Messages (RPC between packages)

[MessageHandler] exposes a method other packages, or the Console, can call by key:

[MessageHandler("GetSunInfo", Description = "Calculates sunrise/sunset for a date and location.")]
public SunInfo GetSunInfo(DateOnly date, int timezone, double latitude, double longitude) { /* ... */ }

Call PackageHost.RegisterMessageHandlers(this) once, typically in OnStart, to wire up every [MessageHandler] method on an instance.

Every registered handler shows up on the Console's Messages page, with a form generated from its declared parameters - useful for testing a handler by hand, and it's the same data source Scheduled Tasks picks a target from:

Key namespacing

The key is namespaced under the package's own name by default: "DayInfo/GetSunInfo" here, not "GetSunInfo". Two unrelated packages picking the same key can't cross-trigger each other's handler. Callers use the qualified key:

var result = await PackageHost.SendMessageAsync<SunInfo>(MessageScope.Create("DayInfo"), "DayInfo/GetSunInfo", new { date, timezone, latitude, longitude });

Pass Shared = true on [MessageHandler] (and shared: true on SendMessage/SendMessageAsync) for a raw, un-namespaced key - for a handler meant as a cross-package convention any caller can reach without knowing this package's name.

Scope

MessageScope targets a single package, a group, the whole Edge, or everyone (Package / Group / Edge / Others / All). Pick the narrowest one that reaches who needs it.

Reaching another package - by name, group, or a broadcast scope - needs an Allow rule on the caller's own credential, which in turn needs the messages:execute permission scope (not to be confused with MessageScope above - unfortunate naming overlap). A package's own credential gets messages:execute automatically; a Console/Consumer credential calling SendMessage from outside a package needs it granted explicitly. See Access control.

Triggering a message on a schedule

Scheduled Tasks fire a message on a cron schedule instead of a human clicking Invoke - same Authorizations/messages:execute check either way.