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LoadFromFile

procedure LoadFromFile(const FileName: string);

Example​

procedure ScriptEvent(var Value: variant);
var
  Blob: TBlobField;
begin
  // DATA_Self and its Payload field must already exist.
  DATA_Self.Edit;
  Blob := TBlobField(DATA_Self.FieldByName('Payload'));
  Blob.LoadFromFile('C:\Fictional\Inbound\payload.bin');
  Value := not Blob.IsNull;
  // The surrounding flow decides when to Post or cancel this edit.
end;

Usage​

LoadFromFile replaces the current record's BLOB value with every byte from a read-only source file.

Parameters

NameTypeDescription
FileNamestring, constSource path visible to the executing Velox process. Relative paths use that process's current directory.

Errors

Missing/denied files, invalid paths, wrong field type, inactive or invalid records, read-only fields, non-editing datasets, allocation failures and file/BLOB read or write errors can raise. There is no rollback to the previous BLOB after the write stream is created; a failure can leave a truncated or partial record buffer. Dataset-specific finalisation can route an error through the host exception handler.

Usage notes

Validate the path and place the dataset in the correct editing state before calling. Use the surrounding flow's normal cancel/recovery path on failure and post/apply only after the load succeeds.

Additional Technical Info​

LoadFromFile reads the complete source file and replaces the BLOB buffer for the current dataset record. It changes dataset state only; it does not post the record, apply updates or commit a database transaction.

The example uses a fictional path and intentionally leaves persistence to the surrounding flow. It is source-reviewed and was not executed by the documentation workflow.

Implementation

Delphi TBlobField.LoadFromFile constructs a TFileStream with fmOpenRead or fmShareDenyWrite, calls LoadFromStream, then frees the file stream in a finally block. LoadFromStream creates a dataset BLOB stream in bmWrite mode and calls installed Delphi TStream.CopyFrom(Source, 0).

For Count zero, Delphi rewinds the file stream to offset zero, obtains its Size and copies that exact number of bytes. On the common client-dataset path, constructing the write BLOB stream requires edit, insert or new-value state and truncates the prior value before copying.

Behaviour

  • The source is opened before the dataset write stream is created. A not-editing failure can therefore occur after the file was successfully opened, although the file is still closed.
  • The open handle denies concurrent write access while the copy runs but permits other readers. A conflicting existing handle prevents the source from opening.
  • On a successful non-empty load, the current client-dataset record buffer contains the file bytes and the field is marked modified when the internal stream closes.
  • Loading a zero-byte file performs no stream write. The common client-dataset path can therefore represent it as null rather than a distinct non-null empty BLOB.

Side effects

Reads a host file, replaces the current record's BLOB buffer and marks the field/dataset modified according to the owning dataset. It never deletes, moves or changes the source file.

Performance and concurrency

The operation is synchronous and linear in file size. Delphi copies in bounded chunks, while the common client dataset also holds the complete modified BLOB in memory. Coordinate dataset cursor/edit access and external file writers.

Related entries

  • LoadFromStream performs the same BLOB replacement from a caller-owned stream.
  • SaveToFile creates or replaces a file from the current BLOB.

External references

Created 2026-07-15