dotnet/wpf · error · ArgumentNullException

reader or compressedData was null in compress

Error message

reader or compressedData was null in compress

What it means

This overload of GorillaCodec.Compress (reading from a BitStreamReader) validates that both the reader and the compressedData output list are non-null, throwing ArgumentNullException with this message. As with the array-based overload, null buffers indicate a caller bug rather than corrupt data.

Solutions

1. Construct the BitStreamReader from a valid, non-null source stream before calling Compress.
2. Initialize compressedData = new List<byte>() before the call.
3. Add explicit null checks for both arguments in the calling method.
4. Use the high-level StrokeCollection serialization APIs so buffers are managed for you.

Example fix

// before
GorillaCodec codec = new GorillaCodec();
codec.Compress(8, reader, GorillaEncodingType.OneByte, count, null);
// after
List<byte> compressedData = new List<byte>();
codec.Compress(8, reader, GorillaEncodingType.OneByte, count, compressedData);

if (reader == null) throw new ArgumentNullException(nameof(reader));
if (compressedData == null) throw new ArgumentNullException(nameof(compressedData));

bool CanCompress(BitStreamReader r, List<byte> outBuf) => r != null && outBuf != null;

try { codec.Compress(bitCount, reader, encodingType, units, outBuf); }
catch (ArgumentNullException ex) { log.Error("Missing BitStreamReader/output buffer", ex); throw; }

When it happens

Trigger: Calling Compress(bitCount, reader, encodingType, unitsToEncode, compressedData) with reader == null (BitStreamReader never constructed or failed construction) or compressedData == null.

Common situations: Custom encoding pipelines where the BitStreamReader creation was skipped or a stream was null before wrapping; refactors that moved buffer initialization away from the call site.

Related errors

• input or compressed data was null in Compress
• bogus GorillaEncodingType passed to compress
• Transform returned unexpected results
• bogus GorillaEncodingType passed to GetDataFromReader
• bogus GorillaEncodingType passed to Uncompress

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/c9ef046b0043e3b7. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/Ink/InkSerializedFormat/GorillaCodec.cs:410

                {
                    writer.Write((uint)input[i], bitCount);
                }
            }
        }

        /// <summary>
        /// Compress - compresses the byte[] being read by the BitStreamReader into compressed data
        /// </summary>
        /// <param name="bitCount">the number of bits to use for each element</param>
        /// <param name="reader">a reader over the byte[] to compress</param>
        /// <param name="encodingType">int, short or byte?</param>
        /// <param name="unitsToEncode">number of logical units to encoded</param>
        /// <param name="compressedData">output write buffer</param>
        internal void Compress(int bitCount, BitStreamReader reader, GorillaEncodingType encodingType, int unitsToEncode, List<byte> compressedData)
        {
            if (null == reader || null == compressedData)
            {
                throw new ArgumentNullException(StrokeCollectionSerializer.ISFDebugMessage("reader or compressedData was null in compress"));
            }
            ArgumentOutOfRangeException.ThrowIfNegative(bitCount);
            ArgumentOutOfRangeException.ThrowIfNegative(unitsToEncode);

            if (bitCount == 0)
            {
                //adjust if the bitcount is 0
                //(this makes bitCount 32)
                switch (encodingType)
                {
                    case GorillaEncodingType.Int:
                        {
                            bitCount = Native.BitsPerInt;
                            break;
                        }
                    case GorillaEncodingType.Short:
                        {
                            bitCount = Native.BitsPerShort;

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