fix: Private nested types in NetworkVariables in CoreCLR

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -18,11 +18,13 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Ensured a useful and human readable error is logged when a NetworkVariable is trying to serialize a private or protected type. (#4239)
+- Ensured all callbacks are wrapped with exception handling to avoid silent errors. (#4161)
+
 ### Security
 
 ### Obsolete
 
-
 ## [3.2.0] - 2026-11-03
 
 ### Added
@@ -42,7 +44,6 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
-- Ensured all callbacks are wrapped with exception handling to avoid silent errors. (#4161)
 - Issue where `NetworkTime` reported a `TickOffset` of almost a whole tick, instead of zero, for some times that land on a tick (for example 0.3 at a tick rate of 10), which put `TickWithPartial` one tick ahead or, in long sessions, `Tick` one tick behind. Times between minus one tick and zero now report tick -1 with a positive `TickOffset`, as other negative times do. (#4228)
 - Issue where bits written with `BitWriter` were lost when a later `BitWriter.TryBeginWriteBits` call in the same bitwise context grew the `FastBufferWriter`. The bytes written earlier in that context were replaced with zeros or uninitialized memory. (#4226)
 - Issue where a growable `FastBufferWriter` created with a size of 0 never returned from its first write or `TryBeginWrite` call that needed the buffer to grow. (#4224)

com.unity.netcode.gameobjects/Documentation~/upgrade-guide.md

@@ -123,6 +123,14 @@ In version 2.x, the `Lerp` and `SmoothDampening` interpolation types smooth by d
 
 Version 3.x introduces the foundations of the unified GameObject layer for Netcode for Entities, known as the hybrid prefab concept. Version 3.x doesn't enable this integration by default for standard GameObject workflows. Standard `NetworkBehaviour`, `NetworkVariable`, and RPC workflows work the same as in version 2.x.
 
+## CoreCLR compilation changes
+
+CoreCLR introduces a new compilation pipeline that changes some custom serialization generation.
+
+## NetworkVariable serialization
+
+The Mono compilation pipeline was capable of generating serialization for private and protected types. With CoreCLR, this is no longer possible and will fail the compilation. Ensure any types used inside NetworkVariables in your project are declared as `public` or `internal`.
+
 ## Additional resources
 
 - [Install Netcode for GameObjects](install.md)

com.unity.netcode.gameobjects/Editor/CodeGen/CodeGenHelpers.cs

@@ -208,6 +208,36 @@ public static bool HasInterface(this TypeReference typeReference, string interfa
             }
         }
 
+        /// <summary>
+        /// Collects every type that assembly scoped code cannot access when it uses <paramref name="type"/>.
+        /// This is each private or protected nested type in its nesting chain, or in the chain of its generic arguments.
+        /// </summary>
+        /// <param name="type">The type the generated code uses.</param>
+        /// <param name="hidden">Receives each inaccessible type, without duplicates.</param>
+        public static void CollectTypesHiddenFromAssembly(this TypeReference type, List<TypeDefinition> hidden)
+        {
+            if (type is GenericInstanceType genericInstance)
+            {
+                foreach (var argument in genericInstance.GenericArguments)
+                {
+                    argument.CollectTypesHiddenFromAssembly(hidden);
+                }
+            }
+            else if (type is TypeSpecification specification)
+            {
+                specification.ElementType.CollectTypesHiddenFromAssembly(hidden);
+                return;
+            }
+
+            for (var definition = type.Resolve(); definition != null && definition.IsNested; definition = definition.DeclaringType)
+            {
+                if ((definition.IsNestedPrivate || definition.IsNestedFamily || definition.IsNestedFamilyAndAssembly) && !hidden.Contains(definition))
+                {
+                    hidden.Add(definition);
+                }
+            }
+        }
+
         /// <summary>
         /// Checks a generic method instantiation against the generic constraints of the method it instantiates.
         /// Mono does not enforce these on the calls codegen emits, but CoreCLR does, and it rejects the entire

com.unity.netcode.gameobjects/Editor/CodeGen/NetworkBehaviourILPP.cs

@@ -437,6 +437,26 @@ private void CreateNetworkVariableTypeInitializers(AssemblyDefinition assembly,
                     continue;
                 }
 
+                // CoreCLR enforces member access on the generic calls emitted below; Mono does not.
+                var hiddenTypes = new List<TypeDefinition>();
+                type.CollectTypesHiddenFromAssembly(hiddenTypes);
+                if (hiddenTypes.Count > 0)
+                {
+#if ENABLE_CORECLR
+                    foreach (var hiddenType in hiddenTypes)
+                    {
+                        m_Diagnostics.AddError($"{type} cannot be used in a network variable - Cannot access the {hiddenType.Name} type to generate serialization as it is {(hiddenType.IsNestedPrivate ? "private" : "protected")}. To fix this, change the access modifier to internal or public.");
+                    }
+                    continue;
+#else
+                    // Log a warning in Mono so we can warn users who haven't yet switched their projects over.
+                    foreach (var hiddenType in hiddenTypes)
+                    {
+                        m_Diagnostics.AddWarning($"NetworkVariable serialization for {type.Name} will fail in CoreCLR! With CoreCLR generated code cannot access the {hiddenType.Name} type to generate serialization as it is {(hiddenType.IsNestedPrivate ? "private" : "protected")}. To fix this, change the access modifier to internal or public.");
+                    }
+#endif
+                }
+
                 if (serializeMethod != null)
                 {
                     instructions.Add(processor.Create(OpCodes.Call, m_MainModule.ImportReference(serializeMethod)));