dotnet/wpf · error · ArgumentException

Unknown path operation attempted.

Error message

Unknown path operation attempted.

What it means

ParserStreamGeometryContext.PackByte throws ArgumentException with 'Unknown path operation attempted.' when a path geometry operation opcode is >= 16 and cannot fit into the packed byte format (the low 4 bits hold the opcode, the high bits hold boolean flags). This is an internal limit of the BAML/path-packing format, so it indicates a corrupted or invalid internal state rather than a caller mistake.

Solutions

1. Verify the BAML/assembly was compiled with a compatible .NET/WPF version and rebuild resources
2. Regenerate the corrupted resource or XAML compilation output (clean + rebuild)
3. If hit in custom parsing code, ensure only defined ParserGeometryContextOpCodes values (< 16) are passed

Example fix

// before
context.PackByte((ParserGeometryContextOpCodes)unknownOp, ...);
// after
if (unknownOp >= ParserGeometryContextOpCodes.OpCodeLimit) throw new InvalidOperationException($"opcode {unknownOp} unsupported");
context.PackByte(knownOp, ...);

try { ParsePath(data); } catch (ArgumentException ex) when (ex.Message.Contains("Unknown path operation")) { /* regenerate */ }

When it happens

Trigger: Internal path-parsing/packing code invoking PackByte with an out-of-range ParserGeometryContextOpCodes value; feeding a corrupted BAML/stream geometry stream whose opcode byte decodes to an undefined operation.

Common situations: Deserializing BAML produced by an incompatible newer WPF version; corrupted XAML/BAML resources; memory corruption or a bug in custom derivation of the parsing pipeline.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• Can't Assign to Known Type attributes
• Could not find prefix for type
• Found unexpected Xmlns BAML record
• NotImplementedException
• SR.AssemblyIdNegative

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/c61baaea52611222. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/Media/ParserStreamGeometryContext.cs:715

        // PackByte
        //      Packs an op-code, and up to 4 booleans into a single byte.
        //
        // Binary format is :
        //      First 4 bits map directly to the op-code.
        //      Next 4 bits map to booleans 1 - 4.
        //
        //          Like this:
        //
        //              7| 6  | 5  | 4  | 3 | 2 | 1 | 0 |
        //           <B4>|<B3>|<B2>|<B1><-  Op Code    ->
        //
        private static byte PackByte(ParserGeometryContextOpCodes opCode, bool bool1, bool bool2, bool bool3, bool bool4)
        {
            byte packedByte = (byte) opCode;

            if (packedByte >= 16)
            {
                throw new ArgumentException(SR.UnknownPathOperationType);
            }

            if (bool1)
            {
                packedByte |= SetBool1;
            }

            if (bool2)
            {
                packedByte |= SetBool2;
            }

            if (bool3)
            {
                packedByte |= SetBool3;
            }

            if (bool4)

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