EndpointReference Error In .NET Aspire 13.0

Hey guys! Ever run into a weird issue after upgrading your .NET Aspire project? Specifically, have you seen your EndpointReference resolve to something like http://: (missing host and port)? Yeah, that's what we're diving into today. Let's break down the problem, what causes it, and how to potentially fix it. This is a common hiccup, especially when dealing with testing and service discovery in your applications. We'll also look at a specific test case that highlights the issue, and what you can do to avoid it. Let's get started!

The Bug: Endpoint Resolution Gone Wrong

So, the core of the problem lies in how EndpointReference is resolving in .NET Aspire 13.0. In the scenario described, after upgrading from an older version (like 9.5), the endpoint, instead of correctly pointing to your in-memory server with its host and port, is resolving to http://:. This means the host and port are missing, which can cause significant issues in your application. For example, if you're using an OpenTelemetry (otel) connector, it won't be able to connect to the intended destination because it doesn't know where to find it. This can cause the application to fail. And, you're left scratching your head!

This behavior is unexpected. In most cases, you would expect the system to throw an error if the host or port are missing. That way, you immediately know something's wrong. You wouldn't want the application to silently fail, right? Instead, the test case in the bug report shows the test code ending up with an empty host and port (http://:). The test then fails with an Assert.That error, which tells us that the value the system got (http://:) did not match the expected value (http://localhost:1234). This is a crucial aspect of testing and shows the importance of ensuring that your endpoints resolve correctly.

The Heart of the Problem

The root cause likely stems from changes in how Aspire handles endpoint resolution, particularly within the WithEnvironment() and WithReference() methods. These methods are commonly used to set up dependencies between services. It's possible that in version 13.0, there's a bug in how these methods are interpreting or handling the endpoint configuration. Specifically, it might be misinterpreting the endpoint information, resulting in the host and port being lost during the resolution process. It is important to remember that these methods help developers manage the relationships between services and ensure they communicate correctly.

Reproducing the Issue: A Step-by-Step Guide

To better understand this, let's look at the provided code snippet, which serves as a test case that can reproduce the problem. Let's break it down step-by-step. The test case sets up an in-memory server, and then tries to connect to it. Let's look at the code:

   [Test]
   public async Task test()
   {
      var builder = DistributedApplicationTestingBuilder.Create()
         .WithTrayportContainerRegistries();

      var dependency = builder
         .AddResource(new FakeResource())
         .WithHttpEndpoint();

      var consumer = builder
         .AddContainer("test", "redis")
         .WithImageRegistry("docker.io")
         .WithReference(dependency.GetEndpoint("http"));

      var endpointAnnotation = dependency.Resource.Annotations.OfType<EndpointAnnotation>().Single();
      endpointAnnotation.AllocatedEndpoint = new AllocatedEndpoint(endpointAnnotation, "localhost", 1234);

      using var app = builder.Build();
      await app.StartAsync();

      var envVars = await consumer.Resource.GetEnvironmentVariableValuesAsync();
      Assert.That(envVars["services__fake__http__0"], Is.EqualTo("http://localhost:1234"));
      // Actual: `http://:`
   }

   public class FakeResource() : Resource("fake"), IResourceWithEndpoints { }

Here's what is happening in the code:

1. Setting up the Environment: The test uses DistributedApplicationTestingBuilder.Create() to build a testing environment. Then the code adds a fake resource using AddResource(new FakeResource()). And it also adds an HTTP endpoint using .WithHttpEndpoint(). This sets up the base for the test.
2. Defining Dependencies: It creates a container and defines its dependencies. Specifically, a container named test (probably representing a service) that depends on the fake resource. Then, it uses the .WithReference() method to link the container to the endpoint of the fake resource.
3. Endpoint Configuration: The code gets the endpoint annotation from the resource. It manually sets the AllocatedEndpoint to localhost:1234. This simulates an endpoint that should be available on localhost at port 1234.
4. Starting the Application: It builds and starts the application using builder.Build() and await app.StartAsync(). This should start all the resources and dependencies.
5. Verifying the Result: The code fetches environment variables for the consumer, then it asserts that the environment variable named services__fake__http__0 equals http://localhost:1234. This check is the important part of the code. That assertion fails. The actual value is http://:. This reveals the bug.

Analyzing the Code and The Issue

The core of the problem is in the assertion: `Assert.That(envVars[