What causes "parent" in microsoft/aspire?

Pass the AzureServiceBusResource created by AddAzureServiceBus as the parent.

microsoft/aspire · error · ArgumentNullException

parent

Error message

parent

What it means

The AzureServiceBusTopicResource constructor throws ArgumentNullException when the parent Azure Service Bus namespace resource is null. A topic must belong to a valid AzureServiceBusResource so connection strings can be composed from the namespace.

Solutions

Pass the AzureServiceBusResource created by AddAzureServiceBus as the parent.
Verify the namespace resource variable is non-null before constructing the topic.
Use the AddAzureServiceBusTopic extension method instead of manual construction.

Example fix

// before
var topic = new AzureServiceBusTopicResource("orders", "orders-topic", GetBusMaybeNull());

// after
var bus = builder.AddAzureServiceBus("bus");
var topic = bus.AddTopic("orders", "orders-topic");

Defensive patterns

Strategy: validation

Validate before calling

if (busResource is null) throw new InvalidOperationException("Create the Azure Service Bus namespace before adding topics.");

Type guard

if (busResource is not null) { var topic = new AzureServiceBusTopicResource(name, topicName, busResource); }

Prevention

Build the namespace with AddAzureServiceBus first, then chain AddTopic.
Avoid constructing resource types manually in app code.
Null-check results of any resource lookup before use.

When it happens

Trigger: Constructing AzureServiceBusTopicResource directly with a null parent argument instead of using builder.AddAzureServiceBusTopic.

Common situations: Custom extension methods that resolve the parent namespace dynamically and pass a null resource; tests that construct topic resources without a backing AzureServiceBusResource.

Related errors

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/c159ea4acd75eb00. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting.Azure.ServiceBus/AzureServiceBusTopicResource.cs:42

[AspireExport(ExposeProperties = true)]
public class AzureServiceBusTopicResource(string name, string topicName, AzureServiceBusResource parent)
    : Resource(name), IResourceWithParent<AzureServiceBusResource>, IResourceWithConnectionString, IResourceWithAzureFunctionsConfig
{
    private string _topicName = ThrowIfNullOrEmpty(topicName);

    /// <summary>
    /// The topic name.
    /// </summary>
    public string TopicName
    {
        get => _topicName;
        set => _topicName = ThrowIfNullOrEmpty(value, nameof(topicName));
    }

    /// <summary>
    /// Gets the parent Azure Service Bus resource.
    /// </summary>
    public AzureServiceBusResource Parent { get; } = parent ?? throw new ArgumentNullException(nameof(parent));

    /// <summary>
    /// Gets the connection string expression for the Azure Service Bus Topic.
    /// </summary>
    public ReferenceExpression ConnectionStringExpression => Parent.GetConnectionString(TopicName, null);

    /// <summary>
    /// ISO 8601 default message timespan to live value. This is the duration
    /// after which the message expires, starting from when the message is
    /// sent to Service Bus. This is the default value used when TimeToLive is
    /// not set on a message itself.
    /// </summary>
    public TimeSpan? DefaultMessageTimeToLive { get; set; }

    /// <summary>
    /// ISO 8601 timeSpan structure that defines the duration of the duplicate
    /// detection history. The default value is 10 minutes.
    /// </summary>

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