Multiple key lookup for Dictionary

Question

I am parsing a file and I would like to store it in a lookup structure in a way that I can lookup with two keys.

I have a User Entity that has name, email and id, types are irrelevant.

I would like to store it in a Dictionary<User, id> so I can get the user id by looking up with User.

I also want the other way around, ie : Dictionary<Id, User>

I can create two structures and do the lookup. That s easy. I d like to do it with a single structure.

I am curious if I can do this with a single structure

I was thinking that I can do:

Dictionary<User, User> , then implement a IEqualityComparer<User>

Is there a better way to do this?

What would be the best practice to implement IEqualityComparer?

Answer 1

Since you store the id in the User object, you don't need a User->id mapping.

Answer 2

I'm not sure your problem description makes sense, for the following reason:

Given a User object you know the ID, as the ID is a property of the User. Therefore, why would you ever need the Dictionary<User, Id>? If you have the Dictionary<Id, User> you can get to the user given the Id, and if you have the user you should already have the ID, making the other dictionary unnecessary.

Or is it the case that sometimes you have an incomplete User object with the Id not populated?

Answer 3

You could easily create two dictionaries since the entries are stored by reference anyway. But as other have said it seams like you would never need to find the ID if you have the user which contains the id.

Answer 4

Whether you need to do this type of mapping or not, you can use a single dictionary to something like what you're asking for. Here's a rough sample:

var dict = new Dictionary<string, object>();
dict["ID_001"] = new User();
dict["USER_??"] = 001; // Need a unique user string to replace the "??"

Of course, you can make up any string you want. And if you want to wrap functions around things, you can avoid having to cast the object each time you get an item. (A static method might work better for you.)

User GetUser(int id, Dictionary<string, object> dict) 
{
    return (User)dict["ID_" + id];
}