dotnet/wpf · error · ArgumentException

Packed button length not equal to expected length

Error message

Packed button length not equal to expected length

What it means

Thrown by StrokeSerializer.SavePackets (invoked from cbWrite) when packing stroke button data for ISF output: the BitWriter produced a byte array whose length differs from the computed expected size (buttonCount*pointCount+7)/8. This is an internal invariant check during serialization.

Solutions

1. Verify any custom Stroke/StrokeData construction sets button counts and point counts consistently
2. Use standard StylusPointCollection-based strokes rather than hand-built packet data
3. If extending ink serialization, ensure buttonCount matches the actual button bits written
4. Report as a bug if hit with stock StrokeCollection data

Example fix

// before
var stroke = new CustomStroke(rawPacketData); // buttonCount mismatch
// after
var stroke = new Stroke(new StylusPointCollection(points));

// Ensure stroke was built from a StylusPointCollection, not raw packet data
bool WellFormedStroke(Stroke s) => s != null && s.StylusPoints != null && s.StylusPoints.Count > 0;

bool WellFormedStroke(Stroke s) => s?.StylusPoints?.Count > 0;

try { stream.Save(ms); }
catch (ArgumentException ex) { throw new InvalidOperationException("Stroke serialization invariant broken", ex); }

When it happens

Trigger: Saving a StrokeCollection to ISF where a Stroke's button data has an internal length inconsistent with its packet description — usually only reachable with programmatically constructed/extended Stroke data.

Common situations: Custom stroke subclasses overriding drawing attributes or packet data incorrectly, ink data built by interop code with mismatched button counts.

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

• Invalid EP in ISF
• Property data must be a non-reference variant compatible…
• Read different size from stream then expected
• ROP data size was not found
• ROP data was incorrectly serialized

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/afa1a5ea33967533. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/Ink/InkSerializedFormat/StrokeSerializer.cs:905

                        //
                        if (bitsToWrite > 0)
                        {
                            bitWriter.Write(byteOfButtonData, bitsToWrite);
                        }
                        if (fullBytesForButtonsPerPacket > 0)
                        {
                            bitsToWrite = Native.BitsPerByte;
                        }
                    }
                }

                // retrieve the button bytes
                byte[] packedButtonData = bitWriter.ToBytes();

                if (packedButtonData.Length !=
                    ((buttonCount * pointCount + 7) / Native.BitsPerByte))
                {
                    throw new ArgumentException(StrokeCollectionSerializer.ISFDebugMessage("Packed button length not equal to expected length"));
                }

                // write out the packed button data to the output stream
                stream.Write(packedButtonData, 0, packedButtonData.Length);
                localBytesWritten += (uint)packedButtonData.Length;
            }
            */

            return localBytesWritten;
        }
#if OLD_ISF
        /// <summary>
        /// Saves the packets data corresponding to a packet property (identified by the guid) into the stream
        /// based on the Compression algorithm and compress header
        /// </summary>
        /// <param name="packetdata">packet data to save</param>
        /// <param name="stream">null to calculate only the size</param>
        /// <param name="compressor"></param>

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