fix: prevent exceptions when multiple connection approval are used [MTT-3496]

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -26,6 +26,10 @@ Additional documentation and release notes are available at [Multiplayer Documen
 - Fixed NetworkManager to cleanup connected client lists after stopping (#1945)
 - Fixed NetworkHide followed by NetworkShow on the same frame works correctly (#1940)
 
+### Changed
+
+- NetworkManager.ConnectionApprovalCallback is now a delegate instead of an event. Adding a second one is forbidden and an error is generated if client code attempts it. (#1941)
+
 ## [1.0.0-pre.8] - 2022-04-27
 
 ### Changed

com.unity.netcode.gameobjects/Runtime/Core/NetworkManager.cs

@@ -367,10 +367,28 @@ public IReadOnlyList<ulong> ConnectedClientsIds
         /// <param name="rotation">The rotation to spawn the client with. If null, the prefab position is used.</param>
         public delegate void ConnectionApprovedDelegate(bool createPlayerObject, uint? playerPrefabHash, bool approved, Vector3? position, Quaternion? rotation);
 
+        public delegate void ConnectionApprovalDelegate(byte[] payload, ulong clientId, ConnectionApprovedDelegate connectionApprovedCallback);
+
         /// <summary>
         /// The callback to invoke during connection approval
         /// </summary>
-        public event Action<byte[], ulong, ConnectionApprovedDelegate> ConnectionApprovalCallback = null;
+        public ConnectionApprovalDelegate ConnectionApprovalCallback
+        {
+            get => m_ConnectionApprovalDelegate;
+            set
+            {
+                if (value != null && value.GetInvocationList().Length > 1)
+                {
+                    Debug.LogError($"Only one connection approval handler is supported in {nameof(ConnectionApprovalCallback)}. Rejecting further adds.");
+                }
+                else
+                {
+                    m_ConnectionApprovalDelegate = value;
+                }
+            }
+        }
+
+        private ConnectionApprovalDelegate m_ConnectionApprovalDelegate = null;
 
         internal void InvokeConnectionApproval(byte[] payload, ulong clientId, ConnectionApprovedDelegate action) => ConnectionApprovalCallback?.Invoke(payload, clientId, action);