主题：[讨论]for each 循环

drs = DataTables("表A").Select("第1级 <> '""'")
For Each dr As DataRow In drs
    Dim jiezhi As List(Of DataRow) = DataTables("表A").Select("第1级 <> '""'" & "and" & "[序号] >'0'")
    For Each jiezhia As DataRow In jiezhi
        If jiezhia("第1级")<> "" Then
            MessageBox.show(jiezhia)
            Exit For
        Else
           MessageBox.show(“123”)
         End If
    Next
Next

在for each 中又用了一个for each （最里面的循环条件满足就exit for），外面的进行下一次循环，但是此时里面一层的for each不执行了

Dim jiezhi As List(Of DataRow) = DataTables("表A").Select("第1级 <> '""' and [序号] > 0")

判断 集合是否为空，我用if jiezhi is nothing then 不行，正确的代码应该怎么写