Axapta multi records selected and operated:多行操作

Objective: We need to select multi sales order and then check each sales line in each sales order. Iused a menuitem button and set multiselected to yes then override clicked methed
目标：要求可以选择多行订单，然后检查每个订单得每一个订单行的净金额是否为空。
用的是一个MenuItem，多选必须设定为Yes，然后重载clicked方法
Code/实现代码 ：

    FormDataSource      fds ;
    SalesTable          _salesTable,_salesTable1,_salesTable2 ;
    SalesLine           _salesLine ;
    Anytype             salesidtemp[] ;
    str                 salesidall,sqltemp ;
    int                 a = 1 ;
    int                 b = 1 ;
    boolean             WarningYes = false ;
    ;

    //super();

    fds = element.dataSource(1);

    _salesTable1     = fds.getFirst(1) ;
    _salesTable2    = fds.getNext() ;

    // Judge single or multi SalesOrder
    //Single SalesOrder process method: use SalesTable_SalesIdAdvanced.text() to get single SalesOrder SalesId.
    if(_salesTable1.SalesId == _salesTable2.SalesId)
    {
        while select _salesLine where  _salesLine.LineAmount == 0 && _salesLine.SalesId == SalesTable_SalesIdAdvanced.text()
        {
            box::warning('Sales Order ID '+ any2str(_salesLine.SalesId) + ' Sales Line '+ int2str(_salesline.HDSH_OrderLineNum) + ' Amount is Null, Please Check it!','Warning!!');
            WarningYes         = true ;
        }
    }
    else
    {
        //It can't process single SalesOrder, only can process multi SalesOrder; use For sentence to get all SalesId
        for( _salesTable=fds.getFirst(1)?fds.getFirst(1): _salesTable ;
                            _salesTable ;
                            _salesTable = fds.getNext())
        {
            salesidtemp[a]              = _salesTable.SalesId ;
            a=a+1 ;
        }
        a = a-1;

        for (b=1 ; b <=a ; b++ )
        {
            while select _salesLine where  _salesLine.LineAmount == 0 && _salesLine.SalesId == salesidtemp[b]
            {
                box::warning('Sales Order ID '+ any2str(_salesLine.SalesId) + ' Sales Line '+ int2str(_salesline.HDSH_OrderLineNum) + ' Amount is Null, Please Check it!','Warning!!');
                WarningYes         = true ;
            }
        }
    }
    if (WarningYes==false)
    {
        box::info('We do not found the record which the amount is null!','Info!!') ;
    }
Question: For{} can't process once i selected only one record. So i judged it first and processed first if only selected one record. Then use For{} to process multi records selected. But Why????
问题：如果只选择单行的话，For{}是不能处理的。所有在For之前增加了一个判断是否只选择了单行的情况，单行单独处理，不知道这个是为什么？