如何从 IGrouping 中获取值

Question

I have a question about IGrouping and the Select() method.

Let's say I've got an IEnumerable<IGrouping<int, smth>> in this way:

var groups = list.GroupBy(x => x.ID);

where list is a List<smth> .

And now I need to pass values of each IGrouping to another list in some way:

foreach (var v in structure)
{
    v.ListOfSmth = groups.Select(...); // <- ???
}

---

Can anybody suggest how to get the values ( List<smth> ) from an IGrouping<int, smth> in such a context?

Answer 1

Since IGrouping<TKey, TElement> implements IEnumerable<TElement> , you can use SelectMany to put all the IEnumerables back into one IEnumerable all together:

List<smth> list = new List<smth>();
IEnumerable<IGrouping<int, smth>> groups = list.GroupBy(x => x.id);
IEnumerable<smth> smths = groups.SelectMany(group => group);
List<smth> newList = smths.ToList();

Here's an example that builds/runs: https://dotnetfiddle.net/DyuaaP

Video commentary of this solution: https://youtu.be/6BsU1n1KTdo

Answer 2

foreach (var v in structure) 
{     
    var group = groups.Single(g => g.Key == v. ??? );
    v.ListOfSmth = group.ToList();
}

---

First you need to select the desired group. Then you can use the ToList method of on the group. The IGrouping is a IEnumerable of the values.

Answer 3

More clarified version of above answers:

IEnumerable<IGrouping<int, ClassA>> groups = list.GroupBy(x => x.PropertyIntOfClassA);

foreach (var groupingByClassA in groups)
{
    int propertyIntOfClassA = groupingByClassA.Key;

    //iterating through values
    foreach (var classA in groupingByClassA)
    {
        int key = classA.PropertyIntOfClassA;
    }
}

Answer 4

From definition of IGrouping :

IGrouping<out TKey, out TElement> : IEnumerable<TElement>, IEnumerable

you can just iterate through elements like this:

IEnumerable<IGrouping<int, smth>> groups = list.GroupBy(x => x.ID)
foreach(IEnumerable<smth> element in groups)
{
//do something
}

Answer 5

var groups = list.GroupBy(x => x.ID);

Can anybody suggest how to get the values (List) from an IGrouping<int, smth> in such a context?

"IGrouping<int, smth> group" is actually an IEnumerable with a key, so you either:

• iterate on the group or
• use group.ToList() to convert it to a List

foreach (IGrouping<int, smth> group in groups)
{
   var thisIsYourGroupKey = group.Key; 
   List<smth> list = group.ToList();     // or use directly group.foreach
}

Answer 6

If you have an IGrouping<GroupItem, ListItem> , and you want to access the items of type ListItem of this group without utilizing a foreach loop, it's very simple. The object of type IGrouping<GroupItem, ListItem> is of type IEnumerable<ListItem> as well, as it is defined as:

public interface IGrouping<out TKey, out TElement> : IEnumerable<TElement>, IEnumerable

So you can simply say:

foreach (IGrouping<GroupItem, ListItem> group in list.GroupBy(x => x.ID))
{
    IEnumerable<ListItem> itemsInThisGroup = group;
    // ...
}

If for some reason, it has to be a List<T> instead of an IEnumerable<T> , you can of course still call itemsInThisGroup.ToList() . But usually it's better not to if you needn't.

Answer 7

Simply do this:

// this will "split" the list into groups
var groups = list.GroupBy(x => x.ID);

// groups is a "collection of lists"
foreach (var sublist in groups)
{
  // now the sublist is only a part of the original list
  // to get which is the value of ID, you can use sublist.Key
}

You don't need Select().GroupBy(expr) makes "a list of lists", kind of.

Answer 8

Assume that you have MyPayments class like

 public class Mypayment
{
    public int year { get; set; }
    public string month { get; set; }
    public string price { get; set; }
    public bool ispaid { get; set; }
}

and you have a list of MyPayments

public List<Mypayment> mypayments { get; set; }

and you want group the list by year. You can use linq like this:

List<List<Mypayment>> mypayments = (from IGrouping<int, Mypayment> item in yearGroup
                                                let mypayments1 = (from _payment in UserProjects.mypayments
                                                                   where _payment.year == item.Key
                                                                   select _payment).ToList()
                                                select mypayments1).ToList();