Code:

public partial class cust_search : System.Web.UI.Page
    {

        private string GridViewSortDirection
        {
            get { return ViewState["SortDirection"] as string ?? "ASC"; }
            set { ViewState["SortDirection"] = value; }
        }

        private string GridViewSortExpression
        {
            get { return ViewState["SortExpression"] as string ?? string.Empty; }
            set { ViewState["SortExpression"] = value; }
        }
        
        protected void Page_Load(object sender, EventArgs e)
        {
       
        }

        DataTable BindData(string searchType, string whereClause, string searchCriteria)
        {
            ContactsCollection cc = new ContactsCollection();
            //custom method in BLL
			DataTable newDt = cc.searchContact("customer", searchCriteria, searchType, whereClause);

            ViewState["gvResults_DataSource"] = newDt;

            return newDt;
        }

        protected void searchGrid_PageIndexChanging(object sender, GridViewPageEventArgs e)
        {
            searchGrid.DataSource = SortDataTable((DataTable)ViewState["gvResults_DataSource"], true);
            searchGrid.PageIndex = e.NewPageIndex;
            searchGrid.DataBind();
        }

        protected void searchGrid_Sorting(object sender, GridViewSortEventArgs e)
        {
            GridViewSortExpression = e.SortExpression;

            int pageIndex = searchGrid.PageIndex;

            searchGrid.DataSource = SortDataTable((DataTable)ViewState["gvResults_DataSource"], false);
            searchGrid.DataBind();

            searchGrid.PageIndex = pageIndex;
        }

        private string GetSortDirection()
        {
            switch (GridViewSortDirection)
            {
                case "ASC":
                    GridViewSortDirection = "DESC";
                    break;
                case "DESC":
                    GridViewSortDirection = "ASC";
                    break;
            }
            return GridViewSortDirection;
        }

        protected DataView SortDataTable(DataTable dataTable, bool isPageIndexChanging)
        {
            if (dataTable != null)
            {
                DataView dataView = new DataView(dataTable);
                if (GridViewSortExpression != string.Empty)
                {
                    if (isPageIndexChanging)
                    {
                        dataView.Sort = string.Format("{0} {1}", GridViewSortExpression, GridViewSortDirection);
                    }
                    else
                    {
                        dataView.Sort = string.Format("{0} {1}", GridViewSortExpression, GetSortDirection());
                    }
                }
                return dataView;
            }
            else
            {
                return new DataView();
            }
        }
    }