场景：公司会对供应商进行评级，临时供应商定为D级，D级供应商限定只能用一次。

def BeforeDoOperation(e):
    opCode=e.Operation.FormOperation.Operation.ToUpperInvariant();
    if(opCode=="SAVE"):
        billObj = this.View.Model.DataObject;
        billNo = billObj["BillNo"];
        Supplier = billObj["SupplierId"];
        SupplierId = Supplier["ID"];
        if Supplier["SupplierBase"][0]["SupplierGrade"] <> None:
            SupplierDJ = Supplier["SupplierBase"][0]["SupplierGrade"]["Number"];
            if SupplierDJ == "D":
                sql = "select FBILLNO from t_PUR_POOrder where FCANCELSTATUS <> 'B' and FDOCUMENTSTATUS <> 'Z' and FSUPPLIERID=" + str(SupplierId) + "order by FCREATEDATE";
                a = DBUtils.ExecuteScalar(this.Context, sql, None);
                if a <> None and a<>billNo:
                    e.Cancel = True;
                    this.View.ShowWarnningMessage("供应商等级为D且被单据"+str(a)+"使用过，单据保存失败！");

效果：