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

Dictionary <<<>>> Hashtable differences:

• Generic <<<>>> Non-Generic
• Needs own thread synchronization <<<>>> Offers thread safe version through Synchronized() method
• Enumerated item: KeyValuePair <<<>>> Enumerated item: DictionaryEntry
• Newer (> .NET 2.0) <<<>>> Older (since .NET 1.0)
• is in System.Collections.Generic <<<>>> is in System.Collections
• Request to non-existing key throws exception <<<>>> Request to non-existing key returns null
• potentially a bit faster for value types <<<>>> bit slower (needs boxing/unboxing) for value types

Dictionary / Hashtable similarities:

• Both are internally hashtables == fast access to many-item data according to key
• Both need immutable and unique keys
• Keys of both need own GetHashCode() method

Similar .NET collections (candidates to use instead of Dictionary and Hashtable):

• ConcurrentDictionary - thread safe (can be safely accessed from several threads concurrently)
• HybridDictionary - optimized performance (for few items and also for many items)
• OrderedDictionary - values can be accessed via int index (by order in which items were added)
• SortedDictionary - items automatically sorted
• StringDictionary - strongly typed and optimized for strings