使用 list<int> 作为 linq group by key

Question

这是来自 LinqPad（因此是 .Dump()）

void Main()
{
    var k1 = new List<int>(){1,2,3,4,5};
    var k2 = new List<int>(){1,2,3,4,5};

    var o1 = new A(){k=k1, objectName="o1"};
    var o2 = new A(){k=k2, objectName="o2"};

    var l = new List<A>();

    l.Add(o1);
    l.Add(o2);

    var b = from a in l 
    group a.objectName by a.k into g
    select new {g.Key, n = String.Join(",",g)};

    b.Dump();
}

public class A {
    public List<int> k;
    public string objectName;
}

问题是它不起作用，上面的代码产生：

我知道为什么会这样，因为列表对象是单独的对象，但我想知道是否有办法告诉 group by 使用列表的内容而不是对象实例。

Answer 1

我完成了以下任务：

var b = l.SelectMany(a => a.k.Select(i =>  new { Key = i, n = a.objectName }))
         .GroupBy(i => i.Key);

Answer 2

public static void Main()
{
    var k1 = new List<int>(){1,2,3,4,5};
    var k2 = new List<int>(){1,2,3,4,5};

    var o1 = new A(){k=k1, objectName="o1"};
    var o2 = new A(){k=k2, objectName="o2"};

    var l = new List<A>();

    l.Add(o1);
    l.Add(o2);

    // Use custom comparer for the list
    var b = l.GroupBy(a => a.k, new ListComparer<int>())
             .Select(g => new 
                {
                    Key = String.Join(",", g.Key.Select(i => i)), 
                    n = String.Join(",",g.Select(i => i.objectName))
                });


    foreach(var item in b)
    {
        Console.WriteLine(string.Format("{0} : {1}", item.Key, item.n));
        // 1,2,3,4,5 : o1,o2
    }
}

public class A
{
    public List<int> k;
    public string objectName;
}


public class ListComparer<T> : IEqualityComparer<List<T>>
{
    // Ignore the order and compare with the sequence value for the equality
    public bool Equals(List<T> left, List<T> right)
    {
        return left.OrderBy(i => i).SequenceEqual(right.OrderBy(i => i));
    }

    public int GetHashCode(List<T> list)
    {
        return list.Count;
    }
}

.Net Fiddle

Answer 3

您可以将数组转换为字符串。

var b = l.GroupBy(o => string.Join(";", o.k))
         .Select(g => new { Key = g.First().k,
                            n = string.Join(",", g.Select(o => o.objectName)) });

使用 list<int> 作为 linq group by key

Use list<int> as a linq group by key

c#

linq

grouping