Grid View Selected key

<asp:GridView ID="gvExamInGr" runat="server" AutoGenerateColumns="False" EmptyDataText="Record Does Not Exist"
                            DataKeyNames="GrNo" OnSelectedIndexChanged="gvExamInGr_SelectedIndexChanged">


GrNo = Convert.ToInt64(gvExamInGr.SelectedDataKey.Value.ToString());