• Distinction between Hash Table and Dictionary

 

In C#, both Hashtable and Dictionary<TKey, TValue> are collections that store key-value pairs, but they differ significantly in terms of type safety, performance, and usage. Here's a detailed comparison:

 

Dictionary: This is a high-level abstract data type that contains key-value pairs. It gives you quick ways to add, remove, and get values based on their unique keys. 

 

A hash table is a specific type of data structure that is often used to make a dictionary.  It uses a hash function to map keys to indices in an array (often called a "bucket array" or "hash map")." Most actions (insertion, deletion, lookup) can be done in O(1) time on average with this mapping.

 

Hashtable VS Dictionary

• Namespace    Hashtable uses System.Collections    Dictionary use System.Collections.Generic
• Generic    Hashtable is Non-generic (stores object types)   Dictionary  is Generic (type-safe)
• Type Safety    Hashtable  is Not type-safe (requires casting)    Dictionary  is Type-safe (compile-time type checking)
• Boxing/Unboxing    Hashtable  Requires boxing/unboxing for value types    in Dictionary  there No boxing/unboxing needed
• Null Keys/Values    Hashtable  Allows null for keys and values  Dictionary   Allows null for values; not for keys
• Thread Safety    Hashtable is Thread-safe for single writer, multiple readers    Dictionary  is Not thread-safe; use ConcurrentDictionary for thread safety
• Performance    Hashtable   is Slower due to boxing/unboxing and lack of type safety    Dictionary is Faster due to type safety and no boxing/unboxing
• Exception on Missing Key    Hashtable Returns null for missing keys  Dictionary  Throws KeyNotFoundException for missing keys
• Enumerated Item   Hashtable is of type DictionaryEntry    Dictionary   is of type KeyValuePair<TKey, TValue>