.NET Hashtable vs. Dictionary: Which Offers Better Performance and Does Order Matter?

.NET Hashtable vs Dictionary: Delving into Performance and Order Preservation

When utilizing hash table data structures in .NET, the choice between Hashtable and Dictionary is crucial. Understanding when and why to employ each option will optimize application performance.

Ambiguity in Order Preservation

Contrary to the belief that Hashtable preserves insertion order while Dictionary sorts items, neither maintains order. Both leverage hash tables internally, relying on chaining (Dictionary) or rehashing (Hashtable) for collision resolution, which inherently disrupts insertion order.

Performance Considerations

Aside from boxing/unboxing overheads, Hashtable and Dictionary exhibit comparable performance. Both employ hash functions to map keys to buckets, enabling efficient key-based lookups regardless of insertion sequence.

However, there might be scenarios where Hashtable outperforms Dictionary. If collision frequency is high, the chaining mechanism in Dictionary can introduce performance degradation. In such cases, the rehashing approach in Hashtable, which seeks alternative hash functions, can prove more effective.

Situational Usage

Selecting Hashtable over Dictionary may be appropriate in the following situations:

• When collision frequency is anticipated to be high.
• When insertion order is critical, as both data structures do not guarantee order preservation.

In most other scenarios, especially in .NET Framework 2.0 environments, Dictionary remains the preferred choice due to its genericity, type safety, and performance parity with Hashtable.

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