Topic exchanges enable flexible messsage routing using wildcard patterns. The binding keys support two special chaarcters: * matches exactly one word, while # matches zero or more words.

Publishing Messages with Topic Exchange

public void PublishTopicMessages()
{
    var connectionHelper = new MQHelper();
    using (var connection = connectionHelper.GetConnection())
    using (var channel = connection.CreateModel())
    {
        // Declare queues
        channel.QueueDeclare("APACRegion", durable: true, exclusive: false, autoDelete: false, arguments: null);
        channel.QueueDeclare("HeadlinesQueue", durable: true, exclusive: false, autoDelete: false, arguments: null);

        // Declare topic exchange
        channel.ExchangeDeclare("RegionalExchange", type: ExchangeType.Topic, durable: true, autoDelete: false, arguments: null);

        // Bind queues with wildcard patterns
        channel.QueueBind("APACRegion", "RegionalExchange", "APAC.*", arguments: null);
        channel.QueueBind("HeadlinesQueue", "RegionalExchange", "*.Headlines", arguments: null);

        // Message 1: matches APACRegion (APAC.*) and HeadlinesQueue (*.Headlines)
        var payload1 = Encoding.UTF8.GetBytes("Breaking news from Asia Pacific");
        channel.BasicPublish("RegionalExchange", routingKey: "APAC.Headlines", basicProperties: null, body: payload1);
        Console.WriteLine($"[APAC.Headlines] dispatched");

        // Message 2: matches APACRegion only
        var payload2 = Encoding.UTF8.GetBytes("Weather update for Asia Pacific");
        channel.BasicPublish("RegionalExchange", routingKey: "APAC.Weather", basicProperties: null, body: payload2);
        Console.WriteLine($"[APAC.Weather] dispatched");

        // Message 3: matches HeadlinesQueue only
        var payload3 = Encoding.UTF8.GetBytes("Breaking news from Europe");
        channel.BasicPublish("RegionalExchange", routingKey: "EU.Headlines", basicProperties: null, body: payload3);
        Console.WriteLine($"[EU.Headlines] dispatched");
    }
}

In this example, the routing key APAC.Headlines matches both queues, so it gets delivered to both subscribers.

Conusming Messages from Topic Exchange

public void ConsumeTopicMessages()
{
    var config = new ConnectionFactory
    {
        HostName = "localhost",
        UserName = "guest",
        Password = "guest"
    };

    using (var connection = config.CreateConnection())
    using (var channel = connection.CreateModel())
    {
        // Queue declaration
        channel.QueueDeclare("HeadlinesQueue", durable: true, exclusive: false, autoDelete: false, arguments: null);

        // Exchange declaration
        channel.ExchangeDeclare("RegionalExchange", type: ExchangeType.Topic, durable: true, autoDelete: false, arguments: null);

        // Binding with wildcard pattern
        channel.QueueBind("HeadlinesQueue", "RegionalExchange", "*.Headlines", arguments: null);

        // Set up consumer
        var messageHandler = new EventingBasicConsumer(channel);
        messageHandler.Received += (sender, eventArgs) =>
        {
            byte[] messageBody = eventArgs.Body.ToArray();
            string content = Encoding.UTF8.GetString(messageBody);
            Console.WriteLine($"Received: [{content}]");
        };

        // Start consuming
        channel.BasicConsume("HeadlinesQueue", autoAck: true, consumer: messageHandler);
    }
}