CreateModelLine.cs

public static ModelCurve CreateModelLine(Document doc,
            XYZ p,
            XYZ q)
        {
            double _eps = 1.0e-9;
            if (p.DistanceTo(q) < _eps) return null;
            XYZ v = q - p;
            double dxy = Math.Abs(v.X) + Math.Abs(v.Y);
            XYZ w = (dxy > _eps)
                ? XYZ.BasisZ
                : XYZ.BasisY;
            XYZ norm = v.CrossProduct(w).Normalize();
            XYZ temp = new XYZ(p.X,p.Y,0);
            Plane plane = Autodesk.Revit.DB.Plane.CreateByNormalAndOrigin(norm, p);
            //MessageBox.Show(plane.Origin.ToString());
            SketchPlane sketchPlane = Autodesk.Revit.DB.SketchPlane.Create(doc, plane);
            Line line = Line.CreateBound(p, q);
            ModelCurve modelCurve = doc.Create.NewModelCurve(line, sketchPlane);
            return modelCurve;
        }