AvaloniaUI/Avalonia · error · InvalidDataException

Invalid bool rule.

Error message

Invalid bool rule.

What it means

Thrown by the private ReadBool helper when the remaining span is empty at the point a boolean flag is expected. ReadBool reads single '0'/'1' characters for arc command flags (large-arc-flag and sweep-flag in an 'A' command). An empty span means the A command's arguments were truncated before both flags were supplied.

Source

Thrown at src/Avalonia.Base/Media/PathMarkupParser.cs:539

                span = span.Slice(1);
            }
            return span;
        }

        private static ReadOnlySpan<char> SkipWhitespace(ReadOnlySpan<char> span)
        {
            int i = 0;
            for (; i < span.Length && char.IsWhiteSpace(span[i]); i++) ;
            return span.Slice(i);
        }

        private static bool ReadBool(ref ReadOnlySpan<char> span)
        {
            span = SkipWhitespace(span);
            
            if (span.IsEmpty)
            {
                throw new InvalidDataException("Invalid bool rule.");
            }
            
            var c = span[0];
            
            span = span.Slice(1);
            
            switch (c)
            {
                case '0':
                    return false;
                case '1':
                    return true;
                default:
                    throw new InvalidDataException("Invalid bool rule");
            }
        }

        private static double ReadDouble(ref ReadOnlySpan<char> span)

View on GitHub (pinned to 11c5427268)

Solutions

1. Provide all seven arc arguments: rx,ry x-axis-rotation large-arc-flag sweep-flag x,y.
2. Use '0' or '1' for both flags (e.g. large-arc-flag=0, sweep-flag=1).
3. Validate the arc token count before parsing if generating path data programmatically.

Example fix

// before (missing sweep flag)
geometry.Parse("M0,0 A5,5 0 1 10,0");
// note: only one bool was supplied in a truncated string like "M0,0 A5,5 0 0"

// after
geometry.Parse("M0,0 A5,5 0 0 1 10,0");

// For each 'A' command, count arguments; require >= 7 tokens
// (rx,ry,rot,large-arc-flag,sweep-flag,x,y). Throw/sanitize if fewer.
static bool ArcHasFlags(ReadOnlySpan<char> arcArgs) => /* parse & ensure 2 flag chars present */ true;

static bool IsBoolFlagChar(char c) => c is '0' or '1';

try { parser.Parse(data); }
catch (InvalidDataException ex) when (ex.Message == "Invalid bool rule.")
{ /* arc flags missing — report truncated A command */ }

When it happens

Trigger: An arc 'A' command missing its boolean flags, e.g. 'M0,0 A5,5 0' (missing large-arc-flag and sweep-flag), or 'M0,0 A5,5 0 0' (missing the sweep-flag).

Common situations: Hand-editing arc path data and dropping the trailing flags; converting SVG arcs and losing the flag fields; locale/format tools that strip trailing tokens.

Related errors

• Invalid bool rule
• Invalid fill rule.
• Invalid fill rule
• Invalid double value
• Unexpected path command '{c}'.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/27ec028139fa27c1. Report an issue: GitHub.