AvaloniaUI/Avalonia · error · ArgumentNullException

owner

Error message

owner

What it means

IResourceProvider.AddOwner registers an IResourceHost (a control, Styles collection, or application) as the owner of this Style's resource dictionary. The method throws ArgumentNullException when owner is null because the resource-ownership lifecycle requires a concrete host to notify on resource changes.

Source

Thrown at src/Avalonia.Base/Styling/StyleBase.cs:160

                    instance.Add(_animations);

                if (canShareInstance)
                {
                    instance.MakeShared();
                    _sharedInstance = instance;
                }
            }

            ao.GetValueStore().AddFrame(instance);
            instance.ApplyAnimations(ao);
            return instance;
        }

        internal virtual void SetParent(StyleBase? parent) => Parent = parent;

        void IResourceProvider.AddOwner(IResourceHost owner)
        {
            owner = owner ?? throw new ArgumentNullException(nameof(owner));

            if (Owner != null)
            {
                throw new InvalidOperationException("The Style already has a parent.");
            }

            Owner = owner;
            _resources?.AddOwner(owner);
        }

        void IResourceProvider.RemoveOwner(IResourceHost owner)
        {
            owner = owner ?? throw new ArgumentNullException(nameof(owner));

            if (Owner == owner)
            {
                Owner = null;
                _resources?.RemoveOwner(owner);

View on GitHub (pinned to 11c5427268)

Solutions

1. Pass a non-null IResourceHost instance to AddOwner.
2. Prefer letting the framework manage ownership via Styles.Add / Styles.Remove rather than calling AddOwner manually.
3. Guard the call site: only invoke AddOwner when you hold a live IResourceHost reference.

Example fix

// before
((IResourceProvider)style).AddOwner(null);

// after
((IResourceProvider)style).AddOwner(ownerHost);

// Validate owner before registering resources.
if (ownerHost is null) throw new ArgumentNullException(nameof(ownerHost));
((IResourceProvider)style).AddOwner(ownerHost);

static bool IsValidResourceOwner(IResourceHost? host) => host is not null;

When it happens

Trigger: Calling ((IResourceProvider)style).AddOwner(null) directly, or a custom IResourceHost whose parenting logic forwards null. In normal Avalonia use the framework calls AddOwner automatically when a style is added to a Styles collection, so this only fires under manual or custom resource wiring.

Common situations: Custom Styles subclass overriding resource plumbing; third-party theming libraries that replicate the IResourceProvider contract; teardown code that calls AddOwner with a field that was already nulled.

Related errors

• The Style already has a parent.
• Resources
• Not selector must have a selector argument.
• dictionary
• items

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/65bf4e4112986af6. Report an issue: GitHub.