What causes "parent" in microsoft/aspire?

Pass the AzureServiceBusTopicResource you created with AddAzureServiceBusTopic directly into the constructor or subscription extension call.

microsoft/aspire · error · ArgumentNullException

parent

Error message

parent

What it means

The AzureServiceBusSubscriptionResource constructor throws ArgumentNullException when the parent topic resource is null. The library requires every subscription resource to be attached to a valid AzureServiceBusTopicResource so its connection string can be derived via the parent hierarchy.

Solutions

Pass the AzureServiceBusTopicResource you created with AddAzureServiceBusTopic directly into the constructor or subscription extension call.
Check that the topic-creating call actually returned a non-null resource before constructing the subscription.
Prefer the AddAzureServiceBusSubscription extension over manual construction.

Example fix

// before
var sub = new AzureServiceBusSubscriptionResource("orders-sub", "orders", GetTopicMaybeNull());

// after
var topic = builder.AddAzureServiceBusTopic("orders", "orders-topic");
var sub = new AzureServiceBusSubscriptionResource("orders-sub", "orders", topic.Resource);

Defensive patterns

Strategy: validation

Validate before calling

if (topicResource is null) throw new InvalidOperationException("Topic resource must be created before the subscription.");

Type guard

if (topicResource is not null) { var sub = new AzureServiceBusSubscriptionResource(name, subName, topicResource); }

Prevention

Always create topics via AddAzureServiceBusTopic before adding subscriptions.
Never pass lookup results that can be null directly into resource constructors.
Prefer extension methods over direct resource construction.

When it happens

Trigger: Calling the AzureServiceBusSubscriptionResource constructor directly (or a low-level extension/API that forwards to it) with a null AzureServiceBusTopicResource parent argument.

Common situations: Hand-constructing subscription resources in custom extension methods or tests; passing the result of a lookup that returned null (e.g. resource not found in the model) into the constructor.

Related errors

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/4bbb42445f1656f7. Report an issue: GitHub.

Appendix: source

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

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

    /// <summary>
    /// The subscription name.
    /// </summary>
    public string SubscriptionName
    {
        get => _subscriptionName;
        set => _subscriptionName = ThrowIfNullOrEmpty(value, nameof(subscriptionName));
    }

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

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

    /// <summary>
    /// A value that indicates whether this queue has dead letter support when
    /// a message expires.
    /// </summary>
    public bool? DeadLetteringOnMessageExpiration { get; set; }

    /// <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>

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