Submit an HCS Message

Submitting a message appends bytes to a topic. Once the network reaches consensus on the message, it receives a sequence number and a consensus timestamp that are immutable and globally ordered.

Simple message

var message = Encoding.UTF8.GetBytes("Hello from Hiero SDK!");
SubmitMessageReceipt receipt = await client.SubmitMessageAsync(topic, message);
Console.WriteLine($"Sequence: {receipt.SequenceNumber}");

Submit-key-gated topics

If the topic was created with a Submitter endorsement, the submit key must sign every message. Use the params overload to attach it:

var receipt = await client.SubmitMessageAsync(new SubmitMessageParams
{
    Topic = topic,
    Message = Encoding.UTF8.GetBytes("Authorized event"),
    Signatory = new Signatory(submitKey)
});

Large messages (segmentation)

Individual messages are limited to about 4 KB. For larger payloads, the SDK can automatically split the data into numbered segments that the mirror node reassembles:

SubmitMessageReceipt[] receipts = await client.SubmitLargeMessageAsync(
    topic,
    largePayload,
    segmentSize: 4000,
    signatory: new Signatory(submitKey));
Console.WriteLine($"Sent {receipts.Length} segments");

Alternatively, manage segments manually with SubmitMessageParams.SegmentIndex and TotalSegmentCount.

Key points

  • The message payload is opaque bytes — the network does not interpret them.
  • The receipt's SequenceNumber increments monotonically per topic. It is the primary ordering key.
  • Subscription consumers on the mirror node receive messages in consensus-timestamp order.

See also