producer.cs

public class KafkaProducerHostedService : IHostedService
{
    private readonly ILogger<KafkaProducerHostedService> _logger;
    private readonly IProducer<Null, string> _producer;

    public KafkaProducerHostedService(ILogger<KafkaProducerHostedService> logger)
    {
        _logger = logger;
        var config = new ProducerConfig()
        {
            BootstrapServers = "localhost:9092"
        };
        _producer = new ProducerBuilder<Null, string>(config).Build();
    }

    public async Task StartAsync(CancellationToken cancellationToken)
    {
        for (var i = 0; i < 100; ++i)
        {
            var value = $"Hello World {i}";
            _logger.LogInformation(value);
            await _producer.ProduceAsync("demo", new Message<Null, string>()
            {
                Value = value
            }, cancellationToken);
        }

        _producer.Flush(TimeSpan.FromSeconds(10));
    }

    public Task StopAsync(CancellationToken cancellationToken)
    {
        _producer?.Dispose();
        return Task.CompletedTask;
    }
}