M2Mqtt library is a .Net Framework/C# based library that provides an MQTT client. You can use it with all .Net Framework version (full, compact and micro) on Windows platform and also on Linux thanks to Mono Project. It is open source and hosted on CodePlex and the assemblies are also deployed on Nuget.

From software architecture perspective, there is only a main class (MqttClient) that you have to use to connect to a broker, subscribe/unsubscribe to topics and publish messages. All other classes are used inside the client itself. For advanced feature, like encryption and authentication, you can configure the library to compile the support for SSL/TLS.