reaching the switch.

Result

wParam	outcome
0 (ICON_SMALL)	survives
1 (ICON_BIG)	survives
2 (ICON_SMALL2)	survives
3	process terminated

Application: ReproLoadIcon.exe
CoreCLR Version: 10.0.1126.37416
.NET Version: 10.0.11
Description: The process was terminated due to an unhandled exception.
Exception Info: System.NotImplementedException: The method or operation is not implemented.
   at Avalonia.Win32.WindowImpl.LoadIcon(Icons type, UInt32 dpi)
   at Avalonia.Win32.WindowImpl.AppWndProc(IntPtr hWnd, UInt32 msg, IntPtr wParam, IntPtr lParam)
   at Avalonia.Win32.WindowImpl.WndProc(IntPtr hWnd, UInt32 msg, IntPtr wParam, IntPtr lParam)
   at Avalonia.Win32.WindowImpl.WndProcMessageHandler(IntPtr hWnd, UInt32 msg, IntPtr wParam, IntPtr lParam)

Byte for byte the stack trace in the original report.

Environment

• Avalonia 12.1.2 (latest on NuGet at the time of writing) and 12.0.5
• .NET 10.0.11, Windows 11 (10.0.26200), x64
• Same code is present on master today; introduced in Improved Windows icon rendering quality #14564 (Feb 2024)

How we hit it without sending anything ourselves

Our app lives in the notification area and hides its window instead of closing
it. Showing that window again from the tray icon killed the process every time.
We never send WM_GETICON ourselves — the shell does, while creating the
taskbar button. The same crash appeared when the window stopped pumping
messages during a long synchronous operation and Windows drew a ghost window
for it, and immediately at startup when the window icon was an .ico.

We are working around it by subclassing the window procedure and answering
WM_GETICON before Avalonia sees it. That works, but it means shipping
P/Invoke to defend against a framework method that throws.

Suggested fix

Either treat an unrecognised wParam as ICON_SMALL, or break and let
DefWindowProc answer — which is the fallback Windows documents anyway. More
generally, a value crossing the boundary from the OS should not reach a
switch whose default arm throws.

repro-avalonia-loadicon.zip