What causes "SR.Format(SR.LengthFormatError, span.ToString())" in dotnet/wpf?

Correct the string to a valid numeric length (e.g. '12' or '12.5'), or use CSS-like units LengthConverter accepts ('in','cm','pt').

dotnet/wpf · error · FormatException

SR.Format(SR.LengthFormatError, span.ToString())

Error message

SR.Format(SR.LengthFormatError, span.ToString())

What it means

LengthConverter.ParseDouble (used by LengthConverter.FromString) wraps a failed double.Parse of a XAML/converter length string in a FormatException with the LengthFormatError message including the offending span. WPF length values must be parseable numbers (optionally with units like 'in', 'cm', 'pt' handled by the converter); a malformed numeric string triggers this.

Solutions

Correct the string to a valid numeric length (e.g. '12' or '12.5'), or use CSS-like units LengthConverter accepts ('in','cm','pt').
Use Double.NaN for 'Auto' instead of the text 'auto'.
In code, parse with the correct CultureInfo (CultureInfo.InvariantCulture) before assigning.
Wrap FromString/parse calls in try-catch on FormatException and surface a friendly message.

Example fix

// before
button.Width = (double)LengthConverter.FromString("12px", culture);
// after
button.Width = (double)LengthConverter.FromString("12", culture);
// or for Auto:
button.Width = double.NaN;

Defensive patterns

Strategy: try-catch

Validate before calling

bool IsValidLength(string s) =>
    double.TryParse(s, NumberStyles.Float, CultureInfo.InvariantCulture, out _);

Type guard

bool TryGetLength(string s, out double value) =>
    double.TryParse(s, NumberStyles.Float, CultureInfo.InvariantCulture, out value);

Try / catch

try
{
    value = (double)LengthConverter.FromString(input, culture);
}
catch (FormatException ex)
{
    Log($"'{input}' is not a valid WPF length: {ex.Message}");
    value = double.NaN; // Auto
}

Prevention

Use invariant culture or the converter's expected format when parsing lengths in code.
Use double.NaN (not 'auto') for Auto sizes.
Never copy CSS units like 'px' into XAML length attributes.
Validate length strings before assigning to dependency properties via converters.

When it happens

Trigger: Setting a length-typed dependency property (Width, Height, etc.) via XAML or a string converter with a value like '12px', 'auto', '1,5' with wrong culture, or any non-numeric text that double.Parse rejects.

Common situations: Typo'd XAML attribute values, culture mismatches (decimal comma vs dot when converting strings in code), CSS-style units copied into XAML, and empty or whitespace strings.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/2218d28b83b187dc. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/LengthConverter.cs:225

            if (valueSpan.IsEmpty)
                return 0;

            return ParseDouble(valueSpan, cultureInfo) * unitFactor;
        }

        private static double ParseDouble(ReadOnlySpan<char> span, CultureInfo cultureInfo)
        {
            // FormatException errors thrown by double.Parse are pretty uninformative.
            // Throw a more meaningful error in this case that tells that we were attempting
            // to create a Length instance from a string.  This addresses windows bug 968884
            try
            {
                return double.Parse(span, cultureInfo);
            }
            catch (FormatException)
            {
                throw new FormatException(SR.Format(SR.LengthFormatError, span.ToString()));
            }
        }


        #endregion

    }
}

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