customvalidator inside gridview doesnot work inside gridview

Question

  CustomValidator custval = new CustomValidator();

            custval = (CustomValidator)sender;

            GridViewRow gvr = custval.NamingContainer as GridViewRow;

            TextBox txtGender = (TextBox)gvr.FindControl("txtGender");

            Label lblRequired = (Label)gvr.FindControl("lblRequired");

            DataTable dt = (DataTable)ViewState["ValidationData"];

            string qry = string.Empty;

            DataRow[] drow;

            qry = qry + " Gender = '" + txtGender.Text + "'";

            //bool _isValid = (string.IsNullOrEmpty(txtNm.Text));

            if (dt.Rows.Count > 0)

            {

                drow = dt.Select(qry);

                if (drow.Length > 0)

                {

args.IsValid=false;

}

else

{

args.IsValid=true;

}