Application class "Autodesk.AutoCAD.ApplicationServices.Application" class exposed a property “RecentDocuments”, which can be used to list out recently opened documents.
[CommandMethod("GetRecentDocument")]
static public void GetRecentDocument()
{
Document doc = Application.DocumentManager.MdiActiveDocument;
Editor ed = doc.Editor;
foreach(RecentDocument recentDoc in Application.RecentDocuments)
{
ed.WriteMessage(recentDoc.Path + "\n");
}
}