Temparary Srore Data in Asp.net

 Step- 1

List<CAT_MST> List = new List<CAT_MST>();

 Step- 2
 protected void btnAdd_Click(object sender, EventArgs e)
        {
            string drpval =drpCategory.SelectedItem.ToString();
            CAT_MST obj = new CAT_MST();
            obj.cat_name3 = drpval;
            if(ViewState["SaveList"]!=null)
            {
                List = ((List < CAT_MST>)ViewState["SaveList"]).ToList();
            }
            List.Add(obj);
            ViewState["SaveList"] = List;
            GridView1.DataSource = ViewState["SaveList"];
            GridView1.DataBind();
        }