c# - Linq 加入空引用

Question

我的查询如下:

(from c in countries
 join r in Db.PaymentRates_VisaImmigrationPermit on c.CountryId equals r.HomeCountryId into countryRates
 from legalRate in countryRates.DefaultIfEmpty()
 join hostC in Db.Countries on legalRate.HostCountryId equals hostC.Id
 select new [...]

我在这一行得到一个空引用异常:

join hostC in Db.Countries on legalRate.HostCountryId equals hostC.Id

...这显然是由这一行引起的:

from legalRate in countryRates.DefaultIfEmpty()

仅当 legalRate 不为 null 时，我如何进行连接；以便在不引发空引用异常的情况下获取我想要的数据？

类似问题:Error in LINQ Left JOIN

Answer 1

您可以使用 DefaultIfEmpty 构造函数设置 legalRate 的默认值:

 from legalRate in 
     countryRates.DefaultIfEmpty(new CountryRate { HostCountryId = int.MaxValue })