After the previous commit (99ac3d0) moved attribute lookup from a hardcoded
dictionary to the dynamic RegistryData, MCC would crash immediately upon
joining a vanilla 1.20.6 server with:
System.ArgumentException: An item with the same key has already been added.
Key: unknown
Root cause: When KnownDataPacks negotiation tells the server that MCC already
has the "minecraft" data pack, the server skips sending RegistryData for
registries it considers "known" — including minecraft:attribute. This left
the dynamic attribute map empty, so every VarInt attribute ID resolved to
"unknown". The EntityProperties packet often contains multiple attributes
(e.g. armor, max_health, movement_speed), and `keys.Add("unknown", ...)` on
the second "unknown" attribute threw ArgumentException.
Two fixes applied:
1. World.GetAttributeNameById(): When the dynamic attribute map is empty
(server didn't send the registry), automatically load the vanilla 1.20.6
default attribute order (22 entries matching Attributes.java registration
order). This mirrors the pattern used for dimensions where defaults are
loaded when RegistryData is not sent. If a modded server sends a custom
attribute registry, the dynamic map takes precedence.
2. Protocol18.cs EntityProperties handler: Change `keys.Add(propertyKey,
propertyValue2)` to `keys[propertyKey] = propertyValue2` to tolerate
duplicate keys defensively, in case an unknown attribute ID still appears.
Tested: MCC now connects to a vanilla 1.20.6 offline-mode server, stays
online for 6+ minutes with no crashes or disconnections. Verified: chat
messages received, inventory listing (item names/counts correct), entity
detection, TPS query, and health query all work correctly.
Made-with: Cursor
b692b13bbc