如何在对象列表上执行组

Question

我有一个目标列表。对象包含另一个列表。我想要在C#中根据内部列表执行组。

我正在使用下面的代码，但它不起作用。

class Abc
{
    public int id { get; set; }
    public List<xyz> courselist { get; set; }
}

class xyz
{
    public string course { get; set; }
}

public void PerformGroupBy 
{
    List<Abc> projects = new List<Abc>();

    var prj1 = new Abc();
    prj1.id = 100;
    prj1.courselist = new List<xyz>();
    prj1.courselist.Add(new xyz { course = "imr101" });
    prj1.courselist.Add(new xyz { course = "imr102" });
    projects.Add(prj1);

    var prj2 = new Abc();
    prj2.id = 101;
    prj2.courselist = new List<xyz>();
    prj2.courselist.Add(new xyz { course = "imr201" });
    prj2.courselist.Add(new xyz { course = "imr102" });
    projects.Add(prj2);

    var prj3 = new Abc();
    prj3.id = 103;
    prj3.courselist = new List<xyz>();
    prj3.courselist.Add(new xyz { course = "imr101" });
    prj3.courselist.Add(new xyz { course = "imr102" });
    projects.Add(prj3);

    var grp1= projects.GroupBy(x => x.courselist).ToList();
    // var grp2 = grp1;
}

预期结果

[
    {
        courselist: [{course:imr101},{course:imr102}]
        ids: [101,103]
    },
    {
        courselist: [{course:imr201},{course:imr102}]
        ids: [102]
    }
]

Answer 1

它没有像预期的那样工作，因为在您的情况下，您需要向您的IEqualityComparer方法提供一个GroupBy。

下面是我为您的案例编写的一篇文章(课程中的订单列表将被忽略，如果这很重要，只需删除OrderBy子句)：

internal class XyzListEqualityComparer : IEqualityComparer<List<xyz>>
{
    public bool Equals(List<xyz> x, List<xyz> y)
    {
        var xCourses = x
            .Select(subX => subX.course)
            .OrderBy(course => course);
        var yCourses = y
            .Select(subY => subY.course)
            .OrderBy(course => course);
        return xCourses.SequenceEqual(yCourses);
    }

    public int GetHashCode(List<xyz> obj)
    {
        int prime = 31;
        int result = 1;
        foreach(var item in obj.Select(course => course.course).OrderBy(course => course)) 
        {
            result = result * prime + item.GetHashCode();
        }
        return result;
    }
}

然后，您只需要编写这个来使用IEqualityComparer进行分组：

var grp1 = projects.GroupBy(x => x.courselist, x => x, new XyzListEqualityComparer()).ToList();

如果您不想使用IEqualityComparer，这里有一个算法可以完成这个任务，并给出您想要的确切输出(对于订单来说，性能不是最好的和相同的东西)。

var groupedItems = projects
    .GroupBy(project => string.Join(",", project.courselist.Select(course => course.course).OrderBy(course => course)))
    .Select(groupedProject => new { courseList = groupedProject.First().courselist, ids = groupedProject.Select(project => project.id) });

Answer 2

要按集合进行分组，必须为GroupBy函数指定适当的GroupBy函数。无论如何，即使没有这个，您也可以生成分组键：

var grp1 = projects.GroupBy(x => string.Join(",", x.courselist.Select(x => x.course))
    .Select(g => new 
    {
        courselist = g.First().courselist,
        ids = g.Select(x => x.Id).ToArray()
    })
    .ToList();