DataTree similar datastructure

I would like to ask help how to implement a similar data structure as datatree.

I have a flatten list of custom class and would like to access items by pair of indices.

For instance item1 would have id 0 and 5.

Essentially a class that imolements Ienumerable, but instead of having indexing of integers, I would like to access items by pair of indices.

Or maybe more simple how can I extend a custom class with properties of Datatree?

You can overload the index operator to take two indices:

class ItemCollection 
  public Item this[int i, int j] 
      // return item based on two indices

Or you can create a dictionary that takes a Tuple<int,int> as key

var dict = new Dictionary<Tuple<int, int>, Item>();

Thanks it works nicely with indexing and tuples, I was trying to write public Item this[int i][int j] , which did not work.