Adding a value to a NetworkList before spawning its networkObject results in a null ref exception

If we try to add something to a network list, BEFORE we spawn its network object, we get a null ref exception. In the code below from NetworkList.cs, m_NetworkBehaviour is null.

internal void MarkNetworkObjectDirty() { m_NetworkBehaviour.NetworkManager.MarkNetworkObjectDirty(m_NetworkBehaviour.NetworkObject); }

This was not the case before the latest release (1.0.1), we were able to manipulate the network list freely before actually spawning its network object.

System.NullReferenceException: Object reference not set to an instance of an object
  at Unity.Netcode.NetworkList`1[T].MarkNetworkObjectDirty () [0x00001] in E:\New folder\MyProject\Library\PackageCache\com.unity.netcode.gameobjects@1.0.1\Runtime\NetworkVariable\Collections\NetworkList.cs:68 
  at Unity.Netcode.NetworkList`1[T].HandleAddListEvent (Unity.Netcode.NetworkListEvent`1[T] listEvent) [0x0000f] in E:\New folder\MyProject\Library\PackageCache\com.unity.netcode.gameobjects@1.0.1\Runtime\NetworkVariable\Collections\NetworkList.cs:499 
  at Unity.Netcode.NetworkList`1[T].Add (T item) [0x0003d] in E:\New folder\MyProject\Library\PackageCache\com.unity.netcode.gameobjects@1.0.1\Runtime\NetworkVariable\Collections\NetworkList.cs:386 

[TheCaveOfWonders]

ok i’ll move forward with Don't modify NetworkVariables before they are spawned would be nice being able to manipulate them before spawn, as part of setting up objects/prefabs.

[jeffreyrainy]

Thanks for the report!

@TheCaveOfWonders The current SDK recommendation is: “Don’t modify NetworkVariables before they are spawned”. There’s a bunch of edge cases we might not be handling correctly if you modify NetworkVariables before (or worse: during) spawning. We made a fix over 1.0.1 so that the exception above doesn’t occur for regular NetworkVariables:

NetworkVariableBase.cs

                if (m_NetworkBehaviour == null)
                {
                    Debug.LogWarning($"NetworkVariable is written to, but doesn't know its NetworkBehaviour yet. " +
                                     "Are you modifying a NetworkVariable before the NetworkObject is spawned?");
                    return;
                }
                m_NetworkBehaviour.NetworkManager.MarkNetworkObjectDirty(m_NetworkBehaviour.NetworkObject);

And yeah, sure enough, we missed the same case for NetworkList. I’m willing to go in NetworkList.MarkNetworkObjectDirty() and add a similar check with a similar log message. This will trade the exception for a warning. But, the usage that @alevastor suggests would result in a pretty high number of warnings.

The initial discussion with the team about having a silent NULL check there, instead of the warning, was that hiding errors was not the right approach. I can double-check, but I doubt we’ll change our mind.

So, take-aways:

• we’ll turn the exception in a warning
• please try to not write to NetworkVariables before spawning
• maybe we would revisit this at some point, but don’t count on it. At the very least, we need to tackle support issues first.