求林中平均值的最大值

• 使用Linq"查询语法"

  var best = from student in students
             group student by student.Id into studentAverage
             let average = studentAverage.Average (s => s.Marks)
             orderby average descending
             select new { id = studentAverage.Key, average };
             // change projection to have only id or average
  

• 使用Linq"方法语法"

  var best =
      students
      .GroupBy (student => student.Id,
                student => student.Marks,
                (id, marks) => new { id, average = marks.Average () })
      .OrderByDescending (studentAverage => studentAverage.average)
      // add a projection (Select) here to have only id or average
      .FirstOrDefault ();
  

  也许更简单的替代方案，但更倾向于只获得Id

  var best =
      students
      .ToLookup (s => s.Id, s => s.Marks)
      .OrderByDescending (marksById => marksById.Average ())
      .FirstOrDefault ();
  // Key will contain the Id but Average must be calculated again
  // var avg = best.Average();
  

• 使用"经典"代码（较长但更清晰的步骤）

  var studentMarks = new Dictionary<int, List<double>> (students.Count);
  foreach (var student in students)
  {
      int id = student.Id;
      List<double> marks;
      if (!studentMarks.TryGetValue (id, out marks))
      {
          marks = new List<double> ();
          studentMarks.Add (id, marks);
      }
      marks.Add (student.Marks);
  }
  double? bestAverage = null;
  int? idOfBest = null;
  foreach (var idAndMarks in studentMarks)
  {
      var average = 0.0;
      foreach (var mark in idAndMarks.Value)
      {
          average += mark;
      }
      average /= idAndMarks.Value.Count;
      if (average > bestAverage)
      {
          bestAverage = average;
          idOfBest = idAndMarks.Key;
      }
  }
  

最佳学生

var best = i.GroupBy(g => g.Id)
            .Select(g => new { 
                StudentID = g.Key, 
                Name = g.First().Name, 
                Average = g.Average(m => m.Marks), 
                })
            .OrderByDescending(g => g.Average)
            .First();
// best = { StudentID = 1, Name = Maddy, Average = 84 }

最佳平均

var best = i.GroupBy(g => g.Id)
            .Select(g => g.Average(m => m.Marks))
            .Max();
// best = 84.0

查询语法（只是因为）

var best = (from student in 
                from grade in grades 
                group grade by grade.Id
             select student.Average(mark=>mark.Marks)).Max();    
// best = 84.0

没有LINQ（我看到一条评论建议你这样做…哎哟）

var dict = new Dictionary<int, List<double>>();
foreach (var g in i )
{
    var id = g.Id;
    if (!dict.ContainsKey(id))
        dict.Add(id, new List<double>());
    dict[id].Add(g.Marks);
}
double best = 0.0;
foreach (var g in dict)
{
    var totalmarks = 0.0;
    foreach (var mark in g.Value)
    {
        totalmarks += (double)mark;
    }
    var average = totalmarks / g.Value.Count;
    if (best < average)
        best = average;
}
// best = 84.0

没有LINQ（稍微优化）

var dict = new Dictionary<int, Tuple<int, double>>();
foreach (var g in i)
{
    var id = g.Id;
    Tuple<int, double> t;
    if (!dict.ContainsKey(id))
        dict.Add(id, t = Tuple.Create(0, 0.0));
    else
        t = dict[id];
    var c = t.Item1 + 1;
    dict[id] =  Tuple.Create(c, (t.Item2 * t.Item1 + g.Marks) / c);
}
double best = 0.0;
foreach (var g in dict)
{
    var m = g.Value.Item2;
    if (best < m)
        best = m;
}
// best = 84.0

var i = new List<Student>();
i.Add(new Student { Id = 1, Name = "Maddy", Subject = "English", Marks = 87 });
i.Add(new Student { Id = 1, Name = "Maddy", Subject = "Science", Marks = 81 });
i.Add(new Student { Id = 2, Name = "Mathew", Subject = "Maths", Marks = 83 });
i.Add(new Student { Id = 2, Name = "Mathew", Subject = "Science", Marks = 80 });

var students = i.GroupBy(p => p.Id).ToDictionary(group => group.Key, group => group.Select(p => p.Marks).Average());
var maxStudent = students.Aggregate((l, r) => l.Value > r.Value ? l : r).Key;

类似的东西？

 var bestStudent = i.GroupBy(i=>i.Id)
    .Select( g=> new Student 
    { 
        Id = g.Key, 
        Name = g.First().Name, 
        Marks = g.Average(i=>i.Marks)) 
    })
    .OrderByDescending(s => s.Marks)
    .FirstOrDefault();