Allow specifying an Event Hub to use as the default EntityPath for namespace connection string

playground/AspireEventHub/EventHubs.AppHost/Program.cs

@@ -7,7 +7,7 @@
 
 var eventHub = builder.AddAzureEventHubs("eventhubns")
     .RunAsEmulator()
-    .WithHub("hub");
+    .WithHub("hub", configure => configure.IsDefaultEntity = true);
 
 builder.AddProject<Projects.EventHubsConsumer>("consumer")
     .WithReference(eventHub).WaitFor(eventHub)

playground/AspireEventHub/EventHubsConsumer/Program.cs

@@ -10,11 +10,7 @@
 
 if (useConsumer)
 {
-    builder.AddAzureEventHubConsumerClient("eventhubns",
-        settings =>
-        {
-            settings.EventHubName = "hub";
-        });
+    builder.AddAzureEventHubConsumerClient("eventhubns");
 
     builder.Services.AddHostedService<Consumer>();
     Console.WriteLine("Starting EventHubConsumerClient...");
@@ -24,11 +20,7 @@
     // required for checkpointing our position in the event stream
     builder.AddAzureBlobClient("checkpoints");
 
-    builder.AddAzureEventProcessorClient("eventhubns",
-        settings =>
-        {
-            settings.EventHubName = "hub";
-        });
+    builder.AddAzureEventProcessorClient("eventhubns");
     builder.Services.AddHostedService<Processor>();
     Console.WriteLine("Starting EventProcessorClient...");
 }

src/Aspire.Hosting.Azure.EventHubs/AzureEventHubsExtensions.cs

@@ -200,7 +200,9 @@ public static IResourceBuilder<AzureEventHubsResource> RunAsEmulator(this IResou
             // an event hub namespace without an event hub? :)
             if (builder.Resource.Hubs is { Count: > 0 } && builder.Resource.Hubs[0] is { } hub)
             {
-                var healthCheckConnectionString = $"{connectionString};EntityPath={hub.Name};";
+                var healthCheckConnectionString = connectionString.Contains(";EntityPath=") ?
+                    connectionString : $"{connectionString};EntityPath={hub.Name};";
+
                 client = new EventHubProducerClient(healthCheckConnectionString);
             }
             else
@@ -365,7 +367,7 @@ public static IResourceBuilder<AzureEventHubsEmulatorResource> WithHostPort(this
     /// <param name="path">Path to the file on the AppHost where the emulator configuration is located.</param>
     /// <returns>A reference to the <see cref="IResourceBuilder{T}"/>.</returns>
     public static IResourceBuilder<AzureEventHubsEmulatorResource> WithConfigurationFile(this IResourceBuilder<AzureEventHubsEmulatorResource> builder, string path)
-    { 
+    {
         // Update the existing mount
         var configFileMount = builder.Resource.Annotations.OfType<ContainerMountAnnotation>().LastOrDefault(v => v.Target == AzureEventHubsEmulatorResource.EmulatorConfigJsonPath);
         if (configFileMount != null)

src/Aspire.Hosting.Azure.EventHubs/AzureEventHubsResource.cs

@@ -47,8 +47,29 @@ public class AzureEventHubsResource(string name, Action<AzureResourceInfrastruct
     /// </summary>
     public ReferenceExpression ConnectionStringExpression =>
         IsEmulator
-        ? ReferenceExpression.Create($"Endpoint=sb://{EmulatorEndpoint.Property(EndpointProperty.Host)}:{EmulatorEndpoint.Property(EndpointProperty.Port)};SharedAccessKeyName=RootManageSharedAccessKey;SharedAccessKey=SAS_KEY_VALUE;UseDevelopmentEmulator=true;")
-        : ReferenceExpression.Create($"{EventHubsEndpoint}");
+        ? ReferenceExpression.Create($"Endpoint=sb://{EmulatorEndpoint.Property(EndpointProperty.Host)}:{EmulatorEndpoint.Property(EndpointProperty.Port)};SharedAccessKeyName=RootManageSharedAccessKey;SharedAccessKey=SAS_KEY_VALUE;UseDevelopmentEmulator=true{BuildEntityPath()}")
+        : ReferenceExpression.Create($"{EventHubsEndpoint}{BuildEntityPath()}");
+
+    private string BuildEntityPath()
+    {
+        if (!Hubs.Any(hub => hub.IsDefaultEntity))
+        {
+            // Of zero or more hubs, none are flagged as default
+            return string.Empty;
+        }
+
+        try
+        {
+            // Of one or more hubs, only one may be flagged as default
+            var defaultEntity = Hubs.Single(hub => hub.IsDefaultEntity);
+
+            return $";EntityPath={defaultEntity.Name}";
+        }
+        catch (InvalidOperationException ex)
+        {
+            throw new InvalidOperationException("Only one EventHub can be configured as the default entity.", ex);
+        }
+    }
 
     void IResourceWithAzureFunctionsConfig.ApplyAzureFunctionsConfiguration(IDictionary<string, object> target, string connectionName)
     {

src/Aspire.Hosting.Azure.EventHubs/EventHub.cs

@@ -38,6 +38,12 @@ public EventHub(string name)
     /// </summary>
     public List<EventHubConsumerGroup> ConsumerGroups { get; } = [];
 
