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Generic Dictionary with multiple keys
I am building a caching system that holds among others a list of Users. 

Sometimes we access the Users by userId (the primary key).
Sometimes we access the Users by name (a string)

Performance is critical, therefore we currently use two generic dictionaries:
One is Dictionary<int?, User>, the other is Dictionary<string, User>

The issue I am trying to solve is the double maintenance of the User objects.
So, if a User changes, we have to remember to update two dictionaries.

Is there another way to have a "duplicate key" Dictionary without duplicating the values (i.e. Users)?

I found some articles about Tuples<> (also called ClassKeys) but this will not work as it requires all key values when accessing the dictionary.  I only have one at a time (userId or Name).

Of course, we could use a simple LINQ statement against the User list (i.e. .Where( u => u.Name == name)), but the list is very large and performance will not be optimal (or is there a way to build an in-memory index?).

Thanks.
Hans Overkamp, Mar 07, 2010
Reply 1
Hi Hans,

You can create 2 dictionaries to solve your issue like this.

The First one is <UserID, UserObject>, same as yours.
The Second one is <UserName, UserID>, which will save your client's user id and user name.

So in your code you can find UserID by UserName, then find its user object by UserID.

In this case you avoid having to update user object twice when it is changed. But the cost is you have to find UserID first if UserName is given.

I hope this helps.

Zac Bai, Mar 09, 2010
Thanks Albert,
Mar 18, 2010
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