What causes "Font weight must be > 0." in AvaloniaUI/Avalonia?

Pass a valid FontWeight such as FontWeight.Normal (400) or FontWeight.Bold (700).

AvaloniaUI/Avalonia · error · ArgumentException

Font weight must be > 0.

Error message

Font weight must be > 0.

What it means

Thrown by the Typeface constructor when the FontWeight argument is zero or negative. FontWeight must be a positive integer (typical values 100–900 in steps of 100). Avalonia validates this up front because the font matching and glyph pipeline require a meaningful weight.

Source

Thrown at src/Avalonia.Base/Media/Typeface.cs:28

    /// </summary>
    [DebuggerDisplay("Name = {FontFamily.Name}, Weight = {Weight}, Style = {Style}")]
    public readonly struct Typeface : IEquatable<Typeface>
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="Typeface"/> class.
        /// </summary>
        /// <param name="fontFamily">The font family.</param>
        /// <param name="style">The font style.</param>
        /// <param name="weight">The font weight.</param>
        /// <param name="stretch">The font stretch.</param>
        public Typeface(FontFamily fontFamily,
            FontStyle style = FontStyle.Normal,
            FontWeight weight = FontWeight.Normal,
            FontStretch stretch = FontStretch.Normal)
        {
            if (weight <= 0)
            {
                throw new ArgumentException("Font weight must be > 0.");
            }
            
            if ((int)stretch < 1)
            {
                throw new ArgumentException("Font stretch must be > 1.");
            }

            FontFamily = fontFamily ?? FontFamily.Default;
            Style = style;
            Weight = weight;
            Stretch = stretch;
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="Typeface"/> class.
        /// </summary>
        /// <param name="fontFamilyName">The name of the font family.</param>
        /// <param name="style">The font style.</param>

View on GitHub (pinned to 11c5427268)

Solutions

Pass a valid FontWeight such as FontWeight.Normal (400) or FontWeight.Bold (700).
Validate any int-derived weight: if ((int)weight <= 0) weight = FontWeight.Normal.
Ensure deserialization/config provides a weight in the 1–999 range.

Example fix

// before
var typeface = new Typeface(family, weight: (FontWeight)0); // throws

// after
var typeface = new Typeface(family, weight: FontWeight.Normal);

Defensive patterns

Strategy: validation

Validate before calling

static Typeface SafeTypeface(FontFamily f, FontStyle st, FontWeight w, FontStretch sr)
{
    if ((int)w <= 0) w = FontWeight.Normal;
    return new Typeface(f, st, w, sr);
}

Type guard

static bool IsValidWeight(FontWeight w) => (int)w > 0;

Prevention

Never cast arbitrary non-positive ints to FontWeight.
Default any unconfigured weight to FontWeight.Normal (400).
Validate deserialized weight values before constructing a Typeface.

When it happens

Trigger: Constructing a Typeface with weight <= 0, e.g. new Typeface(family, weight: (FontWeight)0) or passing a default/uninitialized FontWeight enum that resolves to 0. Also triggered by casting an arbitrary int to FontWeight that is non-positive.

Common situations: Casting a config-driven integer to FontWeight where the value is 0 or negative; default(FontWeight) used before assignment (FontWeight.Normal is 400, but an explicit 0 cast bypasses it); deserialization of a weight field that was not populated.

Related errors

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/7821c6b381c94aed. Report an issue: GitHub.