AvaloniaUI/Avalonia · error · ArgumentOutOfRangeException

Invalid {nameof(TextRunProperties.FontRenderingEmSize)}

Error message

Invalid {nameof(TextRunProperties.FontRenderingEmSize)}

What it means

Thrown by the TextCharacters constructor when the supplied TextRunProperties has a FontRenderingEmSize that is zero or negative. FontRenderingEmSize represents the font size in EM units used to render the text run; it must be a strictly positive number. Avalonia rejects non-positive sizes early because the text layout engine cannot shape or measure glyphs without a valid em size.

Source

Thrown at src/Avalonia.Base/Media/TextFormatting/TextCharacters.cs:35

        private const char WordJoiner = '\u2060';
        private static readonly string s_wordJoinerRun = new string(WordJoiner, 8);

        /// <summary>
        /// Constructs a run for text content from a string.
        /// </summary>
        public TextCharacters(string text, TextRunProperties textRunProperties)
            : this(text.AsMemory(), textRunProperties)
        {
        }

        /// <summary>
        /// Constructs a run for text content from a memory region.
        /// </summary>
        public TextCharacters(ReadOnlyMemory<char> text, TextRunProperties textRunProperties)
        {
            if (textRunProperties.FontRenderingEmSize <= 0)
            {
                throw new ArgumentOutOfRangeException(nameof(textRunProperties), textRunProperties.FontRenderingEmSize,
                    $"Invalid {nameof(TextRunProperties.FontRenderingEmSize)}");
            }

            Text = text;
            Properties = textRunProperties;
        }

        /// <inheritdoc />
        public override int Length
            => Text.Length;

        /// <inheritdoc />
        public override ReadOnlyMemory<char> Text { get; }

        /// <inheritdoc />
        public override TextRunProperties Properties { get; }

        /// <summary>

View on GitHub (pinned to 11c5427268)

Solutions

1. Ensure the TextRunProperties.FontRenderingEmSize is set to a positive value (e.g. 12) before constructing TextCharacters.
2. If building GenericTextRunProperties programmatically, pass a valid typeface with the desired font size in the constructor or set the property explicitly.
3. Validate the font size at the call site with a guard: if (props.FontRenderingEmSize <= 0) throw or assign a sane default.
4. Check upstream style/template binding to confirm a font size is actually resolved at runtime.

Example fix

// before
var props = new GenericTextRunProperties(typeface, 0); // FontRenderingEmSize = 0
var run = new TextCharacters("hello", props); // throws

// after
var props = new GenericTextRunProperties(typeface, 14);
var run = new TextCharacters("hello", props);

static TextCharacters SafeCreate(string text, TextRunProperties props)
{
    if (props.FontRenderingEmSize <= 0)
        throw new ArgumentException("FontRenderingEmSize must be > 0.", nameof(props));
    return new TextCharacters(text, props);
}

static bool IsValidTextRunProperties(TextRunProperties p)
    => p != null && p.FontRenderingEmSize > 0;

When it happens

Trigger: Constructing a new TextCharacters(string, TextRunProperties) or TextCharacters(ReadOnlyMemory<char>, TextRunProperties) where the GenericTextRunProperties.FontRenderingEmSize (or any TextRunProperties implementation) is set to 0, a negative value, or left at a default of 0 on a custom properties object.

Common situations: Creating a custom TextRunProperties implementation whose FontRenderingEmSize defaults to 0; passing a properties object that was only partially initialized; copying properties from a style or template that did not set a font size; unit confusion where a size in points or pixels was never assigned.

Related errors

• {nameof(textLength)} ('0') must be a non-zero value.
• The parameter value must be greater than zero.
• The parameter value cannot be greater than '{MaxFontEmSize}'
• The parameter value must be a number.
• Font weight must be > 0.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/999f7ca24064a897. Report an issue: GitHub.