microsoft/aspire · error · DistributedApplicationException

ConnectionStringAvailableEvent published for resource

Error message

ConnectionStringAvailableEvent published for resource '{resource.Name}', but the connection string was null.

What it means

Aspire's Azure Kusto integration registers a callback on the ConnectionStringAvailableEvent that resolves the resource's connection string. If the event fires but the connection string expression resolves to null, the library throws this DistributedApplicationException because the health-check and lifecycle setup cannot proceed without a valid connection string.

Solutions

1. Ensure the Kusto resource has a connection string: verify any backing parameter/configuration value (user-secrets, appsettings, environment) is set before running the AppHost.
2. If wrapping an existing cluster with an explicit connection string, call WithConnectionString/with a non-null value so ConnectionStringExpression resolves.
3. Check custom code that overrides the connection string expression and make sure it never returns null.
4. Run with dashboard logs and inspect the resource model at the time of the event to confirm which source (parameter/reference) yielded null.

Example fix

// before
var kusto = builder.AddAzureKustoCluster("kusto")
    .WithConnectionString(null);
// after
var kusto = builder.AddAzureKustoCluster("kusto")
    .WithConnectionString(builder.AddParameter("kusto-connection-string"));

var cs = await resource.ConnectionStringExpression.GetValueAsync(ct);
if (string.IsNullOrEmpty(cs)) throw new InvalidOperationException($"Kusto resource '{resource.Name}' has no connection string; check its backing parameter/config.");

if (resource.ConnectionStringExpression is null) { /* resource has no connection string source configured */ }

When it happens

Trigger: The OnConnectionStringAvailable callback fires for the Azure Kusto cluster resource but resource.ConnectionStringExpression.GetValueAsync returns null — e.g. the connection string was never assigned, a referenced parameter resolved to null, or a callback cleared the connection string before the event was published.

Common situations: Running an AppHost where the Kusto connection string is backed by a missing/empty configuration value (user-secrets or appsettings key not set), a RunAsEmulator misconfiguration, or code that overrode the connection string expression after AddAzureKustoCluster.

Understand the failure class

Background: "missing required config value" errors: why libraries refuse to start when a configuration key is empty, unset, or blank — this error's family across 48 libraries.

Related errors

• Connection string for Kusto resource
• Connection string for Kusto resource
• Circular dependency detected
• Connection string is unavailable
• Connection string is unavailable

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/f9dd238512d72f87. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting.Azure.Kusto/AzureKustoBuilderExtensions.cs:243

        return builder.WithEndpoint("http", endpoint =>
        {
            endpoint.Port = port;
        });
    }

    /// <summary>
    /// Adds Kusto-specific health checks and lifecycle management.
    /// </summary>
    private static void AddKustoHealthChecksAndLifecycleManagement(IResourceBuilder<AzureKustoClusterResource> resourceBuilder)
    {
        var resource = resourceBuilder.Resource;

        // Register a health check that will be used to verify Kusto is available
        KustoConnectionStringBuilder? kcsb = null;
        resourceBuilder.OnConnectionStringAvailable(async (resource, evt, ct) =>
        {
            var connectionString = await resource.ConnectionStringExpression.GetValueAsync(ct).ConfigureAwait(false) ??
            throw new DistributedApplicationException($"ConnectionStringAvailableEvent published for resource '{resource.Name}', but the connection string was null.");

            kcsb = GetConnectionStringBuilder(resource, connectionString);
        });

        var healthCheckKey = $"{resource.Name}_check";
        resourceBuilder.ApplicationBuilder
            .Services
            .AddHealthChecks()
            .AddAzureKustoHealthCheck(healthCheckKey, isCluster: true, _ => kcsb!);

        // Execute any setup now that Kusto is ready
        resourceBuilder.OnResourceReady(async (server, evt, ct) =>
        {
            if (kcsb is null)
            {
                throw new DistributedApplicationException($"Connection string for Kusto resource '{server.Name}' is not set.");
            }

View on GitHub (pinned to 25830f84bd)