List<Object> 到层次结构答案

【问题标题】：List<Object> to hierarchyList<Object> 到层次结构
【发布时间】：2016-10-19 07:47:41
【问题描述】：

我有一个List<Data>，其中有数据

public class Data
{
  public string SchoolId {get; set;}
  public string SchoolName {get; set;}
  public string TeacherId {get; set;}
  public string TeacherName {get; set;}
}

列表是扁平的：

SchoolId    SchoolName     TeacherId      TeacherName
1           X              1              Mr X
1           X              2              Mrs Y
2           Y              3              Mr Z
2           Y              1              Mr X

所以基本上一个老师可以属于很多学校。

如何将上面的平面列表转换为List<School>

public class School
{
     public School()
     {
          this.Teachers  = new List<Teacher>();
     }
     public string SchoolId { get; set; }
     public string SchoolName { get; set; }
     public List<Teacher> Teachers {get; set;}
}  


public class Teacher
{
    public string TeacherId { get; set; }
    public string TeacherName { get; set; }
}

【问题讨论】：

标签： c# linq hierarchy hierarchical-data

【解决方案1】：

如果是 Linq to objects，则使用此代码

var result = list.GroupBy(x=>new {x.SchoolId, x.SchoolName})
.Select(x=>
{
    var s = new School();
    s.SchoolId = x.Key.SchoolId;
    s.SchoolName = x.Key.SchoolName;
    s.Teachers.AddRange(x.Select(
        y => new Teacher
        {
            TeacherId = y.TeacherId,
            TeacherName = y.TeacherName
        }
    ));

    return s;
});

请注意，即使在具有相同Id 的Teacher 中，上述代码也会导致重复的Teacher 实例。

【讨论】：

s.Teachers 可能为空
@dotctor 它不应该用于新对象，因为它是在 School 构造函数中初始化的。
你是对的。看School的构造函数就行了。

【解决方案2】：

这种方法的顺序是 O(n^2) 并且不共享教师实例。

var schools = data
    .GroupBy(x => x.SchoolId)
    .Select(group => new School()
    {
        SchoolId = group.Key,
        SchoolName = group.First().SchoolName,
        Teachers = data.Where(x => x.SchoolId == group.Key)
            .Select(x => new Teacher()
            {
                TeacherId = x.TeacherId,
                TeacherName = x.TeacherName
            })
            .ToList()
    })
    .ToList();

如果你想分享老师的实例，那么你可以使用这个

var teachersById = data
    .GroupBy(x => x.TeacherId)
    .Select(group => new Teacher()
    {
        TeacherId = group.Key,
        TeacherName = group.First().TeacherName
    })
    .ToDictionary(x => x.TeacherId);

var schools = data
    .GroupBy(x => x.SchoolId)
    .Select(group => new School()
    {
        SchoolId = group.Key,
        SchoolName = group.First().SchoolName,
        Teachers = teachersById 
            .Where(kv => data
                .Where(x => x.SchoolId == group.Key)
                .Select(x => x.TeacherId)
                .Contains(kv.Key)
            )
            .Select(x => x.Value)
            .ToList()
    })
    .ToList();

【讨论】：

第二个不应该是简单的Teachers = group.Select(x => teachersById[x.TeacherId]).ToList()（真正利用字典）

【解决方案3】：

List<Data> dataList = new List<Data>();

IEnumerable<School> schools = from d in dataList
                              group d by new { SchoolId = d.SchoolId, SchoolName = d.SchoolName } into groupSchool
                              select new School { SchoolId = groupSchool.Key.SchoolId, SchoolName = groupSchool.Key.SchoolName, Teachers =new List<Teacher>(groupSchool.Select(x => new Teacher { TeacherId = x.TeacherId, TeacherName = x.TeacherName })) };

【讨论】：

【解决方案4】：

item.SchoolId注意：它是伪代码（对于简短的代码，我使用带参数的构造函数而不是使用属性）

List<Data> data = //....;
Dictionary<string, Teacher> teachers = new Dictionary<string, Teacher>();
Dictionary<string, School> schools = new Dictionary<string, School>();
foreach (var item in data)
{
   if (item.TeacherId not in teachers)
      teachers.add(item.TeacherId, new Teacher(item.TeacherId, item.TeacherName));
}
foreach (var item in data)
{
   if (item.SchoolId not in schools)
      schools.add(item.SchoolId , item.SchoolName, new School(item.SchoolId , teachers[item.SchoolId]));
}
List<School> gen_schools =     // get values from schools;

附：实际上，您使用了错误的数据库表示形式（您应该将学校的教师分成两个不同的表）。

【讨论】：