按属性分组的列表以及出现多次的列表

Question

I have a list of objects of type student.

 public class Student
 {
    string FirstName
    string Surname
    int Age
 }

I want my to group students with the same surname together and return a list of those surnames where a surname appears more than once. Eg,

 Surname
 Bloggs
 Joe
 Bloggs
 Smith
 Tom
 Smith

My new list would just have

 Bloggs
 Smith

Here is the code I am currently using which works however is there a way in my linq statement to specify to return surnames that appear more than once?

var res = hldList.GroupBy(hld => hld.MyID).ToList();

foreach(var id in res)
{
    var total = id.Count();
    if(total >1)
    // do something
}

Answer 1

Group list on SurName and filter those whose count is greater than 1 :

var duplicateSurNames = ldList.GroupBy(hld => hld.Surname)
                              .Where(g=>g.Count() > 1)
                              .Select(x=>x.Key).ToList();

Answer 2

hldList
  .GroupBy(hld => hld.SurName)
  .Select(hld => new { key = hld.Key, count = hld.Count() })
  .Where(hld => hld.count > 1)
  .ToList();

if you don't want the counts, you can add another projection at the end:

  .Where(hld => hld.count > 1)
  .Select(hld => hld.key)
  .ToList();