Are you using Acad .NET API or ObjectARX API (C++)? AcGePolyline2d is an ObjectARX class, and the corresponding .NET API class is Autdesk.AutoCAD.Geometry.PolylineCurve2d, which has a few Contructors. You can get the needed parameter data from a Polyline and use it in one of the Constructor.
BTW, since you mentioned that you are doing an Windows app, is it an stand-alone EXE? if yes, you cannot use either ObjectARX class or .NET API class i n a stand-alone EXE, even the box has AutoCAD installed. They can only be used in code that run inside AutoCAD (*.arx from Object ARX code/.NET *.dll from .NET API code).