EF core 中用lambda表达式和Linq的一些区别

转眼一看，又过了10几天没有写博客了，主要还是没有什么可以写的，因为遇到的问题都不是很有价值。不过最近发现用lambda表达式，比用Linq的代码量会少一些，而且也方便一些。不过两者都差不多，相差不是特别大。

在EF core中，表和表之中都有对应的关系，例如一对一，一对多，还有多对多。要实现一对一，一对多加上主外键，实现多对多则需要加上一个中间表，这样结构清晰，维护也非常的方便。

那我们在使用EF core 势必要用到非常多的跨表查询，来看看linq是怎样查找数据库中的信息的：

var query = from a in db.ExaminationOrderInfo
                        join b in db.ExaminationOrganization on a.OrganizationId equals b.OrganizationId
                        join c in db.Customerinfo on a.CustomerId equals c.CustomerId
                        join d in db.Supplierinfo on a.OrganizationId equals d.OrganizationId
                        where (a.IsDelete == 0)
                        && (appointmentResult == null || a.AppointmentResult == appointmentResult)
                        && (isPaperReport == null || a.IsPaperReport == isPaperReport)
                        && (examinationStatus == null || a.ExaminationStatus == examinationStatus)
                        && (name == null || a.Name == name)
                        && (id == null || a.IdCard == id)
                        && (organizationName == null || b.Name == organizationName)
                        && (customer == null || c.Name == customer)
                        && (supplierName == null || d.Name == supplierName)
                        orderby a.AppayTime
                        select new ExaminationOrderInfoDto
                        {
                            Type = a.Type == 1 ? "取消预约" : "预约",
                            IdCard = a.IdCard,
                            Name = a.Name,
                            StartTime = a.StartTime.ToString(),
                            EndTime = a.EndTime.ToString(),
                            AppayTime = a.AppayTime,
                            ExaminationTime = a.ExaminationTime.ToString(),
                            AppointmentResult = a.AppointmentResult == 2 ? "成功" : a.AppointmentResult == 1 ? "失败" : "待处理",
                            ExaminationStatus = a.ExaminationStatus == 1 ? "已登记" : "未登记",
                            IsPaperReport = a.IsPaperReport == 1 ? "已打印" : "未打印",
                            ElectronicReport = a.ElectronicReport,
                            Organization = b.Name,
                            Customer = c.Name,
                            SupplierName = d.Name
                        };

可以看出连接了三张表去查询也就是 join....on.... ，然后加上排序，这样代码会偏多一点。

那么我们来看看使用lambda表达式会怎么样：

 var query = db.ExaminationOrderInfo.Where(e => e.IsDelete == 0
            && (appointmentResult == null || e.AppointmentResult == appointmentResult)
            && (isPaperReport == null || e.IsPaperReport == isPaperReport)
            && (examinationStatus == null || e.ExaminationStatus == examinationStatus)
            && (string.IsNullOrEmpty(name) || e.Name == name)
            && (string.IsNullOrEmpty(customer) || e.Customerinfo.Name == customer)
            && (string.IsNullOrEmpty(id) || e.IdCard == id)
            && (string.IsNullOrEmpty(organizationName) || e.ExaminationOrganization.Name == organizationName)
            && (string.IsNullOrEmpty(supplierName) || e.ExaminationOrganization.SupplierInfo.Name == supplierName)).Select(e => new ExaminationOrderInfoDto
            {
                Type = e.Type == 1 ? "取消预约" : "预约",
                IdCard = e.IdCard,
                Name = e.Name,
                Expire = DateTime.Now > e.ExaminationTime ? "是" : "否",
                StartTime = e.StartTime.ToString(),
                EndTime = e.EndTime.ToString(),
                AppayTime = e.AppayTime,
                ExaminationTime = e.ExaminationTime.ToString(),
                AppointmentResult = e.AppointmentResult == 2 ? "成功" : e.AppointmentResult == 1 ? "失败" : "待处理",
                ExaminationStatus = e.ExaminationStatus == 1 ? "已登记" : "未登记",
                IsPaperReport = e.IsPaperReport == 1 ? "已打印" : "未打印",
                ElectronicReport = e.ElectronicReport,
                Organization = e.ExaminationOrganization.Name,
                Customer = e.Customerinfo.Name,
                SupplierName = e.ExaminationOrganization.SupplierInfo.Name
            }).OrderBy(e => e.AppayTime);

我们可以看到使用lambda表达式就没有太多的连接代码了，如果想要查询的话，直接就e.Customerinfo.Name 可以得到Customerinfo表的name了，这是为什么呢，因为ExaminationOrderInfo这个实体中有这个字段：

 public Customerinfo Customerinfo { get; set; }

这就将ExaminationOrderInfo和Customerinfo连接起来了，一个ExaminationOrderInfo对应一个Customerinfo，而相信大家也留意到了ExaminationOrganization这个表了，里面对应的关系是这样的：

public ICollection<ExaminationOrderInfo> ExaminationOrderInfo { get; set; }

也就是说一个ExaminationOrganization对应多个ExaminationOrderInfo。那么还有 e.ExaminationOrganization.SupplierInfo.Name，为何需要这样呢，是因为ExaminationInfo是通过ExaminationOrganization这张表和Supplier连接的。

相信大家已经清楚了这两张查询方式的不同，建议大家两种都去了解和熟悉，这样遇到问题的时候有多种思考方式，不过我一般建议大家用lambda表达式，这样代码非常简洁，易读性也较高。

最后，有什么错误请指出，互相学习。谢谢大家！