Example of an index in Entitas-CSharp

NameIndex.cs

class NameIndex {
	
	Group observedCollection;
	Dictionary<string, Entity> lookup = new Dictionary<string, Entity>();

	public NameIndex(Pool pool){
		observedCollection = pool.GetGroup(Matcher.Name);

		observedCollection.OnEntityAdded += AddEntity;
		observedCollection.OnEntityRemoved += RemoveEntity;
	}

	~NameIndex()
	{
		Cleanup();
	}
	
	public void Cleanup()
    {
        observedCollection.OnEntityAdded -= AddEntity;
		observedCollection.OnEntityRemoved -= RemoveEntity;
		lookup.Clear();
    }


    public Entity FindEntityWithName(string name)
    {
    	return lookup[name];
    }

    protected virtual void AddEntity(Group collection, Entity entity, int index, IComponent component)
	{
		var nameComponent = component as NameComponent;
		if(nameComponent != null){
			if(lookup.ContainsKey(nameComponent.value) && lookup[nameComponent.value] == entity){
				return;
			}

			if(lookup.ContainsKey(nameComponent.value) && lookup[nameComponent.value] != entity){
				throw new Exception("the key " + nameComponent.value + " is not unique. Present on entity: " + entity.creationIndex + " and entity: " + lookup[nameComponent.value].creationIndex);
			}
			entity.Retain(this);
			lookup[nameComponent.value] = entity;
		}

	}

	protected virtual void RemoveEntity(Group collection, Entity entity, int index, IComponent component)
	{
		var nameComponent = component as NameComponent;
		if(nameComponent != null && lookup.ContainsKey(nameComponent.value)){
			lookup[nameComponent.value].Release(this);
			lookup.Remove(nameComponent.value);
		}
	}
}