Immutable collections

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Definition

An immutable collection is a collection of instances that

  • preserves its structure all the time: no addition or removal of elements, no change of order
  • disallows element-level assignments

While still offering APIs to perform mutations: Add, Remove, ...

The immutable collections are designed with two goals in mind:

  • reuse as much memory as possible, to avoid copying, and to reduce pressure on the garbage collector
  • support the same operations offered by mutable collections with competitive time complexities

Immutable Stacks

Cs.svg
var s1 = ImmutableStack<int>.Empty;

// call push, s1 remains unchanged, and it returns a reference to the resulting immutable stack which contains the element 1.
var s2 = s1.Push(1);

// call push, s2 remains unchanged, and it returns a reference to the resulting immutable stack which contains no element.
// s3 and s1 target the same reference: s3 == s1
var s3 = s2.Pop();

Other immutable collection types:

Immutable Lists

Immutable Arrays

Immutable Dictionaries