Eclipse Paho

Eclipse Paho is a MQTT implementation.
Paho is available on various platforms and programming languages:

Java
C#
Go
C
Python
JavaScript
Rust

Example

A simple example of using Paho could be:

client = new MqttClient;
client.connect;
MqttMessage message = new MqttMessage;
message.setPayload);
client.publish;
client.disconnect;