Intro

Hashtable is the non-generic hash table from System.Collections. It is mostly legacy in modern .NET and is usually replaced by Dictionary<TKey, TValue>.

Deeper Explanation

Hashtable stores keys and values as object, so value types are boxed/unboxed.
It still uses hash buckets and collision resolution similar to modern hash-based collections.

Internally, Hashtable uses separate chaining: when two keys hash to the same bucket, they form a linked list at that slot. Lookup traverses the chain until the matching key is found, making worst-case performance O(n) when all keys collide. Dictionary<TKey, TValue> uses open addressing with prime-based probing, which is more cache-friendly and avoids per-collision heap allocations. This is the primary performance reason to prefer Dictionary in new code.

Structure

graph TD
    K1[object key one hash] --> B0[bucket zero]
    K2[object key two hash] --> B1[bucket one]
    B0 --> E1[object key one value ann]
    B1 --> E2[object key two value bob]

Example

var table = new Hashtable
{
    ["user:1"] = "Ann"
};

var value = table["user:1"]; // object

Pitfalls

• No compile-time type safety because everything is object.
• Boxing/unboxing increases allocations and CPU overhead.
• Synchronized() wrappers do not make multi-step operations atomic.

Tradeoffs

• Keep only for interop with old APIs that already require Hashtable.
• Prefer Dictionary<TKey, TValue> for regular code and ConcurrentDictionary for concurrent writes.

Questions

Hash-Based Collections Comparison

Decision rule: do not use Hashtable in new code. Migrate to Dictionary<TKey,TValue> for type safety and better performance.

Links

• Hashtable class — API reference; documents the legacy non-generic hash table.
• When to use generic collections — explains why generic collections replace non-generic ones for type safety and performance.
• Selecting a collection class — decision guide for choosing the right collection type.
• Hashtable implementation in dotnet runtime — source code showing the legacy bucket and collision design.