左连接两个列表并使用 Linq 从右侧维护一个属性

Posted 2023-04-18

【中文标题】左连接两个列表并使用 Linq 从右侧维护一个属性

【英文标题】：Left join on two Lists and maintain one property from the right with Linq

【发布时间】：2016-03-01 21:26:59

【问题描述】：

我有 2 个相同类型的列表。左侧列表：

var leftList = new List<Person>();
leftList.Add(new Person Id = 1, Name = "John", Changed = false);
leftList.Add(new Person Id = 2, Name = "Alice", Changed = false);
leftList.Add(new Person Id = 3, Name = "Mike", Changed = false);

以及正确的列表：

var rightList = new List<Person>();
rightList.Add(new Person Id = 1, Name = "John", Changed = false);
rightList.Add(new Person Id = 3, Name = "Mike", Changed = true);
rightList.Add(new Person Id = 4, Name = "Joshi", Changed = true);

我想做一个左连接，但使用右的Changed属性值。像这样：

Id = 1, Name = "John", Changed = false
Id = 2, Name = "Alice", Changed = false
Id = 3, Name = "Mike", Changed = true // <-- true from the rightList

为此，我不能使用简单的Left Join，也不能使用Concat with GroupBy。

我怎样才能用 linq 做到这一点？谢谢。

【参考方案1】：

这看起来像是一个非常标准的左外连接场景。

我总是将此扩展方法方便地用于左外连接，因此我不必查找如何使用讨厌的查询语法（或记住 GroupJoin 是什么）...

public static class LinqEx

    public static IEnumerable<TResult> LeftOuterJoin<TOuter, TInner, TKey, TResult>(
        this IEnumerable<TOuter> outer, 
        IEnumerable<TInner> inner, 
        Func<TOuter, TKey> outerKeySelector, 
        Func<TInner, TKey> innerKeySelector, 
        Func<TOuter, TInner, TResult> resultSelector)
    
        return outer
            .GroupJoin(inner, outerKeySelector, innerKeySelector, (a, b) => new
            
                a,
                b
            )
            .SelectMany(x => x.b.DefaultIfEmpty(), (x, b) => resultSelector(x.a, b));
    

现在你可以：

leftList.LeftOuterJoin(
     rightList, 
     lft => lft.Id,
     rgt => rgt.Id,
     (lft, rgt) => new PersonId = lft.Id, 
                              Name = lft.Name, 
                              Changed = rgt == null ? lft.Changed : rgt.Changed)

【讨论】：

你会在最后一行得到一个 nullref。更改后应分配rgt == null ? lft.Changed : rgt.Changed

@CameronMacFarland：Ta！

您好，感谢您的帮助，抱歉耽搁了，我在这里很忙。不错的扩展，谢谢分享！

@RenanAraújo 哈哈！没关系。 Resharper 几乎是为我写的！

【参考方案2】：

为什么不试试这样的解决方案：

var query = (from left in leftList
    join right in rightList on left.Id equals right.Id into joinedList
    from sub in joinedList.DefaultIfEmpty()
    select new Person  
        Id = left.Id,
        Name = left.Name,
        Changed = sub == null ? left.Changed : sub.Changed ).ToList();

【参考方案3】：

spender 比我快，我没有任何扩展方法。

没有任何扩展方法：

List<Person> mergedList =  leftList
            .GroupJoin(
                rightList, left => left.Id, right => right.Id, 
                (x, y) => new  Left = x, Rights = y  
            )
            .SelectMany(
                x => x.Rights.DefaultIfEmpty(),
                (x, y) => new Person
                
                    Id = x.Left.Id, 
                    Name = x.Left.Name, 
                    Changed = y == null ? x.Left.Changed : y.Changed
                
            ).ToList();

GroupJoin 进行左外连接操作。

【参考方案4】：

另一种方法是：

//Step 1: Merge the lists while selecting the matching objects from rightList using Last()
var mergedList = leftList.Concat(rightList)
 .GroupBy(x => x.Id)
 .Select(x => x.Last());

//Step 2: Do a inner join between mergedList and leftList to get a left join result as originally required.
var innerJoinQuery = from mPerson in mergedList
                     join leftPerson in leftList on mPerson.Id equals leftPerson.Id
                     select new  Id = leftPerson.Id, Name = mPerson.Name, Changed = mPerson.Changed ;