How to find nearest key ?

[JouriM2]

First of all: sorry I missed this section and abuse issues.

My question: if key not in the database, how to find index (or get itterator) to the key nearest to which I looking for?
F.i. I have a base with:

1:"A"
5:"B"
9:"C"

and im looking for value with key 6.
I need to get position of nearest existing key, 5 or 9 in this sample (low or hi doesnt metter).

[JouriM2]

 /// <summary>
 /// Seeks the iterator to the position where next item is the key.
 /// If key does not exist,
 /// forward iterator's next item is the first greater item (prefix search forward).
 /// reverse iterator's next item is the last smaller item (prefix search backward).
 /// Complexity: O(log(N))
 /// </summary>
 /// <param name="key">The search key</param>
 void Seek(in TKey key);

Im stupid, sorry