building-seq-plug-in-apps

Create a class deriving from SeqApp that implements ISubscribeToAsync<LogEvent>, decorate it with the SeqApp attribute, and process events in OnAsync. Validate settings in OnAttached and abstract external I/O behind a gateway interface for testability.

Implement IPublishJson on a SeqApp subclass, then write CLEF JSON lines to the TextWriter passed to Start. Return immediately from Start using background tasks, synchronize writes with a lock, and block in Stop until publishing finishes.

Use ISubscribeToJsonAsync for high-volume forwarders and relays that process raw CLEF JSON without Serilog deserialization. Use ISubscribeToAsync<LogEvent> when rendering human-readable output with Seq.Syntax templates or inspecting structured properties.

Set setting properties directly on the app instance, call Attach with a TestAppHost from Seq.Apps.Testing, and invoke OnAsync with test events. Inject a test double gateway through an internal constructor to capture sent payloads without network access.

Seq does not resolve package dependencies when installing apps, so all assemblies must be packed into the app's own NUPKG. Exclude Seq.Apps.dll, Serilog.dll, and the app's own assembly since the runtime pins its own copies.