If you are looking for an API to change AutoCAD Civil 3D FeatureLine object's style you can use FeatureLine.StyleName property which gets or sets the Featureline's style name.
And using Autodesk.Civil.DatabaseServices.GeneralSegmentLabel.Create() method we can add Label to a FeatureLine.
In the screenshot below we can see a Civil 3D FeatureLine before changing it's style -
And using the following C# .NET code we can change the style and label it -
using (Transaction ts = AcadApp.DocumentManager.MdiActiveDocument.Database.TransactionManager.StartTransaction())
{
FeatureLine featureLine = ts.GetObject(ftrLineID, OpenMode.ForWrite) as FeatureLine;
// Change the Style to "Flowline"
// Style name used abobe is specific to a DWG file
// FeatureLine.StyleName // Gets or sets the Featureline's style name.
featureLine.StyleName = "Flowline";
// Use Autodesk.Civil.DatabaseServices.GeneralSegmentLabel.Create to add label to a feature line
GeneralSegmentLabel.Create(featureLine.ObjectId, 0.5);
ts.Commit();
}
FeatureLine style after change -
and FeatureLine with label -
