SixLabors/ImageSharp · error · InvalidImageContentException

The ANI file contains a truncated RIFF list.

Error message

The ANI file contains a truncated RIFF list.

What it means

Thrown by AniDecoderCore.ReadList when the remaining bytes between the current stream position and the declared list end are fewer than 4 bytes, so the RIFF list type field cannot be read. The library treats this as corrupt content rather than guessing. It indicates the ANI file declares a RIFF list whose payload is shorter than its declared structure.

Solutions

1. Re-obtain the ANI file from a trusted source and verify it is complete (compare file size/checksum).
2. Validate the RIFF chunk sizes with a hex editor or riff inspection tool before decoding.
3. Catch InvalidImageContentException and reject the file as corrupt instead of attempting decode.

Example fix

// before
var image = Image.Load("broken.ani");
// after
try { var image = Image.Load("broken.ani"); }
catch (InvalidImageContentException ex) { Console.WriteLine($"Corrupt ANI: {ex.Message}"); }

static bool IsPlausibleAni(byte[] bytes) => bytes.Length >= 12 && bytes.AsSpan(0, 4).SequenceEqual("RIFF"u8) && bytes.AsSpan(8, 4).SequenceEqual("ACON"u8) && bytes.Length >= BitConverter.ToUInt32(bytes, 4) + 8;

try { var image = Image.Load(path); } catch (InvalidImageContentException ex) { log.Warn($"Corrupt ANI rejected: {ex.Message}"); }

When it happens

Trigger: Decoding an ANI file via Image.LoadAsync/Image.IdentifyAsync where a RIFF list (RIFF/ 'acon' or 'fram') payload ends fewer than 4 bytes after the list header, i.e. a truncated list type field.

Common situations: Partially downloaded ANI files, files truncated by transfer tools, hand-crafted or fuzzed ANI files with incorrect list sizes in the RIFF chunk headers.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

• The stream does not contain an ANI RIFF container.
• The ANI RIFF container size is invalid.
• The ANI file does not contain an animation header.
• The ANI animation header is truncated.
• The ANI animation header declares an invalid size.

AI-assisted analysis of SixLabors/ImageSharp@59ce6af6fc (2026-09-13). Data as JSON: /api/errors/4d0854ef4c527d90. Report an issue: GitHub.

Appendix: source

Thrown at src/ImageSharp/Formats/Ani/AniDecoderCore.cs:335

        this.aniMetadata.Width = this.header.Width;
        this.aniMetadata.Height = this.header.Height;
        this.aniMetadata.BitCount = this.header.BitCount;
        this.aniMetadata.Planes = this.header.Planes;
        this.aniMetadata.DisplayRate = this.header.DisplayRate;
        this.aniMetadata.Flags = this.header.Flags;
    }

    /// <summary>
    /// Reads a RIFF list type and records or parses its contents.
    /// </summary>
    /// <param name="stream">The ANI stream.</param>
    /// <param name="listEnd">The exclusive end of the list payload.</param>
    private void ReadList(BufferedReadStream stream, long listEnd)
    {
        if (listEnd - stream.Position < sizeof(uint))
        {
            throw new InvalidImageContentException("The ANI file contains a truncated RIFF list.");
        }

        Span<byte> typeData = this.buffer[..sizeof(uint)];
        ReadExactly(stream, typeData, "RIFF list type");
        AniListType type = (AniListType)BinaryPrimitives.ReadUInt32LittleEndian(typeData);

        switch (type)
        {
            case AniListType.Frames:
                // Defer nested decoding until the complete container has supplied any later seq/rate chunks.
                this.frameLists.Add((stream.Position, listEnd));
                break;
            case AniListType.Info when !this.Options.SkipMetadata:
                this.ExecuteAncillarySegmentAction(() => this.ReadInfoList(stream, listEnd));
                break;
        }
    }

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