LINQ中的左外连接 [英] LEFT OUTER JOIN in LINQ

问题描述

如何在不使用 join-on-equals-into 子句的情况下在 C# LINQ 中对对象执行左外连接?有没有办法用 where 子句做到这一点?正确问题:对于内部连接很容易，我有这样的解决方案

How to perform left outer join in C# LINQ to objects without using join-on-equals-into clauses? Is there any way to do that with where clause? Correct problem: For inner join is easy and I have a solution like this

List<JoinPair> innerFinal = (from l in lefts from r in rights where l.Key == r.Key
                             select new JoinPair { LeftId = l.Id, RightId = r.Id})

但是对于左外连接，我需要一个解决方案.我的是这样的，但它不工作

but for left outer join I need a solution. Mine is something like this but it's not working

List< JoinPair> leftFinal = (from l in lefts from r in rights
                             select new JoinPair { 
                                            LeftId = l.Id, 
                                            RightId = ((l.Key==r.Key) ? r.Id : 0
                                        })

其中 JoinPair 是一个类:

public class JoinPair { long leftId; long rightId; }

推荐答案

如上所述:

101 LINQ 示例 - 左外连接

var q =
    from c in categories
    join p in products on c.Category equals p.Category into ps
    from p in ps.DefaultIfEmpty()
    select new { Category = c, ProductName = p == null ? "(No products)" : p.ProductName };

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