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ZLibDecompressString

Function ZLibDecompressString( const Data : TBytes) : string

Example​

procedure ScriptEvent(var Value: variant);
var
  Compressed: TBytes;
begin
  Compressed := ZLibCompressString('Fictional order 1001');
  Value := ZLibDecompressString(Compressed);
end;

Usage​

ZLibDecompressString expands zlib-wrapped bytes and decodes the result as a Velox UTF-16LE Unicode string.

Parameters

NameTypeDescription
DataTBytes, constComplete zlib stream whose expanded payload is expected to be UTF-16LE text.

Behaviour

Data produced by ZLibCompressString is encoded as UTF-16LE without a BOM and is intended to round-trip. Data produced by another system must use exactly that encoding after zlib expansion. A successful zlib checksum does not prove that the payload is valid or intended text.

Errors

All ZDecompress errors, allocation errors and encoding failures propagate. No partial string is returned.

Usage notes

Prefer ZLibDecompress when the protocol specifies an encoding such as UTF-8. Decode the returned bytes using that encoding instead of assuming the UTF-16LE format used by this function.

Additional Technical Info​

ZLibDecompressString expands a zlib-wrapped byte array and decodes the result through Delphi TEncoding.Unicode. It is the intended inverse of ZLibCompressString, not a general-purpose compressed-text detector.

The example round-trips fictional text in memory. It is source-reviewed and is not executed by the documentation workflow.

Implementation

The scripting import calls vxCompression.ZLibDecompressString, which delegates directly to installed System.ZLib.ZDecompressStr. The RTL copies the argument, calls ZDecompress, then passes the expanded bytes to TEncoding.Unicode.GetString.

Edge cases and quirks

  • Empty compressed input raises EZDecompressionError; a valid zlib stream representing an empty string returns '' subject to the underlying empty-array decoding implementation.
  • Gzip, ZIP and raw deflate data are wrapper mismatches.
  • Expanded output is not capped. During decoding, the compressed bytes, expanded bytes and managed Unicode string can coexist.
  • The decoder does not negotiate UTF-8, inspect a content type or normalise Unicode. Use the byte function for any other encoding.
  • Malformed or odd-length UTF-16 payloads are outside the matching-pair contract; do not use decoder output as a validity check for arbitrary binary data.

Side effects

None outside memory allocation.

Performance and concurrency

The operation is synchronous and can allocate several times the compressed size. It uses local state, but an attacker-controlled compression ratio can affect the whole process.

Related entries

External references

Created 2026-07-15