NetworkObject & NetworkBehaviour

GameObjects, NetworkObjects and NetworkBehaviour aren't serializable types so they can't be used in RPCs or NetworkVariables by default.

There are two convenience wrappers which can be used to send a reference to a NetworkObject or a NetworkBehaviour over RPCs or NetworkVariables.

NetworkObjectReference

NetworkObjectReference can be used to serialize a reference to a NetworkObject. It can only be used on already spawned NetworkObjects.

Here is an example of using NetworkObject reference to send a target NetworkObject over an RPC:

public class Weapon : NetworkBehaviour
{
    public void ShootTarget(GameObject target)
    {
        var targetObject = target.GetComponent<NetworkObject>();
        ShootTargetServerRpc(targetObject);
    }

    [ServerRpc]
    public void ShootTargetServerRpc(NetworkObjectReference target)
    {
        if (target.TryGet(out NetworkObject targetObject))
        {
            // deal damage or something to target object.
        }
        else
        {
            // Target not found on server, likely because it already has been destroyed/despawned.
        }
    }
}

Implicit Operators

There are also implicit operators which convert from/to NetworkObject/GameObject which can be used to simplify code. For instance the above example can also be written in the following way:

public class Weapon : NetworkBehaviour
{
    public void ShootTarget(GameObject target)
    {
        ShootTargetServerRpc(target);
    }

    [ServerRpc]
    public void ShootTargetServerRpc(NetworkObjectReference target)
    {
        NetworkObject targetObject = target;
    }
}

note

The implicit conversion to NetworkObject / GameObject will result in Null if the reference can't be found.

NetworkBehaviourReference

NetworkBehaviourReference works similar to NetworkObjectReference but is used to reference a specific NetworkBehaviour component on a spawned NetworkObject.

public class Health : NetworkBehaviour
{
    public NetworkVariable<int> Health = new NetworkVariable<int>();
}

public class Weapon : NetworkBehaviour
{
    public void ShootTarget(GameObject target)
    {
        var health = target.GetComponent<Health>();
        ShootTargetServerRpc(health, 10);
    }

    [ServerRpc]
    public void ShootTargetServerRpc(NetworkBehaviourReference health, int damage)
    {
        if (health.TryGet(out Health healthComponent))
        {
            healthComponent.Health.Value -= damage;
        }
    }
}

How NetworkObjectReference & NetworkBehaviourReference work

NetworkObjectReference and NetworkBehaviourReference are convenience wrappers which serialize the id of a NetworkObject when being sent and on the receiving end retrieve the corresponding with that id. NetworkBehaviourReference sends an additional index which is used to find the right NetworkBehaviour on the NetworkObject.

Both of them are structs implementing the INetworkSerializable interface.