select * from User where 1=1

select * from User where 1=1 and DeptCode like '0041%' and PositionLevel > 3

#region 查询处理
            var query_products = EF.V_OrderDetail.Where(a => a.ActionType == 0 && a.Company_ID == UserInfo.CompanyID);
            if (orderstate.HasValue)
            {
                query_products = query_products.Where(b => b.State==orderstate);
            }
            if (product.HasValue && product.Value > 0)
            {
                query_products = query_products.Where(b => b.Product_ID == product.Value);
            }
            if (datebegin != null && datebegin.IsDatetime())
            {
                DateTime begin = datebegin.ToDateTimeShort();
                query_products = query_products.Where(a => a.ActionTime >= begin);
            }
            if (dateend != null && dateend.IsDatetime())
            {
                DateTime end = dateend.ToDateTimeShort().AddDays(1);
                query_products = query_products.Where(a => a.ActionTime < end);
            }
            if (!string.IsNullOrEmpty(disti_name))
            {
                query_products = query_products.Where(a => a.CustomerCompany.Contains(disti_name));
            }
            if (!string.IsNullOrEmpty(orderSN))
            {
                query_products = query_products.Where(a => a.OrderSN.Contains(orderSN));
            }
            #endregion

------解决方案--------------------
1L正解，如果要想进一步灵活控制查询条件生成，可以Google“Linq 表达式树”。