Skip to main content

GenerateHeader

procedure GenerateHeader;

Example​

procedure PrepareMessageHeaders(const Message: TIdMessage;
  var Value: Variant);
begin
  Message.GenerateHeader;
  Value := Message.ContentType;
end;

Usage​

GenerateHeader updates message counts, resolves the message layout, creates MIME boundary values and prepares the output headers from typed properties and raw header lines.

Additional Technical Info​

GenerateHeader prepares the message's internal registered-but-ungenerated LastGeneratedHeaders list for serialization. It is a mutating normalization pass, not a read-only preview.

The method first recounts MessageParts. If Encoding is meDefault, it permanently sets mePlainText when no parts exist or meMIME when at least one exists; the Encoding setter also selects UUE or MIME AttachmentEncoding. Later part changes do not restore meDefault automatically.

It then normalizes each nonempty part ContentTransfer token. MIME accepts 7bit, 8bit, binary, base64, quoted-printable and binhex40; unknown values become base64. Plain-text layout accepts only UUE/XXE; unknown values become UUE. With any parts present, a message-level ContentTransferEncoding outside 7bit/8bit/binary is cleared.

For MIME layout, the method clears the boundary stack, generates a new random boundary and pushes it as parent -1. Repeated calls therefore replace the prior boundary. When ContentType is empty it stores an inferred value:

  • any attachment produces multipart/mixed;
  • more than one text part and no attachment produces multipart/alternative;
  • otherwise it uses text/plain.

The selected encoder then initializes headers. Header-character encoding/charset comes from Indy's platform default and an internal event hook. The method copies raw Headers into LastGeneratedHeaders and sets/replaces the first standard fields from typed state: From, Subject, To, Cc, Newsgroups, MIME/content headers, Sender, Reply-To, Organization, receipt headers, References, Date, priority headers, Message-ID and In-Reply-To.

Bcc is removed during normal generation and included only while SaveToFile has set an internal flag; direct SaveToStream follows the omission path. Date is current local time when UseNowForDate is true, otherwise the stored Date, then converted to an RFC/GMT string. Normal priority removes all three priority headers. Message-ID is not auto-generated.

A one-part MIME message with empty preamble is marked single-part: message-level content headers are removed so the part encoder can append its headers later. Otherwise text content without a charset uses us-ascii, and multipart output receives the generated boundary parameter.

Finally, ExtraHeaders are appended and may introduce duplicates. They do not reliably override the first standard value. The method does not serialize bytes itself and does not make the object immutable.

Every stage can change live message/part/header state before a later failure. Registry lookup, address/header encoding, date conversion, boundary generation and allocation can raise; there is no rollback. Cost is linear in parts plus header/address content, with extra encoding cost, and the object is not thread-safe. The source-reviewed example was not runtime-tested.

External references

Created 2026-07-15