主题：DataRowState.Deleted的作用？

Dim dt As DataTable = DataTables("订单")
Dim sum As Long
For Each dr As DataRow In dt.DataRows
If dr.RowState = DataRowState.Added Then
sum = sum + dr("数量")
End If
Next
Output.Show(Sum)

但其中的DataRowState.Deleted却不起作用，如：

Dim sum As Long = 0
For Each dr As DataRow In dt.DataRows
If dr.RowState = DataRowState.Deleted Then
sum = sum + 1
End If
Next
Output.Show(Sum)

显示结果为0  

请教狐爸，该如何统计已执行删除但还没保存的记录？？