# CloudAMQP with MQTT and .NET

We recommend [MQTTnet](https://github.com/dotnet/MQTTnet), the .NET Foundation’s MQTT client. The examples are .NET 10 [file-based apps](https://learn.microsoft.com/dotnet/core/sdk/file-based-apps): `dotnet run` installs the MQTTnet package their `#:package` line names. In a project, add it with:

```bash
dotnet add package MQTTnet
```

## Publish and subscribe

Set `MQTT_HOST`, `MQTT_PORT`, `MQTT_USERNAME` and `MQTT_PASSWORD` to the MQTT details on your instance’s details page in the CloudAMQP console. Port 8883 uses TLS. Without them, the examples connect to a local broker.

Save the publisher as `publisher.cs`:

```csharp
#:package MQTTnet@5.2.0.1603
using MQTTnet;
using MQTTnet.Formatter;
using MQTTnet.Protocol;

var port = int.Parse(Environment.GetEnvironmentVariable("MQTT_PORT") ?? "1883");
var options = new MqttClientOptionsBuilder()
    .WithTcpServer(Environment.GetEnvironmentVariable("MQTT_HOST") ?? "localhost", port)
    .WithCredentials(Environment.GetEnvironmentVariable("MQTT_USERNAME") ?? "guest", Environment.GetEnvironmentVariable("MQTT_PASSWORD") ?? "guest")
    .WithTlsOptions(tls => tls.UseTls(port == 8883))
    // MQTTnet defaults to MQTT 5, LavinMQ supports MQTT 3.1.1
    .WithProtocolVersion(MqttProtocolVersion.V311)
    .Build();
using var client = new MqttClientFactory().CreateMqttClient();
await client.ConnectAsync(options);

// QoS 1 completes once the broker has sent its PUBACK
await client.PublishStringAsync("hello", "Hello world!", MqttQualityOfServiceLevel.AtLeastOnce);
Console.WriteLine(" [x] Sent 'Hello world!'");
await client.DisconnectAsync();
```

Save the subscriber as `worker.cs`:

```csharp
#:package MQTTnet@5.2.0.1603
using MQTTnet;
using MQTTnet.Formatter;
using MQTTnet.Protocol;

var port = int.Parse(Environment.GetEnvironmentVariable("MQTT_PORT") ?? "1883");
var options = new MqttClientOptionsBuilder()
    .WithTcpServer(Environment.GetEnvironmentVariable("MQTT_HOST") ?? "localhost", port)
    .WithCredentials(Environment.GetEnvironmentVariable("MQTT_USERNAME") ?? "guest", Environment.GetEnvironmentVariable("MQTT_PASSWORD") ?? "guest")
    .WithTlsOptions(tls => tls.UseTls(port == 8883))
    // MQTTnet defaults to MQTT 5, LavinMQ supports MQTT 3.1.1
    .WithProtocolVersion(MqttProtocolVersion.V311)
    .Build();
using var client = new MqttClientFactory().CreateMqttClient();
await client.ConnectAsync(options);

// Each message is acked when the handler's task completes
client.ApplicationMessageReceivedAsync += e =>
{
    Console.WriteLine($" [x] Received '{e.ApplicationMessage.ConvertPayloadToString()}'");
    return Task.CompletedTask;
};
await client.SubscribeAsync("hello", MqttQualityOfServiceLevel.AtLeastOnce);
Console.WriteLine(" [*] Waiting for messages. To exit press CTRL+C");
await Task.Delay(Timeout.Infinite);
```

Run `dotnet run worker.cs` in one terminal and `dotnet run publisher.cs` in another.

A runnable example is in [dotnet/mqttnet in cloudamqp/examples](https://github.com/cloudamqp/examples/tree/main/dotnet/mqttnet).

## Good to know

- **Put the vhost in the username**, as `vhost:user`. MQTT has no vhost of its own, and the console shows the username that way.
- **Connect with MQTT 3.1.1.** MQTTnet 5 defaults to MQTT 5, which LavinMQ doesn’t support, so set `WithProtocolVersion(MqttProtocolVersion.V311)`.
- **`UseTls()` verifies the server certificate.**
- **Use QoS 1**, so `PublishStringAsync` completes once the broker has sent its PUBACK, and each message is acked when the handler’s task completes.
- **MQTTnet 5 doesn’t reconnect.** Handle `DisconnectedAsync`, connect again and subscribe again.