Data Synchronization

Recommendation: For most use cases, use Synchronized Variables (SyncVars) instead. SyncVars provide a cleaner API with automatic type safety, validation, rate limiting, and built-in prefix handling via NetworkSyncOptions.KeyPrefix.

Use lobby-wide and per-player key-value data for lightweight shared state like versions, flags, and small strings.

Raw API vs SyncVars

SyncVars are recommended over the raw lobby/member data API for most use cases:

Feature	Raw API	SyncVars
Prefix handling	Manual	Automatic via NetworkSyncOptions.KeyPrefix
Type safety	String-only	Type-safe generic T
Validation	Manual	Built-in validators
Rate limiting	Manual	Built-in MaxSyncsPerSecond
Events	Raw change events	Typed OnValueChanged events
Batch updates	SetMyDataBatch()	Auto-sync batching

Use SyncVars for game state, player data, and most mod data. See Synchronized Variables (SyncVars) for details.

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Important: Use Unique Prefixes

Always use custom prefixes for your mod's data keys to avoid collisions with other mods.

// Good: Use a unique prefix for your mod
const string PREFIX = "MyMod_";

client.SetLobbyData($"{PREFIX}version", "1.0.0");
client.SetMyData($"{PREFIX}loadout", "1911");

// Bad: Generic keys may collide with other mods
client.SetLobbyData("version", "1.0.0");  // May conflict!
client.SetMyData("loadout", "1911");      // May conflict!

Tip: SyncVars handle prefixes automatically. Just set KeyPrefix in NetworkSyncOptions:

var options = new NetworkSyncOptions { KeyPrefix = "MyMod_" };
var score = client.CreateHostSyncVar("Score", 0, options);
// Actual Steam key: "MyMod_Score" - no manual prefix needed!

Lobby-wide data (host-only)

// Set by the lobby owner
client.SetLobbyData("mod_version", "1.0.0");

// Read by anyone
string modVersion = client.GetLobbyData("mod_version");

Per-player data

// Local player sets their visible data
client.SetMyData("name", "bob");

// Read for self
string myClass = client.GetMyData("name");

// Read for any specific player
string otherClass = client.GetPlayerData(playerId, "name");

// Read the same key for everyone
Dictionary<CSteamID, string> allClasses = client.GetDataForAllPlayers("name");

Change events

client.OnLobbyDataChanged += (s, e) =>
{
    MelonLogger.Msg($"Lobby data: {e.Key} -> {e.NewValue}");
};

client.OnMemberDataChanged += (s, e) =>
{
    MelonLogger.Msg($"Member {e.MemberId}: {e.Key} -> {e.NewValue}");
};

Version compatibility helper

Version checks are enabled by default. The client stores its library version under a reserved key and triggers OnVersionMismatch when players differ.

client.OnVersionMismatch += (s, e) =>
{
    MelonLogger.Warning($"Version mismatch. Local: {e.LocalVersion}");
};

// Optional: toggle
client.VersionCheckEnabled = true;

// Manual check
bool ok = client.CheckLibraryVersionCompatibility();

Batch updates

client.SetMyDataBatch(new Dictionary<string, string>
{
    ["loadout"] = "1911",
    ["ready"] = "true",
});

When to use P2P instead

• Use data keys for small strings/flags.
• For large payloads (files, images, audio), use the P2P Messaging API.