在 C# 中连接多个对象列表

Question

我有三个列表，其中包含具有以下结构的对象：

List1

 - Status
 - ValueA

List2

- Status
- ValueB

List3

- Status
- ValueC

我想按状态合并列表以获得包含具有以下结构的对象的最终列表：

- Status
- ValueA
- ValueB
- ValueC

并非每个列表都具有所有状态。所以一个简单的（左）连接不会这样做。任何想法如何达到预期的结果？我试过

var result = from first in list1
    join second in list2 on first.Status equals second.Status into tmp1
    from second in tmp1.DefaultIfEmpty()
    join third in list3 on first.Status equals third.Status into tmp2
    from third in tmp2.DefaultIfEmpty()
    select new { ... };

但结果缺少状态。这是完整的 MRE：

using System;
using System.Linq;
using System.Collections.Generic;

public class Program
{
    public static void Main()
    {
        List<A> first = new List<A>() { new A("FOO", 1), new A("BAR", 2) };
        List<B> second = new List<B>() { new B("FOO", 6), new B("BAR", 3) };
        List<C> third = new List<C>() { new C("BAZ", 5) };
        
        var result = from f in first
            join s in second on f.Status equals s.Status into tmp1
            from s in tmp1.DefaultIfEmpty()
            join t in third on f.Status equals t.Status into tmp2
            from t in tmp2.DefaultIfEmpty()
            select new 
            {
                Status = f.Status,
                ValueA = f.ValueA,
                ValueB = s.ValueB,
                ValueC = t.ValueC,
            };

    }
}

public record A(string Status, int ValueA);
public record B(string Status, int ValueB);
public record C(string Status, int ValueC);

Answer 1

这不能使用 left join.First 你必须得到所有 keies，然后使用所有 keies left 加入其他列表：

var keys = first.Select(item => item.Status).ToList();
keys.AddRange(second.Select(item => item.Status));
keys.AddRange(third.Select(item => item.Status));

keys = keys.Distinct().ToList();

var result = (from k in keys JOIN
       f in first on k equals f.Status into tmp0
       from f in tmp0.DefaultIfEmpty()
       join s in second on k equals s.Status into tmp1
       from s in tmp1.DefaultIfEmpty()
       join t in third on k equals t.Status into tmp2
       from t in tmp2.DefaultIfEmpty()
       select new {
             Status = k,
             ValueA = f?.ValueA,
             ValueB = s?.ValueB,
             ValueC = t?.ValueC,
       }
).ToList();

Answer 2

不幸的是，如果一个状态在一个列表中出现多次，那么应该发生什么，导致您的聚合只能为每个状态保存一个值。

One possibility 解决这个问题是：

using System;
using System.Linq;
using System.Collections.Generic;

public class Program
{
    public static void Main()
    {
        List<A> first = new List<A>() { new A("FOO", 1), new A("BAR", 2) };
        List<B> second = new List<B>() { new B("FOO", 6), new B("BAR", 3) };
        List<C> third = new List<C>() { new C("BAZ", 5) };

        var allStates = first.Select(a => a.Status)
            .Concat(second.Select(b => b.Status))
            .Concat(third.Select(c => c.Status))
            .Distinct();
        
        var result = allStates
            .Select(Status => new 
                    {
                        Status,
                        ValueA = first.FirstOrDefault(a => a.Status == Status),
                        ValueB = second.FirstOrDefault(b => b.Status == Status),
                        ValueC = third.FirstOrDefault(c => c.Status == Status),
                    });
        
        foreach (var item in result)
        {
            Console.WriteLine(item);
        }
    }
}

public record A(string Status, int ValueA);
public record B(string Status, int ValueB);
public record C(string Status, int ValueC);

根据必须聚合的项目数量以及每个状态只出现一次或永远不会出现的前提，将您的列表转换为 Dictionary<string, A>、Dictionary<string, B> 等可能有意义. 改进查找并在聚合中做这样的事情：

ValueA = dictFirst.ContainsKey(Status) ? dictFirst[Status] : null

为了进一步改进（此行进行两次查找），您还可以分解出这样的方法

private static T GetValueOrDefault<T>(IReadOnlyDictionary<string, T> dict, string status)
{
    dict.TryGetValue(status, out T value);
    return value;
}

并在 .Select() 方法中用

调用它

ValueA = GetValueOrDefault(firstDict, Status);

可以通过以下方式为列表创建字典：

var firstDict = first.ToDictionary(a => a.Status);

Answer 3

假设每个列表的状态名称都是唯一的，这里是一个解决方案在 switch expressions（自 C# 8.0 起可用）的帮助下在单个查询中：

using System;
using System.Linq;
using System.Collections.Generic;

List<A> first = new List<A>() { new A("FOO", 1), new A("BAR", 2) };
List<B> second = new List<B>() { new B("FOO", 6), new B("BAR", 3) };
List<C> third = new List<C>() { new C("BAZ", 5) };

var result = first
    // concat lists together
    .Cast<object>()
    .Concat(second)
    .Concat(third)
    // group on Status value with help of switch expression
    .GroupBy(el => el switch {
        A a => a.Status,
        B b => b.Status,
        C c => c.Status,
    },
    // project groups with anonymous type
    (Status, group) => new { 
        Status,
        ValueA = group.OfType<A>().Select(a => a.ValueA).Cast<int?>().FirstOrDefault(),
        ValueB = group.OfType<B>().Select(b => b.ValueB).Cast<int?>().FirstOrDefault(),
        ValueC = group.OfType<C>().Select(c => c.ValueC).Cast<int?>().FirstOrDefault()
    }); 

public record A(string Status, int ValueA);
public record B(string Status, int ValueB);
public record C(string Status, int ValueC);

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