Question

What is the best way to index value by two keys in a dictionary. Ex.: having students with unique Id(integer) and username(string) create a dictionary that holds objects of type student indexed by Id and username. Then on a retrieval use either Id or username.
Or may be Dictionary is not a good match?
P.S. I’m aware of tuples but failing to see how they can be used in this scenario.

EDIT: As an alternative to using two keys, I could create a string representation of both id and username separated by unique divider, and then match keys using regex?! Ex.: “1625|user1”

Answer 1

Since you want to be able to retrieve by either, you’ll need two dictionaries.

Assuming the Student type is a reference type, you’ll still only have one Student object per student, so don’t worry about that.

It would be best to wrap the dictionaries in a single object:

public class StudentDictionary
{
  private readonly Dictionary<int, Student> _byId = new Dictionary<int, Student>();
  private readonly Dictionary<string, Student> _byUsername = new Dictionary<string, Student>();//use appropriate `IEqualityComparer<string>` if you want other than ordinal string match
  public void Add(Student student)
  {
    _byId[student.ID] = student;
    _byUsername[student.Username] = student;
  }
  public bool TryGetValue(int id, out Student student)
  {
    return _byId.TryGetValue(id, out student);
  }
  public bool TryGetValue(string username, out Student student)
  {
    return _byUsername.TryGetValue(username, out student);
  }
}

And so on.