在 linq 的外连接中使用过滤器

我有以下实体：

public class Company
    {
        public string CompanyName { get; set; }
        public int ID { get; set; }
    }

public class CompanyCurrency
{
    public int Id { get; set; }
    public int CompanyId { get; set; }
    public decimal Rate { get; set; }
    public int CurrencyId { get; set; }
}

public class Currency
{
    public int ID { get; set; }
    public string Name { get; set; }
}

我需要获取一个国家的货币列表。如果一个国家/地区没有货币条目，我也需要为该缺少的条目添加一行。

我现在的声明是：

var  currencies  = 
from c in Currencies
join cc in CompanyCurrency 
on  c.ID equals cc.CurrencyId 
into jointable
from resultiten in  jointable.DefaultIfEmpty()


select new {c.Name  ,
HasEntry =  resultiten == null ? 0:1,  
rate =  resultiten != null ? resultiten.Rate:0 ,
}  ;

这不是由 CountryID 过滤的。我尝试添加过滤器

from c in Currencies
join cc in CompanyCurrency 
on  c.ID equals cc.CurrencyId 
into jointable
from resultiten in  jointable.DefaultIfEmpty()
where resultiten.CompanyId == 1 || resultiten == null


select new {c.Name  ,
HasEntry =  resultiten == null ? 0:1,  
rate =  resultiten != null ? resultiten.Rate:0 

但对于除公司 ID 1 以外的公司已输入的货币，这不会产生结果。

相应的 SQL 查询将是

select  * 
from [dbo].[Currency] c
left outer join [dbo].[CompanyCurrency] cc
on c.id = cc.Currencyid
and cc.[Companyid] = 1

您需要应用过滤器before the join:

join cc in CompanyCurrency.Where(e => e.CompanyId == 1)

或作为连接的一部分

on new { CurrencyId = c.ID, CompanyId = 1 } equals new { cc.CurrencyId, cc.CompanyId }

For inner join这并不重要，但是对于outer join这很重要（顺便说一句，同样适用于 SQL 查询）。