From a84a95b994060f1d7658c236c5ddc64c53e17bd6 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Mon, 2 Mar 2026 13:27:15 +0000
Subject: [PATCH 1/2] Initial plan


From 993cc710274cb25162be2f173e56060133f5ac39 Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Mon, 2 Mar 2026 10:23:30 -0500
Subject: [PATCH 2/2] Use event delegate composition for thread-safe,
 insertion-ordered handler dispatch

Replace HashSet<SessionEventHandler> with a private event (multicast delegate).
The compiler-generated add/remove accessors use a lock-free CAS loop,
dispatch reads the field once for an inherent snapshot, and invocation
order matches registration order.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 dotnet/src/Session.cs | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/dotnet/src/Session.cs b/dotnet/src/Session.cs
index e906554a..8798565f 100644
--- a/dotnet/src/Session.cs
+++ b/dotnet/src/Session.cs
@@ -44,7 +44,13 @@ namespace GitHub.Copilot.SDK;
 /// </example>
 public partial class CopilotSession : IAsyncDisposable
 {
-    private readonly HashSet<SessionEventHandler> _eventHandlers = new();
+    /// <summary>
+    /// Multicast delegate used as a thread-safe, insertion-ordered handler list.
+    /// The compiler-generated add/remove accessors use a lock-free CAS loop over the backing field.
+    /// Dispatch reads the field once (inherent snapshot, no allocation).
+    /// Expected handler count is small (typically 1–3), so Delegate.Combine/Remove cost is negligible.
+    /// </summary>
+    private event SessionEventHandler? _eventHandlers;
     private readonly Dictionary<string, AIFunction> _toolHandlers = new();
     private readonly JsonRpc _rpc;
     private volatile PermissionRequestHandler? _permissionHandler;
@@ -243,8 +249,8 @@ void Handler(SessionEvent evt)
     /// </example>
     public IDisposable On(SessionEventHandler handler)
     {
-        _eventHandlers.Add(handler);
-        return new ActionDisposable(() => _eventHandlers.Remove(handler));
+        _eventHandlers += handler;
+        return new ActionDisposable(() => _eventHandlers -= handler);
     }
 
     /// <summary>
@@ -256,11 +262,8 @@ public IDisposable On(SessionEventHandler handler)
     /// </remarks>
     internal void DispatchEvent(SessionEvent sessionEvent)
     {
-        foreach (var handler in _eventHandlers.ToArray())
-        {
-            // We allow handler exceptions to propagate so they are not lost
-            handler(sessionEvent);
-        }
+        // Reading the field once gives us a snapshot; delegates are immutable.
+        _eventHandlers?.Invoke(sessionEvent);
     }
 
     /// <summary>
@@ -550,7 +553,7 @@ await InvokeRpcAsync<object>(
             // Connection is broken or closed
         }
 
-        _eventHandlers.Clear();
+        _eventHandlers = null;
         _toolHandlers.Clear();
 
         _permissionHandler = null;