Editor ed = Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument.Editor;
Matrix3d ucs = ed.CurrentUserCoordinateSystem;
 
PromptPointResult ppr = ed.GetPoint("\nSpecify base point: ");
if (ppr.Status != PromptStatus.OK)
    return;
 
Point3d baseUcs = ppr.Value;
CoordinateSystem3d cs = ucs.CoordinateSystem3d;
 
// Transform from UCS to WCS
Matrix3d mat =
    Matrix3d.AlignCoordinateSystem(
    Point3d.Origin,
    Vector3d.XAxis,
    Vector3d.YAxis,
    Vector3d.ZAxis,
    cs.Origin,
    cs.Xaxis,
    cs.Yaxis,
    cs.Zaxis
    );
 
Point3d baseWcs = baseUcs.TransformBy(mat);
Point3d curPt = baseWcs;