microsoft/aspire · error · DistributedApplicationException

was published for the ' ' resource but the connection…

Error message

{nameof(ConnectionStringAvailableEvent)} was published for the '{builder.Resource.Name}' resource but the connection string was null.

What it means

In RunAsEmulator's OnBeforeResourceStarted handler, the blob connection string is resolved asynchronously; if it resolves to null the code throws DistributedApplicationException referencing ConnectionStringAvailableEvent. This is an internal invariant: without a connection string the emulator's BlobServiceClient cannot be created.

Solutions

Check the emulator container started correctly and its endpoint was published
Re-run with updated Aspire packages; if it persists, file an issue (this indicates an internal invariant failure)
Verify RunAsEmulator configuration (port assignments, proxy settings) is unchanged

Defensive patterns

Strategy: try-catch

Validate before calling

var cs = await storage.Resource.GetBlobConnectionString().GetValueAsync(ct);
if (cs is null) { logger.LogError("Blob connection string missing for {Name}", storage.Resource.Name); return; }

Try / catch

try { await startWorkflow(ct); }
catch (DistributedApplicationException ex) { logger.LogError(ex, "Emulator connection string invariant failed"); throw; }

Prevention

Keep RunAsEmulator wiring on current Aspire versions
Do not remove lifecycle hooks that initialize emulator clients
Verify emulator endpoints publish before resource start

When it happens

Trigger: Resource start of an emulated Azure Storage resource where GetBlobConnectionString().GetValueAsync returns null (connection string expression produced no value).

Common situations: Emulator endpoint/proxy not correctly configured, misconfigured emulator settings, or internal wiring regression causing a missing connection string at startup.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/c5ae66056bbd287b. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting.Azure.Storage/AzureStorageExtensions.cs:218

               .WithHttpEndpoint(name: "table", targetPort: 10002)
               .WithAnnotation(new ContainerImageAnnotation
               {
                   Registry = StorageEmulatorContainerImageTags.Registry,
                   Image = StorageEmulatorContainerImageTags.Image,
                   Tag = StorageEmulatorContainerImageTags.Tag
               });

        BlobServiceClient? blobServiceClient = null;
        QueueServiceClient? queueServiceClient = null;

        builder
            .OnBeforeResourceStarted(async (storage, @event, ct) =>
            {
                // The BlobServiceClient and QueueServiceClient are created before the health check is run.
                // We can't use ConnectionStringAvailableEvent here because the resource doesn't have a connection string, so
                // we use BeforeResourceStartedEvent

                var blobConnectionString = await builder.Resource.GetBlobConnectionString().GetValueAsync(ct).ConfigureAwait(false) ?? throw new DistributedApplicationException($"{nameof(ConnectionStringAvailableEvent)} was published for the '{builder.Resource.Name}' resource but the connection string was null.");
                blobServiceClient = CreateBlobServiceClient(blobConnectionString);

                var queueConnectionString = await builder.Resource.GetQueueConnectionString().GetValueAsync(ct).ConfigureAwait(false) ?? throw new DistributedApplicationException($"{nameof(ConnectionStringAvailableEvent)} was published for the '{builder.Resource.Name}' resource but the connection string was null.");
                queueServiceClient = CreateQueueServiceClient(queueConnectionString);
            })
            .OnResourceReady(async (storage, @event, ct) =>
            {
                // The ResourceReadyEvent of a resource is triggered after its health check (AddAzureBlobStorage) is healthy.
                // This means we can safely use this event to create the blob containers.

                _ = blobServiceClient ?? throw new InvalidOperationException($"{nameof(BlobServiceClient)} is not initialized.");
                _ = queueServiceClient ?? throw new InvalidOperationException($"{nameof(QueueServiceClient)} is not initialized.");

                foreach (var container in builder.Resource.BlobContainers)
                {
                    var blobContainerClient = blobServiceClient.GetBlobContainerClient(container.BlobContainerName);
                    await blobContainerClient.CreateIfNotExistsAsync(cancellationToken: ct).ConfigureAwait(false);
                }

View on GitHub (pinned to 25830f84bd)