Why is Dictionary preferred over Hashtable in C#?

Question

In most programming languages, dictionaries are preferred over hashtables. What are the reasons behind that?

Answer 1

The Extensive Examination of Data Structures Using C# article on MSDN states that there is also a difference in the collision resolution strategy:

The Hashtable class uses a technique referred to as rehashing.

Rehashing works as follows: there is a set of hash different functions, H₁ ... H_n, 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 H_n if needed.

The Dictionary uses a technique referred to as chaining.

With rehashing, in the event of a collision the hash is recomputed, and the new slot corresponding to a hash 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.