Is it possible to create some IGrouping object

Question

I have List<IGrouping<string,string>>.

Is is somehow possible to add new item to this list? Or actually, is it possible to create some IGrouping object?

Heredes

The answers below show you how you could do this, but you probably want to considering implementing a Dictionary, Lookup or other structure more suited to your purpose. .NET has plenty of great classes to help with structuring your data and I'm confident that something already exists that will meet your needs. – Amberambergris

As this caused a lot of questions why I do this, here it is: I queried to get result and stored it, and I knew that source has changed and I believed that it was less costly to add new element manually than to query again. – Bobolink

Answer 1

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");

Harpp

So when I instantiate this, I can obviosly set the key, but how do I set the items? – Zingale

Since MyGrouping<TKey, TElement> inherits from List<T>, it has all the properties and methods of that class. See it's documentation here: msdn.microsoft.com/en-us/library/6sh2ey19.aspx – Harpp

Sorry I meant to post a follow up comment. I used AddRange() to dump in an existing list. – Zingale

Probably the easiest approach would be something like: var newGrouping = new List<MyType>().GroupBy(t => t.MyKey); – Xanthene

Enhanced version: ' public class Grouping<TKey, TSource> : List<TSource>, IGrouping<TKey, TSource> { public Grouping(TKey key) : base() => Key = key; public Grouping(int capacity, TKey key) : base(capacity) => Key = key; public Grouping(IEnumerable<TSource> collection, TKey key) : base(collection) => Key = key; public TKey Key { get; private set; } }' – Webster

Not all groups of data can easily or efficiently be formed from a single Linq statement, this solution allows us to build grouping manually, but still use the results in CollectionViewSource for Xaml collection controls. – Ramtil

Answer 2

As of .NET 4.0, there do not appear to be any public types in the BCL that implement the IGrouping<TKey, TElement> interface, so you won't be able to 'new one up' with any ease.

Of course, there's nothing stopping you from:

1. Creating a concrete type yourself that implements the interface, as @Nathan Anderson points out.
2. Getting an instance / instances of IGrouping<TKey, TElement> from a LINQ query such as ToLookup and GroupBy and adding it / them to your list.
3. Calling ToList() on an existing sequence of groups (from ToLookup / GroupBy).

Example:

IEnumerable<Foo> foos = ..
var barsByFoo = foos.ToLookup(foo => foo.GetBar());

var listOfGroups = new List<IGrouping<Foo, Bar>>();

listOfGroups.Add(barsByFoo.First()); // a single group
listOfGroups.AddRange(barsByFoo.Take(3)); // multiple groups

It's not clear why you would want to do this, though.

Stillage

Answer 3

IGrouping<TKey, TElement> CreateGroup<TKey, TElement>(IEnumerable<TElement> theSeqenceToGroup, TKey valueForKey)
{
    return theSeqenceToGroup.GroupBy(stg => valueForKey).FirstOrDefault();
}

Hegira

Excelent and simple solution – Danadanae

Short, but a lot of overhead for something that should be simple. – Pallua

If the source sequence is empty, this returns null instead of an empty group. – Jesu

Answer 4

You can also hack the grouping by not grouping on something within the list:

var listOfGroups = new[] { "a1", "a2", "b1" }
                       .GroupBy(x => x.Substring(0, 1))
                       .ToList();

// baz is not anything to do with foo or bar yet we group on it
var newGroup = new[] { "foo", "bar" }.GroupBy(x => "baz").Single();

listOfGroups.Add(newGroup);

listOfGroups then contains:

a:
    a1, a2
b:
    b1
baz:
    foo, bar

IDEOne example

Chockfull

Answer 5

Based on dovid's answer, I created the following extension method, that creates an instance of IGrouping<TKey, TElement> from an IEnumerable<TElement> and a key of type TKey:

public static IGrouping<TKey, TElement> ToGroup<TKey, TElement>(
    this IEnumerable<TElement> elements,
    TKey keyValue)
{
    return elements.GroupBy(_ => keyValue).FirstOrDefault();
}

It can be called like this:

var fruits = new [] { "Apples", "Bananas" };
var myFruitsGroup = fruits.ToGroup("fruitsKey");

Beware that ToGroup() can return null.

I also created an additional GroupBy extension method:

public static IEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector,
    Func<TSource, IEnumerable<TElement>> elementsSelector)
{
    return source
        .Select(s => elementsSelector(s)
            .ToGroup(keySelector(s)))
        .Where(g => g != default(IGrouping<TKey, TElement>));
}

It can be used like this:

var foodItems = new []
{
    new { Category = "Fruits", Items = new [] { "Apples", "Bananas" } },
    new { Category = "Vegetables", Items = new [] { "Tomatoes", "Broccoli" } },
};
var categoryGroups = foodItems.GroupBy(i => i.Category, i => i.Items);

File

Answer 6

var headers = from header in new[] {
  new { Name = "One", List = new[] { "One 1", "One 2", "One 2" } },
  new { Name = "Two", List = new[] { "Two 1", "Two 2", "Two 2" } }
} from value in header.List group value by header.Name;

Evapotranspiration

Answer 7

The IGrouping interface is for the GroupBy() operator in LINQ. You would normally get an IGrouping object from a LINQ query with a group by clause. It doesn't make much sense to have a list of groupings, though.

Grand

Well, given that the return type of GroupBy is IEnumerable<IGrouping<TKey, TElement>> is actually makes a lot of sense to have a list of groupings, considering that List is just one of the many ways to have an IEnumerable. – Fadein