what is the difference between polycurve and polyline in arcgis engine?

[zhenliu]

I feel polyline and polycurve are both lines reprenting linear features. But I am confused about when to use polyline and when to use polycurve. Are there difference between them ?

[HailiangShen]

Polycurve can be polyline or polygon.

e.g. The following code would accept both polyline and polygon as input.

Public Sub Densify(polyCurve as IPolyCurve)
    polycurve.Densify(...)
End Sub