Question

As we know there are 2 classical strategies to collision resolution: Separate chaining and Open addressing.

I’m wondering which one was chosen for HashTable/Dictionary in .net.

Or there were used some other strategy?

Answer 1

It’s all described in this paper on the MSDN : An Extensive Examination of Data Structures Using C# 2.0

…collision resolution technique called rehasing, which is the
technique used by the .NET Framework’s Hashtable class. In the final
section, we’ll look at the Dictionary class, which uses a collision
resolution technique knows as chaining.
….

… Rehasing works as follows: there is a set of hash different
functions, H1 … Hn, and when inserting or retrieving an item from
the hash table, initially the H1 hash function is used. If this leads
to a collision, H2 is tried instead, and onwards up to Hn if needed.
The previous section showed only one hash function, which is the
initial hash function (H1). The other hash functions are very similar
to this function, only differentiating by a multiplicative factor. In
general, the hash function Hk is defined as:

 Hk(key) = [GetHash(key) + k * (1 + (((GetHash(key) >> 5) + 1) %  (hashsize – 1)))] % hashsize

The Dictionary class differs from the Hashtable class in more ways
than one. In addition to being strongly-typed, the Dictionary also
employs a different collision resolution strategy than the Hashtable
class, using a technique referred to as chaining. Recall that with
probing, in the event of a collision another slot in the list of
buckets is tried. (With rehashing, the hash is recomputed, and that
new slot is tried.) With chaining, however, a secondary data structure
is utilized to hold any collisions. Specifically, each slot in the
Dictionary has an array of elements that map to that bucket. In the
event of a collision, the colliding element is prepended to the
bucket’s list.

Remember only the first sentence is my own 🙂