dotnetcore/CAP · error · ArgumentNullException

Value cannot be null. (Parameter 'topics')

Error message

Value cannot be null. (Parameter 'topics')

What it means

SubscribeAsync requires a non-null collection of topic names to create SNS topics/subscribe SQS queues. It throws ArgumentNullException when topics is null, distinguishing 'null' (bug) from an empty list (no subscriptions).

Solutions

1. Pass a non-null collection; use Array.Empty<string>() or new List<string>() when there is nothing to subscribe
2. Verify consumers/subscribers are registered via services.AddCap(x => x.AddSubscribe(...)) before bootstrapping
3. Guard the call site: if (topics != null) await SubscribeAsync(topics)
4. Catch ArgumentNullException at startup and log which consumer client failed

Example fix

// before
await sqsConsumer.SubscribeAsync(GetTopics()); // GetTopics() returns null
// after
var topics = GetTopics() ?? Enumerable.Empty<string>();
await sqsConsumer.SubscribeAsync(topics);

if (topics is null) throw new InvalidOperationException("topics must not be null; pass an empty collection for none");

bool IsValidTopics(IEnumerable<string>? t) => t is not null;

try { await client.SubscribeAsync(topics); } catch (ArgumentNullException ex) when (ex.ParamName == "topics") { logger.LogError(ex, "SubscribeAsync received null topics"); throw; }

When it happens

Trigger: Calling AmazonSQSConsumerClient.SubscribeAsync(null) directly, or CAP's bootstrapper invoking SubscribeAsync with a null subscriber/topic collection when no consumers were registered but the collection itself was never initialized.

Common situations: Custom IConsumerClient implementations or forks of CAP where the topic collection is built from a dictionary lookup that returned null; also seen when upgrading CAP versions and the startup path changed from empty-list to null semantics.

Related errors

• Value cannot be null. (Parameter 'topicNames')
• Value cannot be null. (Parameter 'configure')
• Value cannot be null. (Parameter 'topics')
• Value cannot be null. (Parameter 'options')
• Value cannot be null. (Parameter 'connectionString')

AI-assisted analysis of dotnetcore/CAP@e52b8508e5 (2026-09-14). Data as JSON: /api/errors/70d07945dc33623e. Report an issue: GitHub.

Appendix: source

Thrown at src/DotNetCore.CAP.AmazonSQS/AmazonSQSConsumerClient.cs:71

        var topicArns = new List<string>();
        foreach (var topic in topicNames)
        {
            var createTopicRequest = new CreateTopicRequest(topic.NormalizeForAws());

            var createTopicResponse = await _snsClient!.CreateTopicAsync(createTopicRequest).ConfigureAwait(false);

            topicArns.Add(createTopicResponse.TopicArn);
        }

        await GenerateSqsAccessPolicyAsync(topicArns).ConfigureAwait(false);

        return topicArns;
    }

    public async Task SubscribeAsync(IEnumerable<string> topics)
    {
        if (topics == null) throw new ArgumentNullException(nameof(topics));

        await ConnectAsync().ConfigureAwait(false);

        await SubscribeToTopics(topics).ConfigureAwait(false);
    }

    public async Task ListeningAsync(TimeSpan timeout, CancellationToken cancellationToken)
    {
        await ConnectAsync().ConfigureAwait(false);

        var request = new ReceiveMessageRequest(_queueUrl)
        {
            WaitTimeSeconds = 5,
            MaxNumberOfMessages = 1
        };

        while (true)
        {

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