dotnetcore/CAP · error · ArgumentNullException

Value cannot be null. (Parameter 'topicNames')

Error message

Value cannot be null. (Parameter 'topicNames')

What it means

ArgumentNullException on 'topicNames' raised inside FetchTopicsAsync's topic-creation path (the guard shown in the constructor context is the pattern reused for topic collections): the sequence of topic names passed to the Kafka admin/client call was null, so metadata for topics could not be fetched. The faulting input is the topics collection supplied to the subscribe/fetch flow.

Solutions

1. Ensure the topic list passed to SubscribeAsync/FetchTopicsAsync is non-null (default to an empty list)
2. Register topics via CAP attributes/configuration so the list is always populated before subscription
3. Null-coalesce at the call site: topics ?? Array.Empty<string>()

When it happens

Trigger: Thrown at src/DotNetCore.CAP.Kafka/KafkaConsumerClient.cs:48 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of dotnetcore/CAP@e52b8508e5 (2026-09-14). Data as JSON: /api/errors/7b56269b123db7ab. Report an issue: GitHub.

Appendix: source

Thrown at src/DotNetCore.CAP.Kafka/KafkaConsumerClient.cs:48

    public KafkaConsumerClient(string groupId, byte groupConcurrent,
        IOptions<KafkaOptions> options, IServiceProvider serviceProvider)
    {
        _groupId = groupId;
        _groupConcurrent = groupConcurrent;
        _semaphore = new SemaphoreSlim(groupConcurrent);
        _serviceProvider = serviceProvider;
        _kafkaOptions = options.Value ?? throw new ArgumentNullException(nameof(options));
    }

    public Func<TransportMessage, object?, Task>? OnMessageCallback { get; set; }

    public Action<LogMessageEventArgs>? OnLogCallback { get; set; }

    public BrokerAddress BrokerAddress => new("kafka", _kafkaOptions.Servers);

    public async Task<ICollection<string>> FetchTopicsAsync(IEnumerable<string> topicNames)
    {
        if (topicNames == null) throw new ArgumentNullException(nameof(topicNames));

        var regexTopicNames = topicNames.Select(Helper.WildcardToRegex).ToList();

        var allowAutoCreate = true;
        if (_kafkaOptions.MainConfig.TryGetValue("allow.auto.create.topics", out var autoCreateValue)
            && bool.TryParse(autoCreateValue, out var parsedValue))
        {
            allowAutoCreate = parsedValue;
        }

        if (allowAutoCreate)
        {
            try
            {
                var config = new AdminClientConfig(_kafkaOptions.MainConfig)
                { BootstrapServers = _kafkaOptions.Servers };

                using var adminClient = new AdminClientBuilder(config).Build();

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