public class Lines
    {
        public string X1 { get; set; }
        public string X2 { get; set; }
        public string Y1 { get; set; }
        public string Y2 { get; set; }
        public string Z1 { get; set; }
        public string Z2 { get; set; }
        public string diameter { get; set; }
        public string material { get; set; }
    }

List<Lines> linesobj = new List<Lines>();
for (int i = 0; i < 20; i++)
        { 
            linesobj.Add(new Lines(){X1=“1”, X2=“2”, Y1=“3”, Y2=“4”, Z1=”5“, Z2=”6“, diameter=”7“, material=”8”}); 
        }

JavaScriptSerializer jss = new JavaScriptSerializer();
string myJson = jss.Serialize(linesobj);
return myJson;