问一下这种linq怎么写

[-一个大坑]

a是主表，b是为了让a表和c表有对应关系，如果a表的PaymentCode 有值时，a表、b表、c表联合查询，如果a表的PaymentCode 为空时，就不用和c表联合查询，AgentName ，ChannelCategory 为string.Empty就行

                              (from a in _service.GetList()
                               join b in _DlrPaymentService.GetList() on a.PaymentCode equals b.PaymentCode
                               into paymentCode
                               from payment in paymentCode.DefaultIfEmpty()
                               join c in _DlrBdAgentService.GetList() on payment.AgentName equals c.AgentName
                               into agentName
                               from agent in agentName.DefaultIfEmpty()
                               select new DLRRP04Model
                               {
                                   Emplid = a.Emplid,
                                   Deptid = a.Deptid,
                                   Cname = a.Cname,
                                   Pstatus = a.Pstatus == null ? string.Empty : EnumExtension.GetDescription((InterviewStatus)((int)a.Pstatus)),
                                   AgentName = agent == null ? string.Empty : agent.AgentShortName,
                                   ChannelCategory = payment==null?string.Empty:payment.ChannelCategory
                               })

我现在写的PaymentCode 为空时查询会报错"Object reference not set to an instance of an object."

[正怒月神]

left join 然后去除空值，不可以吗？ 我看你已经left join 了

[-一个大坑]

引用 2 楼 正怒月神 的回复:

left join 然后去除空值，不可以吗？
我看你已经left join 了

不行，我把a表的PaymentCode 改为null查询就报错，有数据就是正常的

[娃都会打酱油了]

DefaultIfEmpty有个重载方法允许你自己 定义默认值， 你可以设置一个非空值，这样保证与c 进行join时不会产生null异常，然后在select的地方对这个默认值做特殊处理

[-一个大坑]

引用 4 楼 娃都会打酱油了 的回复:

DefaultIfEmpty有个重载方法允许你自己 定义默认值， 你可以设置一个非空值，这样保证与c 进行join时不会产生null异常，然后在select的地方对这个默认值做特殊处理

awhere这样写长了查询会报错？

Expression<Func<dlrRP04Model, bool>> awhere
// (dlrRP04Model.InterviewDate==null ? true : o.InterviewDate == dlrRP04Model.InterviewDate) &&
            // (string.IsNullOrEmpty(dlrRP04Model.Plant) ? true : o.Plant == dlrRP04Model.Plant) &&
            // (string.IsNullOrEmpty(dlrRP04Model.Deptid) ? true : o.Deptid == dlrRP04Model.Deptid) &&
            // (string.IsNullOrEmpty(dlrRP04Model.Emplid) ? true : o.Emplid == dlrRP04Model.Emplid) &&
            // (dlrRP04Model.Hdate==null ? true : o.Hdate == dlrRP04Model.Hdate) &&
            // (dlrRP04Model.Pstatus==null ? true : o.Pstatus == dlrRP04Model.Pstatus) &&
            // (string.IsNullOrEmpty(dlrRP04Model.ChannelCategory) ? true : o.PaymentCodeNavigation.ChannelCategory==dlrRP04Model.ChannelCategory) &&
            // (string.IsNullOrEmpty(drpChannelItem) ? true : o.PaymentCodeNavigation.ChannelItem==drpChannelItem);

public List<T> GetList(Expression<Func<T, bool>> @where)
        {
            return _repository.Get(@where).ToList();
        }

 public virtual bool Exist(Expression<Func<T, bool>> @where = null)
        {
            return Get(where).Any();
        }

[-一个大坑]

引用 1 楼 caozhy 的回复:

出于可读性的考虑，应该分2个linq来写，用if判断

这样吗？感觉写的有点长

var dlrRP04List1 = (from a in _service.GetList(o => o.PaymentCode == null)
                                join b in _DlrPaymentService.GetList() on a.PaymentCode equals b.PaymentCode
                                into paymentCode
                                from payment in paymentCode.DefaultIfEmpty()
                                select new DLRRP04Model
                                {
                                    Emplid = a.Emplid,
                                    Plant = a.Plant,
                                    Deptid = a.Deptid,
                                    Cname = a.Cname,
                                    Sex = a.Sex,
                                    Nation = a.Nation,
                                    Identitycard = a.Identitycard,
                                    InterviewDate = a.InterviewDate,
                                    Hdate = a.Hdate,
                                    PredeliverDate = a.PredeliverDate,
                                    Pstatus = a.Pstatus == null ? string.Empty : EnumExtension.GetDescription((InterviewStatus)((int)a.Pstatus)),
                                    AgentName = string.Empty,
                                    ChannelCategory = payment == null ? string.Empty : payment.ChannelCategory
                                });

            var dlrRP04List2 = (from a in _service.GetList(o => o.PaymentCode != null)
                                join b in _DlrPaymentService.GetList() on a.PaymentCode equals b.PaymentCode
                                into paymentCode
                                from payment in paymentCode.DefaultIfEmpty()
                                join c in _DlrBdAgentService.GetList() on payment.AgentName equals c.AgentName
                                into agentName
                                from agent in agentName.DefaultIfEmpty()
                                select new DLRRP04Model
                                {
                                    Emplid = a.Emplid,
                                    Plant = a.Plant,
                                    Deptid = a.Deptid,
                                    Cname = a.Cname,
                                    Sex = a.Sex,
                                    Nation = a.Nation,
                                    Identitycard = a.Identitycard,
                                    InterviewDate = a.InterviewDate,
                                    Hdate = a.Hdate,
                                    PredeliverDate = a.PredeliverDate,
                                    Pstatus = a.Pstatus == null ? string.Empty : EnumExtension.GetDescription((InterviewStatus)((int)a.Pstatus)),
                                    AgentName = agent == null ? string.Empty : agent.AgentShortName,
                                    ChannelCategory = payment == null ? string.Empty : payment.ChannelCategory
                                });

            var dlrRP04List = dlrRP04List1.Concat(dlrRP04List2).ToList();

[threenewbee]

出于可读性的考虑，应该分2个linq来写，用if判断