Linq，IQueryable得到最频繁的元素对象

Question

My main problem lies in understanding IGrouping<int, SubForm> . I want the most frequent object ( MyClass ). This is the code I have now:

var subForm =
            classes.GroupBy(c => c.SubFormId)
            .OrderByDescending(sf => sf.Count())
            .FirstOrDefault();

This returns IGrouping<int, SubForm> . To get the actual object, I have to do another FirstOrDefault() but the compiler shows that there might be a null exception. This is the code to get the actual subform, can it be done better?

var subForm =
            classes.GroupBy(c => c.SubFormId)
            .OrderByDescending(sf => sf.Count())
            .FirstOrDefault().FirstOrDefault().SubForm;

Answer 1

You could just check to make sure there is at least one item in your classes collection (so it is guaranteed there is at least one group) either at the end or the beginning:

if(classes.Any())
{
    var subForm = classes.GroupBy(c => c.SubFormId)
                         .OrderByDescending(sf => sf.Count())
                         .First().First().SubForm;

}

Or:

var topGroup = classes.GroupBy(c => c.SubFormId)
                      .OrderByDescending(sf => sf.Count())
                      .FirstOrDefault();

if(topGroup!=null)
  subForm = item.First().SubForm;

Answer 2

I think you want:

 var sfGroup = classes.GroupBy(c => c.SubFormId)
            .OrderByDescending(sf => sf.Count())                
            .FirstOrDefault();

  int count = sfGroup.Count();
  MyClass subForm = sfGroup.FirstOrDefault();

Answer 3

you could just use First . It would throw an exception if nothing is found.

Answer 4

var subForm = classes.GroupBy(c => c.SubFormId)
    .OrderByDescending(sf => sf.Count())
    .FirstOrDefault();

return subForm == null ? default(SubForm) : subForm.Select(s => s.SubForm);

Answer 5

You are missing a select:

var subForm = (
    from c in classes
    group c by c.SubFormId into g
    select g.Key)
    .FirstOrDefault();