iOfficeAI/OfficeCLI · error · ArgumentException

Invalid font size: '{value}'. Minimum font size is 0.5pt (on

Error message

Invalid font size: '{value}'. Minimum font size is 0.5pt (one half-point).

What it means

Thrown by ParseFontSize when the size is positive but below 0.5pt. OOXML stores font size as half-points (w:sz must be >= 1), so anything below 0.5pt would round to val=0 on write and produce schema-invalid OOXML that Word rejects. The library rejects up front with the same message shape as the other guards.

Source

Thrown at src/officecli/Core/ParseHelpers.cs:375

    /// Returns double to preserve fractional sizes for correct unit conversion.
    /// </summary>
    public static double ParseFontSize(string value)
    {
        var trimmed = value.Trim();
        if (trimmed.EndsWith("pt", StringComparison.OrdinalIgnoreCase))
            trimmed = trimmed[..^2].Trim();
        if (trimmed.Contains(','))
            throw new ArgumentException($"Invalid font size: '{value}'. Comma is not allowed — use '.' as decimal separator (e.g., '10.5').");
        if (!double.TryParse(trimmed, CultureInfo.InvariantCulture, out var result) || double.IsNaN(result) || double.IsInfinity(result))
            throw new ArgumentException($"Invalid font size: '{value}'. Expected a finite number (e.g., '12', '10.5', '14pt').");
        if (result <= 0)
            throw new ArgumentException($"Invalid font size: '{value}'. Font size must be greater than 0.");
        // OOXML w:sz/w:szCs/w:fontSize are half-points and must be >= 1.
        // Anything below 0.5pt would round to val=0 on write, producing
        // schema-invalid OOXML. Reject up front with the same shape as
        // the "<= 0" guard above.
        if (result < 0.5)
            throw new ArgumentException($"Invalid font size: '{value}'. Minimum font size is 0.5pt (one half-point).");
        // OOXML caps user-entered font size at 1638pt (Word) and Office
        // renderers stop honoring values past ~4000pt anyway. Anything
        // larger silently overflows the int32 the writers cast to (PPTX
        // writes pt × 100, Word writes pt × 2 as half-points), producing
        // negative w:sz / a:rPr@sz values Word rejects on open. Reject
        // up front with the same shape as the lower-bound guards.
        if (result > 4000)
            throw new ArgumentException($"Invalid font size: '{value}'. Maximum font size is 4000pt (Office cap).");
        return result;
    }

    /// <summary>
    /// BUG-R4B(BUG1): Leniently parse an integer-valued OOXML attribute that
    /// some producers emit with a fractional part (e.g. <c>w:w="0.0"</c> /
    /// <c>w:w="9440.0"</c>). The Open XML SDK's typed accessors (e.g.
    /// <c>Int32Value.Value</c>) parse the raw string lazily and throw a bare
    /// <see cref="FormatException"/> ("The input string '0.0' was not in a
    /// correct format") the first time the value is read. Reading the raw

View on GitHub (pinned to 1ced45e900)

Solutions

Use a size of at least 0.5pt.
Clamp the computed size: Math.Max(pts, 0.5).
Re-check scale factors that can shrink text below readable sizes.

Example fix

// before
var pts = baseSize * 0.01; // -> 0.12
ParseFontSize(pts.ToString(CultureInfo.InvariantCulture)); // throws 295

// after
var pts = Math.Max(baseSize * 0.01, 0.5);
ParseFontSize(pts.ToString(CultureInfo.InvariantCulture));

Defensive patterns

Strategy: validation

Validate before calling

const double MinPt = 0.5;
pts = Math.Max(pts, MinPt);

Type guard

static bool IsAtLeastHalfPoint(double v) => v >= 0.5 && !double.IsNaN(v) && !double.IsInfinity(v);

Try / catch

try { pts = ParseFontSize(s); }
catch (ArgumentException ex) when (ex.Message.Contains("0.5pt"))
{ pts = 0.5; }

Prevention

Clamp computed sizes to >= 0.5pt.
Re-check scale factors that can shrink text below readable sizes.
Unit-test the lower boundary explicitly.

When it happens

Trigger: Passing "0.25" or "0.3". A fractional size from a scaling factor that shrank the font below half a point.

Common situations: Responsive/scaling code that multiplies by a small factor; converting from a unit where the result is sub-half-point; debug values entered during testing.

Related errors

AI-assisted analysis of iOfficeAI/OfficeCLI@1ced45e900 (2026-08-13). Data as JSON: /api/errors/8b118106b89fb5a4. Report an issue: GitHub.