c# - 有没有办法使用 LINQ 将重复项目的集合与其子集合合并？

Question

示例类:

public class Pallet
{
    public int Id { get; set; }
    public List<Location> Locations { get; set; }
}

public class Location
{
    public int Id { get; set; }
}

鉴于此输入:

var input = new List<Pallet>
{
    new Pallet
    {
        Id = 1,
        Locations = new List<Location>
        {
            new Location { Id = 1 },
            new Location { Id = 3 }
        }
    },
    new Pallet
    {
        Id = 2,
        Locations = new List<Location>
        {
            new Location { Id = 2 }
        }
    },
    new Pallet
    {
        Id = 1,
        Locations = new List<Location>
        {
            new Location { Id = 1 },
            new Location { Id = 4 }
        }
    },
};

是否有一种很好的 LINQ 方法可以将重复项(在父集合和子集合中)折叠为与此等效的方法？

var output = new List<Pallet>
{
    new Pallet
    {
        Id = 1,
        Locations = new List<Location>
        {
            new Location { Id = 1 },
            new Location { Id = 3 },
            new Location { Id = 4 }
        }
    },
    new Pallet
    {
        Id = 2,
        Locations = new List<Location>
        {
            new Location { Id = 2 }
        }
    }
};

是的，我可以遍历集合并手动合并项目，但我很好奇 LINQ 是否会/可以提供更具表现力的东西。

Answer 1

您可以使用 GroupBy将匹配的 ID 收集在一起，然后是 Select创建新集合:

var output = input.GroupBy(pallet => pallet.Id)
    .Select(grp => new Pallet {
        Id = grp.Key,
        Locations = grp.SelectMany(pallet => pallet.Locations).Distinct().ToList()
    }).ToList();

上面的一个问题是 Distinct() 只有在类类型上才能正常工作，如果你为它提供 IEqualityComparer<Location> ，或者让“位置”类实现 IEquatable<Location> (也覆盖 object.GetHashCode ):

public class Location : IEquatable<Location>
{
    public int Id { get; set; }

    public bool Equals(Location other)
    {
        //Check whether the compared object is null.  
        if (object.ReferenceEquals(other, null)) return false;

        //Check whether the compared object references the same data.  
        if (object.ReferenceEquals(this, other)) return true;

        return Id == other.Id;
    }

    public override int GetHashCode()
    {
        return Id.GetHashCode();
    }
}

或者，您可以只使用第二个 GroupBy 而不是所有的爵士乐在每个组中选择第一个位置:

var output = input.GroupBy(pallet => pallet.Id)
    .Select(grp => new Pallet {
        Id = grp.Key,
        Locations = grp.SelectMany(pallet => pallet.Locations)
            .GroupBy(location => location.Id)
            .Select(location => location.First())
            .ToList()
    }).ToList();