仅使用每个键从具有多个键的字典中检索值

Question

I have this situation where I am using a dictionary to store entities. Each entity has two ID's that I want to index by when retrieving values. One is generated as the object is created (I'll call this ID) and the other is read from an external source (I'll call this Handle). I've played around with ToLookup() and that is very, very slow. I've created a custom key object to contain both identifiers, but then it seems that basic use of a dictionary means you have to know both to retrieve the associated value. I need a very fast solution if I only know one or the other. Any help would be appreciated. I also tried creating a class that would internally contain two dictionaries, but then I guess the pointer references add to the memory footprint, so I'm not sure this is the right approach either. One of the reasons I'm somewhat concerned about the memory footprint is because I'm compiling this as a dll so that it can be loaded into a third-party system. I don't have any control over the amount of memory to be used, so the more I use, the less I can do. If this isn't clear enough I can provide some oversimplified examples. Just let me know which to provide.

Answer 1

The only way that you're going to be able to efficiently look up a value given just one of two possible types of keys is to have two dictionaries, and that is inherently going to require twice as much memory (approximately) as a single lookup. There's no real way around that.

If using that much memory isn't an option then you have to resort to far less efficient searching algorithms, such as a linear search.

The choice is yours, execution speed or memory. You have to pick one.

Answer 2

Well, I think I've come up with the best compromise I think I could have come up with considering. I ended up using the main dictionary with an internal sub-dictionary to be used as a lookup. The sub-dictionary stores the ID and is indexed by the Handle. I can then create a custom indexer for the Handle, use the lookup dictionary to find the ID, then call the base indexer to retrieve the actual value. This makes it a little lighter-weight than duplicating dictionaries. If anyone else is interested (or has other comments to either improve or show me a better direction), here's a very simplified example:

public class MultiKeyDictionary : Dictionary<EntityID, string>
{
    private Dictionary<string, EntityID> _handleLookup = new Dictionary<string, EntityID>();
    public Dictionary<string, EntityID> HandleLookup
    {
        get { return _handleLookup; }
        set { _handleLookup = value; }
    }

    public string this[string handle]
    {
        get
        {
            EntityID id = this.HandleLookup[handle];
            return base[id];
        }
    }

    public void Add(EntityID id, string handle, string value)
    {
        base.Add(id, value);
        this.HandleLookup.Add(handle, id);
    }
}