AvaloniaUI/Avalonia · error · InvalidOperationException

Invalid selector: Template selector must be followed by cont

Error message

Invalid selector: Template selector must be followed by control selector.

What it means

Thrown by Style.ValidateSelector (invoked from the Selector setter) when the assigned selector is a bare TemplateSelector with nothing following it. The Template selector (`/template/`) only makes sense when followed by a selector addressing a part inside the template; a lone TemplateSelector has no target part and is rejected at assignment time.

Source

Thrown at src/Avalonia.Base/Styling/Style.cs:96

                        SelectorMatch.NeverThisInstance);

                activity?.AddTag(Diagnostic.Tags.SelectorResult, match.Result);

                if (match.IsMatch)
                {
                    Attach(target, match.Activator, type, Selector is not OrSelector);
                }

                result = match.Result;
            }

            return result;
        }

        private static Selector? ValidateSelector(Selector? selector)
        {
            if (selector is TemplateSelector)
                throw new InvalidOperationException(
                    "Invalid selector: Template selector must be followed by control selector.");
            return selector;
        }
    }
}

View on GitHub (pinned to 11c5427268)

Solutions

1. Follow the Template selector with a part selector, e.g. `s.OfType<Button>().Template().OfType<ContentPresenter>()`.
2. In AXAML, write `Button /template/ ContentPresenter` rather than just `Button /template/`.
3. If you did not intend template traversal, remove the Template selector.

Example fix

// before
new Style(s => s.OfType<Button>().Template())

// after
new Style(s => s.OfType<Button>().Template().OfType<ContentPresenter>())

// Reject a bare TemplateSelector before assigning to Style.Selector.
if (selector is TemplateSelector)
    throw new InvalidOperationException("Template selector must be followed by a part selector.");

static bool IsCompleteTemplateChain(Selector? s) => s is not TemplateSelector;

When it happens

Trigger: Assigning `s.Template()` alone as a Style's Selector (no subsequent OfType/Class/etc.), or AXAML `Selector="... /template/"` with nothing after the template token.

Common situations: Truncated selector after refactoring; AXAML where the post-template part was deleted; programmatic selector building that stops at Template().

Related errors

• ControlTemplate styles cannot contain multiple template sele
• Need more than one query to OR.
• Need more than one selector to OR.
• ControlTheme style may not directly contain a child or desce
• Child styles must have a nesting selector.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/31c919274bc470c7. Report an issue: GitHub.