Accessibility on itself

[codecat]

I am using the MacOS accessibility client APIs (AXUIElement etc.) to access elements from certain programs. This is done through a separate thread, and works well. My program is an Avalonia app, and as soon as it tries to access its own UI elements, I get an exception:

System.InvalidOperationException: Call from invalid thread
   at Avalonia.Threading.Dispatcher.<VerifyAccess>g__ThrowVerifyAccess|16_0()
   at Avalonia.Threading.Dispatcher.VerifyAccess()
   at Avalonia.AvaloniaObject.VerifyAccess()
   at Avalonia.Controls.Control.GetOrCreateAutomationPeer()
   at Avalonia.Automation.Peers.ControlAutomationPeer.CreatePeerForElement(Control element)
   at Avalonia.Native.WindowBaseImpl.GetAutomationPeer()
   at Avalonia.Native.WindowBaseImpl.WindowBaseEvents.Avalonia.Native.Interop.IAvnWindowBaseEvents.get_AutomationPeer()
   at Avalonia.Native.Interop.Impl.__MicroComIAvnWindowBaseEventsVTable.GetAutomationPeer(Void* this)
--- End of stack trace from previous location ---
   at Avalonia.Native.DispatcherImpl.RunLoop(CancellationToken token)
   at Avalonia.Threading.DispatcherFrame.Run(IControlledDispatcherImpl impl)
   at Avalonia.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
   at Avalonia.Threading.Dispatcher.MainLoop(CancellationToken cancellationToken)
   at Avalonia.Controls.ApplicationLifetimes.ClassicDesktopStyleApplicationLifetime.Start(String[] args)
   at Avalonia.ClassicDesktopStyleApplicationLifetimeExtensions.StartWithClassicDesktopLifetime(AppBuilder builder, String[] args, Action`1 lifetimeBuilder)

It works fine if I use the AXUIElement APIs through the UI thread instead. Might this be a bug, or is this intentional and expected?

[IanRawley]

Anything interacting directly with the UI (either the controls themselves, or the values in the VM that they're bound to) needs to do the interaction on the UI thread. The work can be done in another thread, but anything that directly changes the UI needs to happen in the right place.

It sounds like you're trying to use a separate thread to interact with the Avalonia controls, in which case all those interactions will need to be dispatched to the UI thread.

[codecat]

Right, that's what I am assuming. I just wasn't expecting the intermediate accessibility layer to also rely on this being done from the UI thread.

[IanRawley]

Why wouldn't it? AXUIElement is from outside the Avalonia framework, so there's no reason to assume it's following Avalonia's rules.