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How to set a Last Will & Testament

The Last Will and Testament support of MQTT can be used to notify subscribers that your client is offline.

For a more in-depth explanation, see What is MQTT Last Will and Testament (LWT)? – MQTT Essentials: Part 9.

Using LastWillAndTestament​

This example instantiates the LastWillAndTestament in the HiveMQClientOption class. This is then sent to the broker in the connect operation.

// Specify the Last Will and Testament specifics in HiveMQClientOptions
var options = new HiveMQClientOptions
{
    LastWillAndTestament = new LastWillAndTestament("custom/lastwill/topic", QualityOfService.AtLeastOnceDelivery, "last will message"),
};

// Optionally set extended properties on the Last Will and Testament message
options.LastWillAndTestament.WillDelayInterval = 1;
options.LastWillAndTestament.PayloadFormatIndicator = 1;
options.LastWillAndTestament.MessageExpiryInterval = 100;
options.LastWillAndTestament.ContentType = "application/text";
options.LastWillAndTestament.ResponseTopic = "response/topic";
options.LastWillAndTestament.CorrelationData = new byte[] { 1, 2, 3, 4, 5 };
options.LastWillAndTestament.UserProperties.Add("userPropertyKey", "userPropertyValue");

// ConnectAsync will transmit the Last Will and Testament configuration.
var client = new HiveMQClient(options);
connectResult = await client.ConnectAsync().ConfigureAwait(false);

// The Last Will and Testament message will be sent to the "custom/lastwill/topic" topic if your clients get
// unexpectedly disconnected or alternatively, if your client disconnects with `DisconnectWithWillMessage`
var disconnectOptions = new DisconnectOptions { ReasonCode = DisconnectReasonCode.DisconnectWithWillMessage };
var disconnectResult = await client.DisconnectAsync(disconnectOptions).ConfigureAwait(false);

Because the client above disconnected with DisconnectReasonCode.DisconnectWithWillMessage, subscribers to the last/will topic will receive the Last Will and Testament message as specified above.

The LastWillAndTestament Builder Class​

As an ease-of-use alternative, the HiveMQtt client offers a LastWillAndTestamentBuilder class to more easily instantiate a LastWillAndTestament class.

var lwt = new LastWillAndTestamentBuilder()
            .WithTopic("last/will")
            .WithPayload("last will message")
            .WithQualityOfServiceLevel(QualityOfService.AtLeastOnceDelivery)
            .WithContentType("application/text")
            .WithResponseTopic("response/topic")
            .WithCorrelationData(new byte[] { 1, 2, 3, 4, 5 })
            .WithPayloadFormatIndicator(MQTT5PayloadFormatIndicator.UTF8Encoded)
            .WithMessageExpiryInterval(100)
            .WithUserProperty("userPropertyKey", "userPropertyValue")
            .WithWillDelayInterval(1)
            .Build();

// Setup & Connect the client with LWT
var options = new HiveMQClientOptions
{
    LastWillAndTestament = lwt,
};

var client = new HiveMQClient(options);
connectResult = await client.ConnectAsync().ConfigureAwait(false);

See Also​