c# - 使用 LINQ 从列表列表中获取不同元素的计数

Question

我有一个结构，它是一个列表列表。下面是示例:

学生表

public partial class Students
{
        public string FirstName { get; set; }
        public string MiddleName { get; set; }
        public string LastName { get; set; }
        public int Marks { get; set; }
        public virtual ICollection<StudentAddresses> StudentAddresses{ get; set; }
}

public partial class StudentAddresses
{
    public string HouseArea {get; set;}
    public string HouseCity {get; set;}
    public string HouseZipCode {get; set;}
}

var student = context.Students
              .Include(i => i.StudentAddresses)
              .Where(c => c.Marks > 50);

我需要获得不同的 HouseZipCode 和相同的学生人数。

下面的代码不工作:

var test = student.GroupBy(g => g.StudentAddresses.Select(gs => gs.HouseZipCode ))
    .Select(s => new StudentModel { ZipCode= s.Key, StudentNumbers = s.Count() })
    .OrderBy(o => o.HouseZipCode )
    .ToList();

Answer 1

你快到了:

var test = student
    .SelectMany(s => s.StudentAddresses) // get all addresses from students
    .GroupBy(adr => adr.HouseZipCode) // group addresses by zip code
    .Select(grp => new StudentModel { ZipCode= grp.Key, StudentNumbers = grp.Count() }) // count how many addresses have same zip code
    .OrderBy(o => o.ZipCode)
    .ToList();