What is a Hashtable?

by

The Hashtable class is a collection that stores key-value pairs. It organizes the pairs based on the hash code of each key and uses it to access elements in the collection.

In a .NET interview, if you’re asked about a Hashtable, you could provide the following answer:

A Hashtable in .NET is a data structure that stores key/value pairs. It uses a hash function to compute an index into an array of buckets or slots from which the desired value can be found. This data structure provides fast retrieval and insertion operations, typically O(1) time complexity on average, making it efficient for large datasets.

Here are some key points about Hashtable:

  1. Key-Value Pair Storage: Hashtable stores data in the form of key-value pairs, where each key is unique and maps to a specific value.
  2. Hashing Mechanism: It employs a hashing mechanism to compute an index for each key. This index determines the location in the underlying array where the key-value pair will be stored or retrieved.
  3. Fast Operations: Retrieval and insertion operations are fast on average, with O(1) time complexity, although this may degrade to O(n) in the worst case scenario due to hash collisions. However, collision resolution strategies such as chaining or open addressing are implemented to handle such cases.
  4. Dynamic Resizing: Hashtables typically dynamically resize themselves as needed to maintain efficiency during insertion and deletion operations. This ensures that the load factor (the ratio of the number of elements to the number of buckets) remains within acceptable bounds.
  5. Thread Safety: The standard Hashtable class in .NET is not inherently thread-safe for multiple writers. For thread-safe operations, you may use the Hashtable.Synchronized method or prefer the ConcurrentDictionary class in .NET.
  6. Usage: Hashtables are commonly used in scenarios where fast lookup and insertion operations are required, such as caching, implementing associative arrays, or building dictionaries.

Overall, a Hashtable provides an efficient way to store and retrieve data based on keys, making it a fundamental data structure in .NET programming for various applications.