Differences and Similarities: Dictionary vs. Hashtable in C# Explained

Introduction

Developers often come across the Dictionary and Hashtable classes when working with collections. While they serve a similar purpose of storing key-value pairs, the two have significant differences and similarities. Understanding these distinctions is crucial for selecting the appropriate collection for specific scenarios.

The article demonstrates the dissimilarities and similarities between Dictionary and Hashtable and explores alternative collections offered in the .NET framework.

Differences Between Dictionary and Hashtable:

Generic vs Non-Generic

The dictionary is a generic collection introduced in .NET 2.0, allowing type safety and compile-time checks. On the other hand, Hashtable is a non-generic collection that has been around since .NET 1.0.

Thread Synchronization

The dictionary requires manual synchronization when accessed from multiple threads. In contrast, Hashtable offers a thread-safe version through the Synchronized() method, automatically synchronising access.

Enumerated Item

Dictionary returns KeyValuePair objects when enumerating items, while Hashtable returns DictionaryEntry objects.

Namespace and Version

The dictionary resides in the System.Collections.Generic namespace, while Hashtable is located in System.Collections. This reflects their respective introduction periods within the .NET framework.

Exception Handling

Requesting a non-existing key from Dictionary throws an exception, whereas Hashtable returns null in such cases.

Performance

The dictionary is generally considered faster for value types due to its avoidance of boxing and unboxing. On the other hand, Hashtable incurs a slight performance penalty when dealing with value types.

Similarities Between Dictionary and Hashtable:

Internal Hashtable Implementation

Both Dictionary and Hashtable internally use hashtables, allowing fast data access based on keys.

Immutable and Unique Keys

Both collections require keys to be immutable and unique.

GetHashCode()

Keys in both Dictionary and Hashtable should have their own implementation of the GetHashCode() method.

Alternative .NET Collections:

In addition to Dictionary and Hashtable, the .NET framework offers several alternative collections that may be suitable for specific scenarios:

ConcurrentDictionary

This collection provides thread-safe access and can be safely accessed by multiple threads concurrently.

HybridDictionary

Optimized for performance, the HybridDictionary offers efficient storage for a few items and extensive collections.

OrderedDictionary

To retrieve the values in this collection, an integer index based on the order of item addition can be used.

SortedDictionary

Items in this collection are automatically sorted according to the keys.

StringDictionary

This collection is strongly typed and optimized for storing strings. However, it has been deprecated in favour of using Dictionary<string, string>.

Conclusion

Understanding the differences and similarities between Dictionary and Hashtable is crucial for making informed decisions when selecting the appropriate collection for a given scenario. While Dictionary offers type safety and improved performance, Hashtable provides thread-safe options.

Additionally, exploring alternative collections like ConcurrentDictionary, HybridDictionary, OrderedDictionary, SortedDictionary, and StringDictionary expands the developer’s toolkit for efficient data storage and retrieval in C#.

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