Storing array to session

protected void CompetencyRetrieve(object sender, EventArgs e)
   {
       int index = Convert.ToInt32(maxindex.Text); // ERROR HERE

           DropDownList box1 = (DropDownList)Gridview1.Rows[index].Cells[1].FindControl("DropDownList1");
           DropDownList box2 = (DropDownList)Gridview1.Rows[index].Cells[2].FindControl("DropDownList2");
           DropDownList box3 = (DropDownList)Gridview1.Rows[index].Cells[3].FindControl("DropDownList3");

           var Trainings = DataService.Instance.getTraining(box2.Text, box1.Text);
           if (Trainings.Rows.Count != 0)
           {
               box3.DataSource = Trainings;
               box3.DataTextField = "ProgramsDesc";
               box3.DataValueField = "Programs_id";
               box3.DataBind();

               //session here

           }
           else
           {
               box3.Items.Clear();
           }


   }