AvaloniaUI/Avalonia · error · InvalidOperationException

Object is not yet attached to the compositor

Error message

Object is not yet attached to the compositor

What it means

Thrown by CompositionCustomVisualHandler.VerifyAccess when the handler's _host is null — i.e. the handler is not yet attached to a ServerCompositionCustomVisual. Most protected members (EffectiveSize, CompositionNow, SendNextRender) call VerifyAccess first, so any access before the handler is attached to a CompositionCustomVisual fails fast.

Source

Thrown at src/Avalonia.Base/Rendering/Composition/CompositionCustomVisualHandler.cs:47

        _inRender = true;
        _currentTransformedClip = currentTransformedClip;
        _currentTransform = drawingContext.CurrentTransform;
        try
        {
            OnRender(drawingContext);
        }
        finally
        {
            _inRender = false;
        }
    }

    public abstract void OnRender(ImmediateDrawingContext drawingContext);

    void VerifyAccess()
    {
        if (_host == null)
            throw new InvalidOperationException("Object is not yet attached to the compositor");
        _host.Compositor.VerifyAccess();
    }

    void VerifyInRender()
    {
        VerifyAccess();
        if (!_inRender)
            throw new InvalidOperationException("This API is only available from OnRender");
    }

    protected Vector EffectiveSize
    {
        get
        {
            VerifyAccess();
            return _host!.Size;
        }
    }

View on GitHub (pinned to 11c5427268)

Solutions

1. Access handler members only from OnRender/OnMessage (which run after attachment), not from the constructor or external code.
2. In tests, attach the handler to a real or mock ServerCompositionCustomVisual before exercising it.
3. Do not hold and later use a handler reference from a detached visual.

Example fix

// before
var handler = new MyHandler();
var size = handler.EffectiveSize; // throws: not attached
// after
var visual = compositor.CreateCustomVisual(handler); // attaches handler
// size is only read inside handler.OnRender

static bool IsAttached(CompositionCustomVisualHandler h) => h is not null; // real check: only call from OnRender/OnMessage

When it happens

Trigger: Reading EffectiveSize/CompositionNow or calling rendering APIs on a CompositionCustomVisualHandler instance that was never attached, or before Attach was called (e.g. constructing a handler and querying it directly without going through CompositionCustomVisual).

Common situations: Instantiating a handler standalone in a unit test or helper and calling its members; using a handler detached from its visual; accessing handler state in the constructor before the compositor attaches it.

Related errors

• This API is only available from OnRender
• Animation has no key frames
• Visual was invalidated during the render pass
• Unable to obtain ANativeWindow
• TopLevel.InternalView was not expected to be null.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/a4907ffc3b0ffd5e. Report an issue: GitHub.