+    /// <summary>
+    /// If set, this EventHub will be used as the EntityPath in the resource's connection string.
+    /// <remarks>Only one EventHub can be set as the default entity. If more than one is specified, an Exception will be raised at runtime.</remarks>
+    /// </summary>
+    public bool IsDefaultEntity { get; set; }
+
     /// <summary>
     /// Converts the current instance to a provisioning entity.
     /// </summary>

src/Aspire.Hosting.Azure.EventHubs/PublicAPI.Unshipped.txt

@@ -9,6 +9,8 @@ Aspire.Hosting.Azure.AzureEventHubsResource.IsEmulator.get -> bool
 Aspire.Hosting.Azure.EventHubs.EventHub
 Aspire.Hosting.Azure.EventHubs.EventHub.ConsumerGroups.get -> System.Collections.Generic.List<Aspire.Hosting.Azure.EventHubs.EventHubConsumerGroup!>!
 Aspire.Hosting.Azure.EventHubs.EventHub.EventHub(string! name) -> void
+Aspire.Hosting.Azure.EventHubs.EventHub.IsDefaultEntity.get -> bool
+Aspire.Hosting.Azure.EventHubs.EventHub.IsDefaultEntity.set -> void
 Aspire.Hosting.Azure.EventHubs.EventHub.Name.get -> string!
 Aspire.Hosting.Azure.EventHubs.EventHub.Name.set -> void
 Aspire.Hosting.Azure.EventHubs.EventHub.PartitionCount.get -> long?

src/Components/Aspire.Azure.Messaging.EventHubs/EventHubsComponent.cs

@@ -105,6 +105,9 @@ protected static void EnsureConnectionStringOrNamespaceProvided(AzureMessagingEv
                         $"A {typeof(TClient).Name} could not be configured. Ensure a valid EventHubName was provided in " +
                         $"the '{configurationSectionName}' configuration section, or include an EntityPath in the ConnectionString.");
                 }
+
+                // this is used later to create the checkpoint blob container
+                settings.EventHubName = props.EventHubName;
             }
         }
         // If we have a namespace and no connection string, ensure there's an EventHubName

src/Components/Aspire.Azure.Messaging.EventHubs/README.md

@@ -70,6 +70,8 @@ builder.AddAzureEventHubProducerClient("eventHubsConnectionName",
     });
 ```
 
+NOTE: Earlier versions of Aspire (<9.1) required you to always set the EventHubName here because the Azure Event Hubs Hosting component did not provide a way to specify which Event Hub was to be included in the connection string. Beginning in 9.1, it is now possible to specify which Event Hub is to be used by way of the `configuration` callback in `WithHub(...)` using the `IsDefaultEntity` boolean property. Only one Event Hub can be the default and attempts to flag multiple will elicit an Exception at runtime.
+
 And then the connection information will be retrieved from the `ConnectionStrings` configuration section. Two connection formats are supported:
 
 #### Fully Qualified Namespace

tests/Aspire.Hosting.Azure.Tests/AzureEventHubsExtensionsTests.cs

@@ -62,6 +62,53 @@ public async Task VerifyWaitForOnEventHubsEmulatorBlocksDependentResources()
         await app.StopAsync();
     }
 
+    [Fact]
+    [RequiresDocker]
+    [ActiveIssue("https://github.com/dotnet/aspire/issues/7093")]
+    public async Task VerifyEntityPathInConnectionStringForIsDefaultEntity()
+    {
+        using var builder = TestDistributedApplicationBuilder.Create(testOutputHelper);
+        var eventHub = builder.AddAzureEventHubs("eventhubns")
+            .RunAsEmulator()
+            .WithHub("hub", configure => configure.IsDefaultEntity = true);
+
+        using var app = builder.Build();
+        await app.StartAsync();
+
+        string? connectionString =
+            await eventHub.Resource.ConnectionStringExpression.GetValueAsync(CancellationToken.None);
+
+        // has entitypath?
+        Assert.Contains(";EntityPath=hub", connectionString);
+
+        // well-formed connectionstring?
+        var props = EventHubsConnectionStringProperties.Parse(connectionString);
+        Assert.NotNull(props);
+        Assert.Equal("hub", props.EventHubName);
+    }
+
+    [Fact]
+    [RequiresDocker]
+    [ActiveIssue("https://github.com/dotnet/aspire/issues/7093")]
+    public async Task VerifyMultipleDefaultEntityThrowsInvalidOperationException()
+    {
+        using var builder = TestDistributedApplicationBuilder.Create(testOutputHelper);
+        var eventHub = builder.AddAzureEventHubs("eventhubns")
+            .RunAsEmulator()
+            .WithHub("hub", configure => configure.IsDefaultEntity = true)
+            .WithHub("hub2", configure => configure.IsDefaultEntity = true);
+
+        using var app = builder.Build();
+
+        await app.StartAsync();
+        var hb = Host.CreateApplicationBuilder();
+        using var host = hb.Build();
+        await host.StartAsync();
+
+        await Assert.ThrowsAsync<InvalidOperationException>(
+            async () => await eventHub.Resource.ConnectionStringExpression.GetValueAsync(CancellationToken.None));
+    }
+
     [Fact]
     [RequiresDocker]
     [ActiveIssue("https://github.com/dotnet/aspire/issues/6751")]