AvaloniaUI/Avalonia · error · FormatException

Invalid unit: {part.ToString()}

Error message

Invalid unit: {part.ToString()}

What it means

Thrown by TransformParser.ParseUnit when a unit token does not match any of the supported units. The recognized units are deg, grad, rad, turn, and px (mapped in s_unitMapping). Any other unit string — or a value with an unrecognized suffix — causes this FormatException, which echoes the offending token.

Source

Thrown at src/Avalonia.Base/Media/Transformation/TransformParser.cs:374

            throw new FormatException($"Invalid value {value.Value} {unitString} for {function}");
        }

        private static void ThrowFormatInvalidValueCount(TransformFunction function, int count)
        {
            throw new FormatException($"Invalid format. {function} expects {count} value(s).");
        }

        private static Unit ParseUnit(in ReadOnlySpan<char> part)
        {
            foreach (var (name, unit) in s_unitMapping)
            {
                if (part.Equals(name.AsSpan(), StringComparison.OrdinalIgnoreCase))
                {
                    return unit;
                }
            }

            throw new FormatException($"Invalid unit: {part.ToString()}");
        }

        private static TransformFunction ParseTransformFunction(in ReadOnlySpan<char> part)
        {
            foreach (var (name, transformFunction) in s_functionMapping)
            {
                if (part.Equals(name.AsSpan(), StringComparison.OrdinalIgnoreCase))
                {
                    return transformFunction;
                }
            }

            return TransformFunction.Invalid;
        }

        private static double ToRadians(in UnitValue value)
        {
            return value.Unit switch

View on GitHub (pinned to 11c5427268)

Solutions

1. Use only supported units: px for lengths, deg/grad/rad/turn for angles.
2. Convert unsupported units (em, %, pt) to px before building the transform string.
3. Check the echoed '{part}' token in the message to find the offending unit.

Example fix

// before
var t = TransformOperations.Parse("translate(2em, 1em)"); // 'em' unsupported

// after
var t = TransformOperations.Parse("translate(32px, 16px)");

static readonly HashSet<string> s_validUnits =
    new() { "deg", "grad", "rad", "turn", "px", "" };

static bool HasValidUnits(string transform)
    /* check each numeric token's unit suffix against s_validUnits */;

try { return TransformOperations.Parse(s); }
catch (FormatException ex) when (ex.Message.Contains("Invalid unit"))
{ /* convert unsupported unit (em, %, pt) to px and retry */ }

When it happens

Trigger: Supplying a value with a unit suffix not in {deg, grad, rad, turn, px}, e.g. '10em', '5pt', '2%', or a bare unknown token. The ParseUnit loop finds no match and throws.

Common situations: Using CSS units Avalonia's transform parser does not support (em, rem, %, pt, vw); typos in unit names ('degress'); assuming browser-CSS unit coverage that the parser does not implement.

Related errors

• Invalid value {value.Value} {unitString} for {function}
• Invalid transform string: '{s}'.
• Too many provided values.
• Invalid format. {function} expects {count} value(s).
• Invalid text trimming string: '{s}'.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/2d57095b28471680. Report an issue: GitHub.