AvaloniaUI/Avalonia · error · ArgumentException

Specified family is not supported.

Error message

Specified family is not supported.

What it means

FontFamily.Parse throws ArgumentException when the input string is null or empty. A font family specification must contain at least a family name or a source identifier (e.g. 'FamilyName', 'avares://...#Family', or '#Source'). Empty/null input cannot identify any font family.

Source

Thrown at src/Avalonia.Base/Media/FontFamily.cs:210

        /// <exception cref="ArgumentException">
        /// Specified family is not supported.
        /// </exception>
        public static FontFamily Parse(string s) => Parse(s, null);

        /// <summary>
        /// Parses a <see cref="T:Avalonia.Media.FontFamily"/> string.
        /// </summary>
        /// <param name="s">The <see cref="T:Avalonia.Media.FontFamily"/> string.</param>
        /// <param name="baseUri">Specifies the base uri that is used to resolve font family assets.</param>
        /// <returns></returns>
        /// <exception cref="ArgumentException">
        /// Specified family is not supported.
        /// </exception>
        public static FontFamily Parse(string s, Uri? baseUri)
        {
            if (string.IsNullOrEmpty(s))
            {
                throw new ArgumentException("Specified family is not supported.", nameof(s));
            }

            return new FontFamily(baseUri, s);
        }

        /// <summary>
        /// Returns a <see cref="string" /> that represents this instance.
        /// </summary>
        /// <returns>
        /// A <see cref="string" /> that represents this instance.
        /// </returns>
        public override string ToString()
        {
            if (Key != null)
            {
                return Key + "#" + FamilyNames;
            }

View on GitHub (pinned to 11c5427268)

Solutions

1. Provide a non-empty font family string (e.g. a system font name or an avares: source).
2. Guard against null/empty before parsing: if (string.IsNullOrEmpty(s)) use a fallback family.
3. Check XAML bindings and resource definitions for empty FontFamily values.

Example fix

// before
var ff = FontFamily.Parse(fontNameFromConfig, null); // fontNameFromConfig is ""

// after
var name = string.IsNullOrEmpty(fontNameFromConfig) ? "Segoe UI" : fontNameFromConfig;
var ff = FontFamily.Parse(name, null);

if (string.IsNullOrEmpty(s))
    throw new ArgumentException("Font family name required.", nameof(s));
var ff = FontFamily.Parse(s, baseUri);

When it happens

Trigger: Calling FontFamily.Parse(s, baseUri) where s is null or empty string. Often reached from XAML when a FontFamily attribute is empty or bound to a null/empty value.

Common situations: A XAML FontFamily attribute left empty or set to "{}". Binding FontFamily to a property that evaluates to null/empty. Loading font family names from config with missing entries. A theme/style that omits the font family value.

Related errors

• IterationCount can't be a negative number.
• Invalid brush string: '{s}'.
• Unknown CacheMode: {s}
• Invalid color string: '{s}'.
• Invalid color string: '{s.ToString()}'.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/4135f262e904e3cf. Report an issue: GitHub.