AvaloniaUI/Avalonia · error · ArgumentOutOfRangeException

The parameter value must be a number.

Error message

The parameter value must be a number.

What it means

ValidateFontSize rejects NaN because NaN propagates through every arithmetic operation and silently corrupts all subsequent layout metrics (line heights, advance widths, hit-testing rectangles). NaN cannot produce a valid glyph layout.

Source

Thrown at src/Avalonia.Base/Media/FormattedText.cs:114

            InvalidateMetrics();
        }

        private static void ValidateFontSize(double emSize)
        {
            if (emSize <= 0)
            {
                throw new ArgumentOutOfRangeException(nameof(emSize), "The parameter value must be greater than zero.");
            }

            if (emSize > MaxFontEmSize)
            {
                throw new ArgumentOutOfRangeException(nameof(emSize), $"The parameter value cannot be greater than '{MaxFontEmSize}'");
            }

            if (double.IsNaN(emSize))
            {
                throw new ArgumentOutOfRangeException(nameof(emSize), "The parameter value must be a number.");
            }
        }

        private static void ValidateFlowDirection(FlowDirection flowDirection, string parameterName)
        {
            if ((int)flowDirection < 0 || (int)flowDirection > (int)FlowDirection.RightToLeft)
            {
                throw new InvalidEnumArgumentException(parameterName, (int)flowDirection, typeof(FlowDirection));
            }
        }

        private int ValidateRange(int startIndex, int count)
        {
            if (startIndex < 0 || startIndex > _text.Length)
            {
                throw new ArgumentOutOfRangeException(nameof(startIndex));
            }

View on GitHub (pinned to 11c5427268)

Solutions

1. Filter with double.IsFinite(emSize) before passing to FormattedText; substitute a sensible default for non-finite values.
2. Fix the upstream computation producing NaN (guard divisors, check for 0/0).
3. Add a numeric IValueConverter that maps NaN/Infinity to a fallback font size.
4. Validate deserialized font sizes against a schema/range and reject missing fields.

Example fix

// before
var emSize = (a / b) * scale; // b may be 0 -> NaN propagates
var ft = new FormattedText(text, font, emSize, ...);

// after
var raw = (a / Math.Max(b, 1)) * scale;
var emSize = double.IsFinite(raw) ? raw : 12.0;
var ft = new FormattedText(text, font, emSize, ...);

double em = double.IsFinite(requested) ? requested : 12.0;
em = em > 0 ? em : 12.0;

When it happens

Trigger: Binding FontSize to 0.0/0.0 or any computation that yields NaN; reading FontSize from a deserialized source where the value was missing and defaulted to NaN; a multiplication of Infinity * 0 in a zoom calculation.

Common situations: A division by zero in a view-model that feeds FontSize; JSON deserialization of a missing numeric field returning NaN on some serializers; XAML binding to a non-numeric source converted by a faulty IValueConverter.

Related errors

• The parameter value must be greater than zero.
• The parameter value cannot be greater than '{MaxFontEmSize}'
• 'MaxTextHeight' property value cannot be NaN.
• Parameter must be greater than or equal to zero.
• 'MaxTextHeight' property value must be greater than zero.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/058bdcdaa863c7a3. Report an issue: GitHub.