"Dictionary" with key as part of stored elements?

Question

Seems like I forgot a typename ... Something inside my head keeps telling me that I have stumbled across a Dictionary that basicly uses a user-defined Comparer for lookups. But I somehow can't find that class in the depths of .Net anymore.

As I have no real idea how to describe what I am looking for without describing a possible implmenentation, I will also give an example of what I wan't to do.

I am basicly looking for a way to achieve the following (pseudocode):

class CustomId
{
   // [...]
}
class Element
{
   CustomId id;
}
Container<CustomId, Element> myContainer = new Container(myCustomComparer)
myContainer.Add(new Element()) // No key specified
myElement = myContainer[new CustomId(...)]; // Retrieval with custom id

Maybe this technically isn't quite a dictionary, but I hope the idea is clear. Basicly the key is part of the stored element.

Answer 1

I think you are looking for KeyedCollection<TKey, TItem>. Create a class that inherits from this class and override the GetKeyForItem() method.

Answer 2

It's actually quite easy. you just have to inherit from KeyedCollection<TKey, TItem>:

public class ElementCollection : KeyedCollection<CustomId, Element>
{
    public override CustomId GetKeyForItem(Element element)
    {
        return element.id;
    }
}

Answer 3

You could do a couple of things:

Use a simple List and use Linq to retrieve objects by CustomID:

var elements = myContainer.Where(e=>e.CustomID == new CustomID(...)).ToList();

Or, extend the generic Dictionary class to provide an Add overload that knows how to project the key from the given value:

public class AutoDictionary<K,V> : Dictionary<K,V>
{
   public Func<V,K> KeyGenerator { get; set; }
   public void Add(V value)
   {
      Add(KeyGenerator(V),V);
   }
}

...

var myContainer = new AutoDictionary<CustomId, Element>();
myContainer.KeyGenerator = e=> e.id;
myContainer.Add(myElement);
var elementFromDictionary = myContainer[myElement.id]; //will be the same instance as myElement