DataTable.Copy()

DataTable dtpocopy = dtPO.Copy();
DataRow[] dr = dtpocopy .Select("客户名称='" + cusName + "' and 机种代码='" + modelNo + "'", "订单数 Desc");

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DataRow[] dr = dtPO.Copy().Select("客户名称='" + cusName + "' and 机种代码='" + modelNo + "'", "订单数 Desc");