Line Type attribute implementation in RhinoCommon

I am trying to implement hidden lineType when I draw a line, but it is not working yet… am I missing anything?

The fraction of code is as below. Please have a look.

        System.Drawing.Color ObjectColor1 = System.Drawing.Color.Blue;
         Rhino.DocObjects.ObjectAttributes att=new Rhino.DocObjects.ObjectAttributes();
         att.ColorSource = ColorSource1;
         att.ObjectColor = ObjectColor1;
         att.LinetypeSource = Rhino.DocObjects.ObjectLinetypeSource.LinetypeFromObject;
         att.LinetypeIndex = 7;

     arcn_A is a given curve in the code..................  

Curve[] curves = arcn_A.Offset(p, Vector3d.ZAxis, 21.52,
doc.ModelAbsoluteTolerance, CurveOffsetCornerStyle.None);

        foreach (var offset_curve in curves)

It is not giving hidden line… It is giving a normal line.

Solved ! it works like this below. First create a layer with linetype dotted and then draw the line in that layer automated…
Rhino.DocObjects.Layer layer = new Rhino.DocObjects.Layer();
layer.Color = System.Drawing.Color.Blue;
layer.LinetypeIndex = doc.Linetypes.Count - 1;