Collection Performance: Creating Immutable Sorted Collections

Updated January 2024

Two immutable sorted collections are available: ImmutableSortedDictionary and ImmutableSortedSet<>. Below is an example illustrating how to construct an immutable sorted collection using a builder.

var builder = ImmutableSortedSet.CreateBuilder<Person>();

foreach (var person in people)
{
    _ = builder.Add(person);
}

var collection = builder.ToImmutable();

You also have the option to utilize CreateRange().

var collection = ImmutableSortedDictionary.CreateRange(personDictionary);

Benchmark Results

Here are the performance results. As evident, the ImmutableSortedDictionary<,> consistently shows lower performance compared to the ImmutableSortedSet<> in both scenarios. Furthermore, using CreateRange() proves to be more performant than employing CreateBuilder() with foreach().

I’d like to emphasize that employing a regular List<> with the Sort() method tends to be more efficient than opting for an immutable sorted collection.

Allocations: ImmutableSortedDictionary<,>: 1,008 – 114,801 bytes. ImmutableSortedSet<>: 856 – 98,392  bytes. ImmutableList<> + Sort(): 1,808 – 213,137 bytes.

Allocations: ImmurableSortedDictionary<,>: 2,536 – 262,745 bytes. ImmutableSortedSet<>: 1,008 – 114, 800 bytes.

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