What is the implementing class for IGrouping?

Question

I am trying create a WCF Data Services ServiceOperation that does grouping on the server side and then sends the data down to the client.

When I try to call it (or even connect to the service) I get an error. It says that it can't construct an interface.

The only interface I am using is IGrouping.

What is a/the actual class for this interface?

---

Update:

I checked the type while debugging a sample app and it told me it was:

System.Linq.Lookup<TKey,TElement>.Grouping

But what assembly is it in?

Pericardium

See #5073455 – Distinguished

WCF Data Services ServiceOperation? Or WCF operation? – Tiu

System.Core has all the LINQ implementations. – Tiu

@JeffN825 - I forgot to clarify that. It is a WCF Data Services ServiceOperation. I updated my question. – Pericardium

System.Core (in System.Core.dll) msdn.microsoft.com/en-us/library/bb344977.aspx – Rodi

That has IGrouping. But I don't see Grouping (or anything that implements IGrouping) in there. (Maybe I am just blind?) – Pericardium

@Pericardium Can you click on it and hit F12 (In VS) to navigate to it's definition? – Rodi

@JonC - No that does not work. – Pericardium

Answer 1

Several types in the BCL implement IGrouping, however they are all internal and cannot be accessed except by the IGrouping interface.

But an IGrouping is merely an IEnumerable<TElement> with an associated key. You can easily implement an IGrouping that is backed by a List<TElement> and that should not be hard to serialize across a call boundary:

public class Grouping<TKey, TElement> : IGrouping<TKey, TElement> {

  readonly List<TElement> elements;

  public Grouping(IGrouping<TKey, TElement> grouping) {
    if (grouping == null)
      throw new ArgumentNullException("grouping");
    Key = grouping.Key;
    elements = grouping.ToList();
  }

  public TKey Key { get; private set; }

  public IEnumerator<TElement> GetEnumerator() {
    return this.elements.GetEnumerator();
  }

  IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); }

}

After applying the GroupBy operator you can create a list of Grouping instances:

var listOfGroups =
  source.GroupBy(x => ...).Select(g => new Grouping<TKey, TElement>(g)).ToList();

Hest

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

@aBetterGamer: I believe that the question is not about how to create an IGrouping which is quite easy as you point out but how to recreate "something" that implements IGrouping on the other side of a serialization boundary. – Hest

The answer helps me to grasp how IGrouping<key,value> can yield multiple values internally +1. – Quantic

Answer 2

Here is probably the most basic and generic implementation of IGrouping. Its constructor takes a key and a set of values.

public class Grouping<TKey, TElement> : IGrouping<TKey, TElement>
{
    private readonly TKey key;
    private readonly IEnumerable<TElement> values;

    public Grouping(TKey key, IEnumerable<TElement> values)
    {
        if (values == null)
            throw new ArgumentNullException("values");
        this.key = key;
        this.values = values;
    }

    public TKey Key
    {
        get { return key; }
    }

    public IEnumerator<TElement> GetEnumerator()
    {
        return values.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

Antherozoid

Answer 3

I checked the type with a sample app and it is this:System.Linq.Lookup<TKey,TElement>.Grouping. But what assembly is it in?

It's a type nested in System.Linq.Lookup<TKey,TElement>; internal to the System.Core assembly.

 var groupingType = "1".GroupBy(x => x).Single().GetType().GetGenericTypeDefinition();
 Console.WriteLine("Type: " + groupingType);
 Console.WriteLine("Public: " + groupingType.IsPublic);
 Console.WriteLine("Assembly: " + groupingType.Assembly);

Output:

Type: System.Linq.Lookup`2+Grouping[TKey,TElement]
Public: False
Assembly: System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089

---

As of .NET 4.0, there is no public type in the core .NET framework that implements System.Linq.IGrouping<TKey,TElement>. If you need such a type (say that's serializable), you'll probably have to roll one yourself, unfortunately.

Gaucherie