Is it possible to create some IGrouping object

.net, c#, collections

Solution

If you really wanted to create your own `IGrouping<TKey, TElement>`, it is a simple interface to implement:

public class Grouping<TKey, TElement> : List<TElement>, IGrouping<TKey, TElement>
{
    public Grouping(TKey key) : base() => Key = key;
    public Grouping(TKey key, int capacity) : base(capacity) => Key = key;
    public Grouping(TKey key, IEnumerable<TElement> collection)
        : base(collection) => Key = key;
    public TKey Key { get; }
}

Note: you shouldn't try to allow the `Key` to be settable, mainly because the key should be managed by the collection that this Grouping is contained within.

This class inherits from `List<T>` and implements the `IGrouping` interface. Aside of the requirement of being an `IEnumerable` and `IEnumerable<TElement>` (which `List<T>` satisfies) the only property to implement is `Key`.

You could create `List` of these groups from scratch:

var groups = new List<Grouping<string, string>>();
groups.Add(new Grouping<string,string>("a", new string [] { "apple" }));
groups.Add(new Grouping<string,string>("p", new string [] { "peach", "pear" }));
groups.Add(new Grouping<string,string>("o", new string [] { "orange" }));

// inline variant:
groups = new List<Grouping<string, string>>
{
    new Grouping<string, string>("a", new string[] { "apple" }),
    new Grouping<string, string>("p", new string[] { "peach", "pear" }),
    new Grouping<string, string>("o", new string[] { "orange" }),
};

Or you could use this structure to append new groups to the results of a previous Linq `GroupBy` expression that has been evaluated into a list:

var strings = new string [] { "apple", "peach", "pear" };
var groups = strings.GroupBy(x => x.First().ToString()).ToList();
…
// Inject a new item to the list, without having to re-query
groups.Add(new Grouping<string,string>("o", new string [] { "orange" }));

If you need to add Items to the groups resolved from an IGrouping expression you can cast the Linq results into a `List` of `Grouping`:

var strings = new string [] { "apple", "peach", "orange" };
var groupExpression = strings.GroupBy(x => x.First().ToString());
var editableGroups = groupExpression.Select(x => new Grouping<string,string>(x.Key, x)).ToList();
…
// Add "pear" to the "p" list, with a check that the group exits first.
var pGroup = editableGroups.FirstOrDefault(x => x.Key == "p");
if (pGroup == null)
    editableGroups.Add(new Grouping<string, string>("p", new string[] { "pear" }));
else
    pGroup.Add("pear");

Problem

Original source