oracle - InvalidOperationException 与 ExecuteOracleScalar()

Question

我需要从数据库中选择值：

    string AttrIdent = "";           

    OracleCommand sel_attr = new OracleCommand(@"

    select 
        IDENTIFIER_ 
    from 
        TBL_A_CUSTOM_FORM_BASE_ATTR
    where 
        TEMPLATEUUID in(
                        select UUID 
                        from TBL_A_CUSTOM_FORM_TEMPLATE
                        where PARENTUUID =:projid) 
        and IDENTIFIER_ in ('xxx',
                            'yyy',
                            'zzz')", conn);

    sel_attr.Parameters.Add("projid", OracleType.NVarChar).Value = id;
    sel_attr.CommandType = CommandType.Text;
    AttrIdent = Convert.ToString(sel_attr.ExecuteOracleScalar());

但我得到了 InvalidOperationException

 AttrIdent = Convert.ToString(sel_attr.ExecuteOracleScalar());

我做错了什么？