如何计算按嵌入式列表分组的列表项

Question

I have a class like

class MyClass
{
    public DateTime Date { get; set; }
    public List<int> IdList { get; set; }

    public MyClass(DateTime initDate)
    {
        Date = initDate;
        IdList = new List<int>();
    }
}

and need to count the number of entries in a List<MyClass> , grouped by each int in IdList .

I have experimented with various Linq constructs, but I cannot get anything to work. Here is what I have so far:

List<MyClass> myc = new List<MyClass>();
myc.Add(new MyClass(new DateTime(2016, 1, 1)) { IdList = new List<int> { 1, 2 } });
myc.Add(new MyClass(new DateTime(2016, 1, 2)) { IdList = new List<int> { 1, 3 } });
myc.Add(new MyClass(new DateTime(2016, 1, 3)) { IdList = new List<int> { 1, 4 } });
myc.Add(new MyClass(new DateTime(2016, 1, 4)) { IdList = new List<int> { 5, 6 } });
myc.Add(new MyClass(new DateTime(2016, 1, 5)) { IdList = new List<int> { 2, 3 } });

var grouped = from p in myc
    group p by p.IdList into g
    select new { Id = g.Key, Count = g.Count() };
foreach (var x in grouped)
{
    Console.WriteLine("ID: {0}, Count: {1}", x.Id, x.Count);
}

// Expecting output like:
// ID: 1, Count: 3
// ID: 2, Count : 2
// etc.

If there was a single int Id property in MyClass , it would be straightforward, but I cannot work out how to use the List<int> . Is there any alternative to writing nested loops and populating a Dictionary? Thanks for any help.

Answer 1

You can use SelectMany

var grouped = myc.SelectMany(x => x.IdList).GroupBy(x => x);
foreach (var i in g)
{
    Console.WriteLine(string.Format("Id: {0}, Count: {1}", i.Key,i.Count()));
}

This should give you the output you're looking for.

Answer 2

I don't know if I've understand your requeriment correctly. But try this and let me know:

var groupedIds = myc.SelectMany(x => x.IdList.Select(i => i))
    .GroupBy(x => x)
    .ToList();

The full fiddle here

And here SelectMany documentation so you know what this code means.

Hope this helps!