FeaturedCollection Performance in .NET 9

.NET 9 Performance Comparison: Hashtable vs Dictionary vs List vs Collection

.NET 9 includes several performance improvements, particularly in collections like Hashtable, Dictionary, List, and HashSet. This post compares their performance and suitability for different use cases.

---

Dictionary<TKey, TValue> vs Hashtable

• Dictionary<TKey, TValue> is a generic collection that provides O(1) average-time complexity for lookups.
• Hashtable is a non-generic collection, requiring boxing/unboxing for value types, making it less efficient.

List

• List<T> is an ordered collection with fast index-based access but slower search times compared to hash-based structures.

Other Collections

• ArrayList is a non-generic collection that stores objects and suffers from boxing/unboxing overhead.
• HashSet<T> provides unique elements with fast lookups, making it useful for membership tests.

---

Benchmark Code

public class CollectionPerformance
{
    private Dictionary<int, int> dictionary;
    private Hashtable hashtable;
    private List<int> list;
    private HashSet<int> hashSet;
    private int searchKey = 999999;

    public void Setup()
    {
        dictionary = new Dictionary<int, int>();
        hashtable = new Hashtable();
        list = new List<int>();
        hashSet = new HashSet<int>();

        for (int i = 0; i < 1000000; i++)
        {
            dictionary[i] = i;
            hashtable[i] = i;
            list.Add(i);
            hashSet.Add(i);
        }
    }

    public void DictionaryLookup() => dictionary.ContainsKey(searchKey);

    public void HashtableLookup() => hashtable.ContainsKey(searchKey);

    public void ListLookup() => list.Contains(searchKey);

    public void HashSetLookup() => hashSet.Contains(searchKey);
}

Test Environment

• Processor: 13th Gen Intel Core i7-1370P, 1 CPU, 20 logical and 14 physical cores
• .NET SDK: 9.0.103
• Runtime: .NET 9.0.2 (9.0.225.6610), X64 RyuJIT AVX2

---

Benchmark Results

Method	Mean	Error	StdDev	Median	P95	Gen0	Allocated
DictionaryLookup	1.806 ns	0.1874 ns	0.4936 ns	1.660 ns	3.375 ns	-	-
HashtableLookup	9.726 ns	0.3621 ns	1.0620 ns	9.591 ns	11.761 ns	0.0019	24 B
ListLookup	112,984.793 ns	2,973.6506 ns	8,483.9931 ns	115,142.621 ns	123,135.237 ns	-	-
HashSetLookup	3.272 ns	0.1454 ns	0.4218 ns	3.317 ns	3.922 ns	-	-

---

Analysis & Key Takeaways

• Dictionary<TKey, TValue> offers the best balance of speed and memory efficiency for key-based lookups.
• Hashtable is significantly slower due to boxing/unboxing overhead and additional memory allocation.
• List performs poorly in large datasets as it relies on O(n) linear search, making it unsuitable for frequent lookups.
• HashSet is one of the fastest structures for lookups and ideal when uniqueness is required.

---

Conclusion

For most key-value lookups:

• Use Dictionary<TKey, TValue> for the best trade-off between speed and memory.
• Avoid Hashtable unless legacy support is required.
• Use List only when preserving order is necessary.
• Prefer HashSet when unique items and fast lookups are needed.