AvaloniaUI/Avalonia · error · ArgumentNullException

Resources

Error message

Resources

What it means

The Styles.Resources property assigns the IResourceDictionary backing this Styles collection. The getter lazily initializes a ResourceDictionary (get => _resources ?? (Resources = new ResourceDictionary())), so a null dictionary is never a valid state. The setter throws ArgumentNullException because a null value would break lazy initialization and leave resource lookups with no dictionary to query.

Source

Thrown at src/Avalonia.Base/Styling/Styles.cs:63

            private set
            {
                if (_owner != value)
                {
                    _owner = value;
                    OwnerChanged?.Invoke(this, EventArgs.Empty);
                }
            }
        }

        /// <summary>
        /// Gets or sets a dictionary of style resources.
        /// </summary>
        public IResourceDictionary Resources
        {
            get => _resources ?? (Resources = new ResourceDictionary());
            set
            {
                value = value ?? throw new ArgumentNullException(nameof(Resources));

                var currentOwner = Owner;

                if (currentOwner is not null)
                {
                    _resources?.RemoveOwner(currentOwner);
                }

                _resources = value;

                if (currentOwner is not null)
                {
                    _resources.AddOwner(currentOwner);
                }
            }
        }

        bool ICollection<IStyle>.IsReadOnly => false;

View on GitHub (pinned to 11c5427268)

Solutions

1. To clear resources, assign an empty dictionary: styles.Resources = new ResourceDictionary();
2. If a dictionary is already assigned, call styles.Resources.Clear() to remove entries without replacing the dictionary.
3. Remove any Resources="{x:Null}" from XAML and ensure bindings that feed Resources never yield null (use a fallback value of an empty ResourceDictionary).

Example fix

// before
styles.Resources = null;

// after
styles.Resources = new ResourceDictionary();

// Never assign null; assign an empty dictionary to clear.
styles.Resources = newResources ?? new ResourceDictionary();

static bool IsValidResourceDictionary(IResourceDictionary? dict) => dict is not null;

When it happens

Trigger: Assigning styles.Resources = null directly in C#, or a binding/XAML expression that resolves Resources to null (e.g. Resources="{x:Null}"). Also reachable via code paths that intend to 'clear' resources by nulling them.

Common situations: Attempted resource clearing via null assignment instead of clearing the dictionary contents; data-bound resource dictionaries whose source becomes null; migration of older code that assumed nullable Resources; XAML authored with explicit {x:Null}.

Related errors

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

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/e7ec0d0980649556. Report an issue: GitHub.