IExternalCommand
The Revit .NET interface a class implements to expose a single command button. Implements Execute(...) and returns Result.Succeeded/Failed.
IExternalCommand is the entry-point interface for a Revit external command. Your class implements one method — Execute(ExternalCommandData commandData, ref string message, ElementSet elements) — and returns Result.Succeeded, Result.Failed, or Result.Cancelled. Combined with a transaction inside Execute, this is the canonical way to add a button-triggered command to Revit. For lifecycle events (startup, shutdown), implement IExternalApplication instead.
Related terms
Autodesk's Building Information Modeling (BIM) authoring tool. Automated primarily with Dynamo and the Revit .NET API.
A managed assembly (C#/VB.NET DLL) that uses Autodesk's .NET API to add commands and behaviour to AutoCAD or Civil 3D.
The XML file Revit reads at startup to discover and load .NET add-ins from the user's Addins folder.