C# 列出联接列表并从另一个列表中筛选

需要一些帮助来构造正确的带有连接的LINQ查询

我有以下设置：

public class Student
{
    public string StudentName { get; set; }
    public string StudentEmail { get; set; }
}

public class Enrolled
{
    public Student Student { get; set; }
    public string Subject { get; set; }
}

public class Dropped
{
    public Student Student { get; set; }
    public string Subject { get; set; }
}


// Populating the List.
// Setup the Student List
List<Student> lstStudents = new List<Student>();

var John = new Student()
{
    StudentEmail = "john@stanford.edu",
    StudentName = "John Wayne"
};

var Maya = new Student()
{
    StudentEmail = "maya@stanford.edu",
    StudentName = "Maya Agnes"
};

var Eric = new Student()
{
    StudentEmail = "eric@stanford.edu",
    StudentName = "Eric James"
};

var Ellen = new Student()
{
    StudentEmail = "ellen@stanford.edu",
    StudentName = "Ellen Page"
};

lstStudents.Add(John);
lstStudents.Add(Maya);
lstStudents.Add(Eric);
lstStudents.Add(Ellen);

// Setup the Enrolled List
List<Enrolled> lstEnrolled = new List<Enrolled>();

// John
var JohnMath = new Enrolled() { Student = John, Subject = "Math" };
var JohnScience = new Enrolled() { Student = John, Subject = "Science" };
var JohnEnglish = new Enrolled() { Student = John, Subject = "English" };

// Maya
var MayaMath = new Enrolled() { Student = Maya, Subject = "Math" };

// Eric
var EricMath = new Enrolled() { Student = Eric, Subject = "Math" };
var EricScience = new Enrolled() { Student = Eric, Subject = "Science" };
var EricSocial = new Enrolled() { Student = Eric, Subject = "Social" };

// Ellen
var EllenMath = new Enrolled() { Student = Ellen, Subject = "Math" };
var EllenScience = new Enrolled() { Student = Ellen, Subject = "Science" };
var EllenEnglish = new Enrolled() { Student = Ellen, Subject = "English" };
var EllenSocial = new Enrolled() { Student = Ellen, Subject = "Social" };

lstEnrolled.Add(JohnMath);
lstEnrolled.Add(JohnScience);
lstEnrolled.Add(JohnEnglish);
lstEnrolled.Add(MayaMath);
lstEnrolled.Add(EricMath);
lstEnrolled.Add(EricScience);
lstEnrolled.Add(EricSocial);
lstEnrolled.Add(EllenMath);
lstEnrolled.Add(EllenScience);
lstEnrolled.Add(EllenEnglish);
lstEnrolled.Add(EllenSocial);

// Setup the Dropped List
List<Dropped> lstDropped = new List<Dropped>();

// John dropped Math
var JohnDropMath = new Dropped() { Student = John, Subject = "Math" };

// Eric dropped Social
var EricDropSocial = new Dropped() { Student = Eric, Subject = "Social" };

// Ellen Dropped Math
var EllenDropMath = new Dropped() { Student = Ellen, Subject = "Math" };

lstDropped.Add(JohnDropMath);
lstDropped.Add(EricDropSocial);
lstDropped.Add(EllenDropMath);

到目前为止，我已经构建了以下内容：

var StudentsAndCurrentSubjects = (from st in lstStudents
                                  join en in lstEnrolled 
                                      on st.StudentName equals en.Student.StudentName
                                  select new
                                  {
                                      st.StudentName,
                                      en.Subject
                                  })
                                  .ToList();

但它给了我每个人每个主题的结果

关于如何从列表中排除掉的项目，我遇到了一个难题

我正在考虑遍历删除的列表，如：

foreach(d in lstDropped) 
{
    // Logic to Remove it from the StudentsAndCurrentSubjects 
}

但是我觉得它效率很低（特别是如果我有很多行的话）

我也不知道如何将主题连接在一行中

这里需要帮助

---

谢谢。

我们只需要首先从注册列表中删除掉的科目。然后按学生分组。 请注意，我们有每个学生的列表，它有

student

和

Subject

作为其字段，我们只需从每个学生中选择第一个学生的名字（因为他们都是相同的），并将这些科目连接在一起

var result = lstEnrolled.Where(e => !lstDropped.Any(d => d.Student == e.Student && d.Subject == e.Subject))  //omit dropped courses
     .GroupBy(x => x.Student) // group results by students
     .Select(x => new {
            Name = x.First().Student.StudentName.Split(' ').First(), 
            Subjects = string.Join(", ", x.Select(e => e.Subject)) 
       }).ToArray();

用于打印：

foreach(var x in result)
Console.WriteLine($"{x.Name}\t{x.Subjects}");

---

你就快到了。联接是正确的，您只需实际应用所需的分组：

var StudentsAndCurrentSubjects = (from st in lstStudents
                                  join en in lstEnrolled 
                                      on st.StudentName equals en.Student.StudentName
                                  select new
                                  {
                                      st.StudentName,
                                      en.Subject
                                  })
                                  .GroupBy(s => s.StudentName, d => d.Subject)
                                  .Select(grp => new { StudentName = grp.Key, Subjects = grp.ToList() })
                                  .ToList();

然后，您可以使用上面的投影来显示它。可能是这样的：

foreach (var grouping in StudentsAndCurrentSubjects) {
    var studentName = grouping.StudentName;
    var subjects = string.Join(", ", grouping.Subjects);
    Console.WriteLine($"{studentName}\t{subjects}");
}

---

使用

into

关键字尝试

GroupJoin

var studentsAndCurrentSubjects = (
  from student in lstStudents
  join enrollment in lstEnrolled
    on student equals enrollment.Student
    into enrollments
  join drop in lstDrop
    on student equals drop.Student
    into drops
  let classes = enrollments.Select(e => e.Subject).Except(drops.Select(d => d.Subject))
  select new
  {
    Student = student.StudentName,
    Subjects = string.Join(", ", classes)
  })
  .ToList();

或者，您可以使用

let

进行子查询。与其他解决方案相比，对于较大的列表，这将显著降低性能，但在数据库中应该可以

var studentsAndCurrentSubjects = (
  from student in lstStudents
  let enrollments = lstEnrolled.Where(x => student == x.Student).Select(x => x.Subject)
  let drops = lstDrops.Where(x => student == drop.Student).Select(x => x.Subject)
  let classes = enrollments.Except(drops)
  select new
  {
    Student = student.StudentName,
    Subjects = string.Join(", ", classes)
  })
  .ToList();

---

很好的解决方案。2个想法：#1可以使用

Name=x.Key.StudentName

作为名称#2个没有课程的学生将不会出现在输出中，这可能是需要的。OP明确定义了他想要的输出。我在代码中添加了一个实时演示，您可以检查。

var studentsAndCurrentSubjects = (
  from student in lstStudents
  let enrollments = lstEnrolled.Where(x => student == x.Student).Select(x => x.Subject)
  let drops = lstDrops.Where(x => student == drop.Student).Select(x => x.Subject)
  let classes = enrollments.Except(drops)
  select new
  {
    Student = student.StudentName,
    Subjects = string.Join(", ", classes)
  })
  .ToList();