fix: add better error messages for a couple of error cases

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -16,6 +16,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Changed
 
+- Better error message when using generic IEquatable in a generic INetworkSerializable class and updated documentation with workaround. (#3739)
 - The `NetworkManager` functions `GetTransportIdFromClientId` and `GetClientIdFromTransportId` will now return `ulong.MaxValue` when the clientId or transportId do not exist. (#3707)
 - Changed NetworkShow to send a message at the end of the frame and force a NetworkVariable synchronization prior to generating the CreateObjectMessage as opposed to waiting until the next network tick to synchronize the show with the update to NetworkVariables. (#3664)
 - Changed NetworkTransform now synchronizes `NetworkTransform.SwitchTransformSpaceWhenParented` when it is updated by the motion model authority. (#3664)
@@ -37,6 +38,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 - Fixed NetworkTransform state synchronization issue when `NetworkTransform.SwitchTransformSpaceWhenParented` is enabled and the associated NetworkObject is parented multiple times in a single frame or within a couple of frames. (#3664)
 - Fixed issue when spawning, parenting, and immediately re-parenting when `NetworkTransform.SwitchTransformSpaceWhenParented` is enabled. (#3664)
 - Fixed issue where the disconnect event and provided message was too generic to know why the disconnect occurred. (#3551)
+- Exception when the network prefab list in the network manager has uninitialized elements. (#3739)
 
 ### Security
 

com.unity.netcode.gameobjects/Documentation~/advanced-topics/serialization/inetworkserializable.md

@@ -181,3 +181,55 @@ public struct MyStructB : MyStructA
     }
 }
 ```
+
+## Generic IEquatable network variables
+
+Generic `INetworkSerializable` types with generic `IEquatable` are not supported, implemented as `public class NotSupported<T> : INetworkSerializable, IEquatable<NotSupported<T>>` where the type would be passed in during declaration like `NetworkVariable<NotSupported<int>> myVar;`.
+
+The recommended workaround for this would be to create the generic class as usual but add a virtual method for handling the serialization of the type. Then wrap this generic `INetworkSerializable` in a derived class which then needs to have a serializable type defined where the implementation for the serialization is provided.
+
+For example:
+
+```csharp
+public class MyGameData<T> : INetworkSerializable
+{
+    // This needs to be a serializable type according to what network variables support
+    public T Data;
+
+    protected virtual void OnNetworkSerialize<T2>(BufferSerializer<T2> serializer) where T2 : IReaderWriter
+    {
+    }
+
+    public void NetworkSerialize<T2>(BufferSerializer<T2> serializer) where T2 : IReaderWriter
+    {
+        OnNetworkSerialize(serializer);
+    }
+}
+
+public class GameDataWithLong : MyGameData<long>, IEquatable<GameDataWithLong>
+{
+    // Potential additional data
+    public int AdditionalData;
+
+    protected virtual bool OnEquals(GameDataWithLong other)
+    {
+        return other.Data.Equals(other);
+    }
+    public bool Equals(GameDataWithLong other)
+    {
+        return OnEquals(other);
+    }
+
+    protected override void OnNetworkSerialize<T2>(BufferSerializer<T2> serializer)
+    {
+        serializer.SerializeValue(ref AdditionalData);
+        serializer.SerializeValue(ref Data);
+    }
+}
+```
+
+Then declare this network variable like so:
+
+```csharp
+NetworkVariable<GameDataWithLong> myVar = new NetworkVariable<GameDataWithLong>();
+```

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

@@ -409,7 +409,14 @@ private void CreateNetworkVariableTypeInitializers(AssemblyDefinition assembly,
                     }
                     else
                     {
-                        m_Diagnostics.AddError($"{type}: Managed type in NetworkVariable must implement IEquatable<{type}>");
+                        foreach (var typeInterface in type.Resolve().Interfaces)
+                        {
+                            if (typeInterface.InterfaceType.Name.Contains(typeof(IEquatable<>).Name) && typeInterface.InterfaceType.IsGenericInstance)
+                            {
+                                m_Diagnostics.AddError($"{type}: A generic IEquatable '{typeInterface.InterfaceType.FullName}' is not supported.");
+                            }
+                        }
+                        m_Diagnostics.AddError($"{type}: Managed type in NetworkVariable must implement IEquatable<{type}>.");
                         equalityMethod = new GenericInstanceMethod(m_NetworkVariableSerializationTypes_InitializeEqualityChecker_ManagedClassEquals_MethodRef);
                     }
 

com.unity.netcode.gameobjects/Editor/NetworkManagerEditor.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using Unity.Netcode.Editor.Configuration;
 using UnityEditor;
 using UnityEngine;
@@ -301,6 +302,10 @@ private void DisplayNetworkManagerProperties()
                     {
                         EditorGUILayout.HelpBox("You have no prefab list selected. You will have to add your prefabs manually at runtime for netcode to work.", MessageType.Warning);
                     }
+                    else if (m_NetworkManager.NetworkConfig.Prefabs.NetworkPrefabsLists.All(x => x == null))
+                    {
+                        EditorGUILayout.HelpBox("All prefab lists selected are uninitialized. You will have to add your prefabs manually at runtime for netcode to work.", MessageType.Warning);
+                    }
                     EditorGUILayout.PropertyField(m_PrefabsList);
                 }
 

com.unity.netcode.gameobjects/Runtime/Configuration/NetworkPrefabs.cs

@@ -94,6 +94,7 @@ internal void Shutdown()
         public void Initialize(bool warnInvalid = true)
         {
             m_Prefabs.Clear();
+            NetworkPrefabsLists.RemoveAll(x => x == null);
             foreach (var list in NetworkPrefabsLists)
             {
                 list.OnAdd += AddTriggeredByNetworkPrefabList;

com.unity.netcode.gameobjects/Tests/Editor/NetworkManagerConfigurationTests.cs

@@ -294,6 +294,22 @@ public void WhenModifyingPrefabListUsingPrefabsAPI_ModificationIsLocal()
             }
         }
 
+        [Test]
+        public void WhenThereAreUninitializedElementsInPrefabsList_NoErrors()
+        {
+            var networkConfig = new NetworkConfig();
+
+            networkConfig.Prefabs.NetworkPrefabsLists = new List<NetworkPrefabsList> { null };
+
+            networkConfig.InitializePrefabs();
+
+            // Null elements will be removed from the list so it should be empty
+            Assert.IsTrue(networkConfig.Prefabs.NetworkPrefabsLists.Count == 0);
+            Assert.IsTrue(networkConfig.Prefabs.Prefabs.Count == 0);
+
+            networkConfig.Prefabs.Shutdown();
+        }
+
         [Test]
         public void WhenModifyingPrefabListUsingPrefabsListAPI_ModificationIsShared()
         {