From fe3d192516e7342a5d3d509631e880823af2274f Mon Sep 17 00:00:00 2001
From: "aspire-repo-bot[bot]"
 <268009190+aspire-repo-bot[bot]@users.noreply.github.com>
Date: Tue, 18 Aug 2026 14:17:09 +0000
Subject: [PATCH 1/3] docs: document ArgumentException for Subscribe<T> with
 interface/abstract event types

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 .../src/content/docs/app-host/eventing.mdx      | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/src/frontend/src/content/docs/app-host/eventing.mdx b/src/frontend/src/content/docs/app-host/eventing.mdx
index bafa25341..a25928a5d 100644
--- a/src/frontend/src/content/docs/app-host/eventing.mdx
+++ b/src/frontend/src/content/docs/app-host/eventing.mdx
@@ -113,6 +113,23 @@ builder.Eventing.Subscribe<AfterResourcesCreatedEvent>(
   callbacks like `onResourceReady`.
 </Aside>
 
+<Aside type="caution">
+  `Eventing.Subscribe<T>()` requires `T` to be a concrete event type.
+  Subscribing to an interface or abstract class, such as
+  `IDistributedApplicationEvent`, throws an `ArgumentException` because
+  the internal lookup key is `typeof(T)` while `PublishAsync<T>` is always
+  invoked with the concrete event type — so an interface or abstract
+  subscription could never fire. Subscribe to the concrete event type
+  instead, for example `BeforeStartEvent` or `AfterResourcesCreatedEvent`.
+
+  ```csharp
+  // Throws ArgumentException: Cannot subscribe to interface or abstract
+  // type 'IDistributedApplicationEvent'. Subscribe to a concrete event
+  // type instead.
+  builder.Eventing.Subscribe<IDistributedApplicationEvent>((e, ct) => { ... });
+  ```
+</Aside>
+
 When the AppHost is run, by the time the Aspire dashboard is displayed, you should see the following log output in the console:
 
 ```plaintext {2,12} data-disable-copy

From dc8763ea15df695fcd935a0e2463de2db8bf291d Mon Sep 17 00:00:00 2001
From: David Pine <7679720+IEvangelist@users.noreply.github.com>
Date: Tue, 18 Aug 2026 09:25:20 -0500
Subject: [PATCH 2/3] docs: clarify concrete event subscriptions

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
---
 .../src/content/docs/app-host/eventing.mdx    | 23 +++++++++----------
 1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/src/frontend/src/content/docs/app-host/eventing.mdx b/src/frontend/src/content/docs/app-host/eventing.mdx
index a25928a5d..e28ff7541 100644
--- a/src/frontend/src/content/docs/app-host/eventing.mdx
+++ b/src/frontend/src/content/docs/app-host/eventing.mdx
@@ -116,18 +116,17 @@ builder.Eventing.Subscribe<AfterResourcesCreatedEvent>(
 <Aside type="caution">
   `Eventing.Subscribe<T>()` requires `T` to be a concrete event type.
   Subscribing to an interface or abstract class, such as
-  `IDistributedApplicationEvent`, throws an `ArgumentException` because
-  the internal lookup key is `typeof(T)` while `PublishAsync<T>` is always
-  invoked with the concrete event type — so an interface or abstract
-  subscription could never fire. Subscribe to the concrete event type
-  instead, for example `BeforeStartEvent` or `AfterResourcesCreatedEvent`.
-
-  ```csharp
-  // Throws ArgumentException: Cannot subscribe to interface or abstract
-  // type 'IDistributedApplicationEvent'. Subscribe to a concrete event
-  // type instead.
-  builder.Eventing.Subscribe<IDistributedApplicationEvent>((e, ct) => { ... });
-  ```
+  `IDistributedApplicationEvent`, throws an `ArgumentException` with the
+  message `Cannot subscribe to interface or abstract type
+  'IDistributedApplicationEvent'. Subscribe to a concrete event type instead.`
+  Subscribe to a concrete type such as `BeforeStartEvent` or
+  `AfterResourcesCreatedEvent`.
+
+```csharp title="AppHost.cs"
+builder.Eventing.Subscribe<IDistributedApplicationEvent>(
+    static (@event, cancellationToken) => Task.CompletedTask);
+```
+
 </Aside>
 
 When the AppHost is run, by the time the Aspire dashboard is displayed, you should see the following log output in the console:

From f780e1363bd932cda5a223f9b571ed3880b77931 Mon Sep 17 00:00:00 2001
From: David Pine <dapine@microsoft.com>
Date: Tue, 18 Aug 2026 09:56:56 -0500
Subject: [PATCH 3/3] Correct concrete event subscription guidance

Remove the example that subscribes to the forbidden interface and preserve the exact ArgumentException message in one concise caution.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 32c349b4-907d-42e9-aad8-2f0edc267779
---
 .../src/content/docs/app-host/eventing.mdx       | 16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/src/frontend/src/content/docs/app-host/eventing.mdx b/src/frontend/src/content/docs/app-host/eventing.mdx
index e28ff7541..b748841ce 100644
--- a/src/frontend/src/content/docs/app-host/eventing.mdx
+++ b/src/frontend/src/content/docs/app-host/eventing.mdx
@@ -115,18 +115,10 @@ builder.Eventing.Subscribe<AfterResourcesCreatedEvent>(
 
 <Aside type="caution">
   `Eventing.Subscribe<T>()` requires `T` to be a concrete event type.
-  Subscribing to an interface or abstract class, such as
-  `IDistributedApplicationEvent`, throws an `ArgumentException` with the
-  message `Cannot subscribe to interface or abstract type
-  'IDistributedApplicationEvent'. Subscribe to a concrete event type instead.`
-  Subscribe to a concrete type such as `BeforeStartEvent` or
-  `AfterResourcesCreatedEvent`.
-
-```csharp title="AppHost.cs"
-builder.Eventing.Subscribe<IDistributedApplicationEvent>(
-    static (@event, cancellationToken) => Task.CompletedTask);
-```
-
+  Subscribing to an interface or abstract class throws an `ArgumentException`.
+  For example, subscribing to `IDistributedApplicationEvent` produces
+  `Cannot subscribe to interface or abstract type 'IDistributedApplicationEvent'. Subscribe to a concrete event type instead.`
+  Use a concrete type such as `BeforeStartEvent` or `AfterResourcesCreatedEvent`.
 </Aside>
 
 When the AppHost is run, by the time the Aspire dashboard is displayed, you should see the following log output in the console: