层层判断仍未排除的异常：“未将对象引用设置到对象的实例”

Question

dim sStr as string=","
try
dim dt as datatable =getData(1)
If dt.Rows.Count > 0 Then
 Dim drRows() As DataRow = dt.Select("F1 like 'BCX/%' ")
 
 If drRows.Length > 0 Then
     Dim i As Integer
     For i = 0 To drRows.Length - 1
         sStr += DBNULL2Str(drRows(i).Item("F1") + ","
     Next
 End If
End If

Catch ex As Exception
   log(ex) 
end Try

以上运行时，偶尔会出现log “System.NullReferenceException: 未将对象引用设置到对象的实例”
错误 代码竟然是这行： sStr += DBNULL2Str(drRows(i).Item("F1") + ","
由于调试正常，批量客户运行时，会这样，无法调试。
请问有什么可能？

Answer 1

有可能是dt获取时，并发访问产生的错误。

Answer 2

drRows(i).Item("F1") 这个值有可能为 nothing.