c# - 如何在 C# 中按列表列表分组并将其合并为单个列表而不重复？

Question

我有一个名为 studentdetails 的类和一个名为 students 的属性，它是 studentdetails 的列表。

public class studentdetails 
{
    public int SubjectId  {get ; set; }
    public int studentId  { get; set; } 
    public int ClassId  { get; set; }     
}

List<studentdetails> students = new List<studentdetails>()
{ 
    new studentdetails() { studentId = 1, SubjectId = 1, ClassId = 1 },
    new studentdetails() { studentId = 2, SubjectId = 2, ClassId = 1 },
    new studentdetails() { studentId = 3, SubjectId = 1, ClassId = 2 },
    new studentdetails() { studentId = 1, SubjectId = 3, ClassId = 2 },
    new studentdetails() { studentId = 1, SubjectId = 3, ClassId = 1 }
};

我必须创建一个包含学生列表(其中包含作为属性的 sbject 列表)作为属性的类列表，而不重复类、学生和主题(如果它已经存在)。

例子:

public class Class
{
    public int ClassId  { get; set; }
    public Lits<student> students  { get; set; }
}
public class student
{
    public int StudentId  { get; set; }
    public Lits<Subject> subjects  { get; set; }
}
public class Subject 
{
    public int SubjectId   { get; set; }
}

例如:
引用以上学生详情

class(1) -student(1) -subject(1)
                     -subject(3)
         -student(2) -subject(2)

class(2) -student(3) -subject(1)
         -student(1) -subject(3)

Answer 1

下一个方法可以用来解决这个问题:

List<Class> classes = students
    // This GroupBy creates groups by ClassId:
    // (ClassId) -> (List of Students).
    .GroupBy(s => s.ClassId)
    .Select(c => new Class
    {
       ClassId = c.Key,
       Students = c
          // This GroupBy for given Class creates groups by StudentId:
          // (StudentId) -> (List of Subjects).
          .GroupBy(s => s.StudentId)
          .Select(s => new Student
          {
             StudentId = s.Key,
             Subjects = s
                // This GroupBy for given Class and Student removes
                // duplicate values of SubjectId. If you can guarantee
                // that for given Class and Student will not be duplicate
                // values of SubjectId then you can remove this GroupBy.
                // If you remove this GroupBy then you need to change 
                // expression inside Select to the following:
                // new Subject { SubjectId = t.SubjectId }.
                .GroupBy(t => t.SubjectId)
                .Select(t => new Subject { SubjectId = t.Key })
                .ToList()
          }).ToList()
    }).ToList();

这里是 complete sample这表明了这种方法。

@Thaks 在评论中问道:

If there is class name, student name and aсtivity name along with ids. Then how can I map accordingly to the instance?

如果您还需要映射 id 以外的属性那么你应该使用 GroupBy 的下一个重载方法: GroupBy(keySelector, comparer) .使用这种方法，我们可以使用 StudentDetails 的实例。类作为键并指定 comparer为他们。

首先我们应该创建 comparer , 实现接口(interface) IEqualityComparer 的类.在我们的示例中，我们可以使用单个 comparer类来执行所有三个 GroupBy操作，因为我们所有的 GroupBy使用 int Id 执行操作属性(property)。通常每个 GroupBy操作使用自己的 comparer因为大多数时候不同 GroupBy使用不同的键(不同的数据类型，不同数量的分组属性)执行操作。以下是我们如何实现 comparer :

// For demo I simplified implementation of the Equals and GetHashCode
// methods by excluding null checks. In the documentation of 
// IEqualityComparer you can find implementation with null checks.
public class ComparerById<T> : IEqualityComparer<T>
{
    private readonly Func<T, int> _keySelector;

    public ComparerById(Func<T, int> keySelector) => _keySelector = keySelector;

    public bool Equals(T x, T y) => _keySelector(x) == _keySelector(y);

    public int GetHashCode(T obj) => _keySelector(obj);
}

然后使用这个 comparer我们可以执行所需的 GroupBy :

List<Class> classes = students
    // Now key of each group has type StudentDetails, therefore later we
    // will be able to use properties of StudentDetails such as ClassName.
    // Here to compare keys of type StudentDetails we use comparer:
    // new ComparerById<StudentDetails>(s => s.ClassId);
    // It means that we create groups by ClassId.
    .GroupBy(s => s, new ComparerById<StudentDetails>(s => s.ClassId))
    .Select(c => new Class
    {
        ClassId = c.Key.ClassId,
        ClassName = c.Key.ClassName,
        Students = c
            // Here we create groups by StudentId.
            .GroupBy(s => s, new ComparerById<StudentDetails>(s => s.StudentId))
            .Select(s => new Student
            {
                StudentId = s.Key.StudentId,
                StudentName = s.Key.StudentName,
                Subjects = s
                    // Here we create groups by SubjectId.
                    .GroupBy(t => t, new ComparerById<StudentDetails>(t => t.SubjectId))
                    .Select(t => new Subject {SubjectId = t.Key.SubjectId, SubjectName = t.Key.SubjectName})
                    .ToList()
            }).ToList()
    }).ToList();

这里是 complete sample这表明了这种方法。