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ftVarBytes

ftVarBytes = 13

Example​

function IsVariableBinary(Field: TField): Boolean;
begin
  Result := Field.DataType = ftVarBytes;
end;

Usage​

ftVarBytes identifies a variable-length inline byte sequence with a declared maximum Size and a length-prefixed Velox field buffer.

Additional Technical Info​

ftVarBytes is declaration ordinal 13 of TFieldType. It identifies variable-length binary data stored in the normal record buffer rather than as a streamed large object.

The installed RTL maps it to TVarBytesField. A new field defaults to Size = 16; live provider metadata supplies the actual maximum. Its internal DataSize is Size + SizeOf(Word) because Delphi's field buffer includes a two-byte length prefix. That prefix is an RTL buffer detail, not automatically part of the logical byte value or an external wire format.

Use byte-array access and preserve the actual logical length. Dataset compatibility checks require matching Size. Both current setters have a special case when an ordinary byte array or Variant array has exactly DataSize (Size + 2) elements: they assume the array already contains a native length prefix. That is not a valid maximum-length logical value; a logical value is at most Size bytes.

The special branch then constructs only SizeOf(Pointer) bytes before calling SetData in native format, rather than passing the complete prefixed buffer. This is a current installed-RTL quirk and can supply incomplete native data to the provider. Do not manufacture or pass a Size + 2 array to trigger it. Pass an unprefixed logical byte array no longer than Size so the normal non-native path is used, and let the field/provider create its internal prefix.

The enum conveys no encoding or packed structure. Null, zero-length and all-zero values are distinct. Choose ftBytes for an exact fixed-length sequence and ftBlob when size or streaming requirements exceed ordinary record-buffer use.

The example only inspects metadata and was source-reviewed; no byte-array conversion, dataset, database, runtime or image test ran.

External references

Created 2026-07-15