AvaloniaUI/Avalonia · error · InvalidOperationException

Descendant selector must be preceded by a selector.

Error message

Descendant selector must be preceded by a selector.

What it means

Error "Descendant selector must be preceded by a selector." thrown in AvaloniaUI/Avalonia.

Source

Thrown at src/Avalonia.Base/Styling/DescendentSelector.cs:16

using System;
using Avalonia.LogicalTree;
using Avalonia.Styling.Activators;

#nullable enable

namespace Avalonia.Styling
{
    internal class DescendantSelector : Selector
    {
        private readonly Selector _parent;
        private string? _selectorString;

        public DescendantSelector(Selector? parent)
        {
            _parent = parent ?? throw new InvalidOperationException("Descendant selector must be preceded by a selector.");
        }

        /// <inheritdoc/>
        internal override bool IsCombinator => true;

        /// <inheritdoc/>
        internal override bool InTemplate => _parent.InTemplate;

        /// <inheritdoc/>
        internal override Type? TargetType => null;

        public override string ToString(Style? owner)
        {
            if (_selectorString == null)
            {
                _selectorString = _parent.ToString(owner, true) + ' ';
            }

View on GitHub (pinned to 11c5427268)

Solutions

1. Ensure the selector string has a selector before the descendant combinator (a space); e.g. 'Button TextBlock' instead of starting with a space.
2. Fix programmatically built selectors so a DescendentSelector is always constructed with a preceding selector.

When it happens

Trigger: Raised when a descendant selector (whitespace combinator) appears without a preceding selector in a style selector string. Defend by ensuring selector strings start with a type or name selector before any descendant combinator.

Common situations: See trigger scenarios.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/fd1a27a569e2ec78. Report an issue: GitHub.