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?

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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?

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();

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();
    }
}

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

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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.