Explain what immutable collections are and what their advantages are?

Immutable collections are collections whose contents cannot be changed after creation. In C#, for example, IReadOnlyList<T>, ImmutableList<T> from the System.Collections.Immutable library.

Advantages:

• Thread safety: Immutable collections can be safely used in multithreaded environments without locks.
• Predictability: Data does not change, which simplifies debugging and understanding the code.
• Functional style: Promotes the use of pure functions and reduces side effects.

Example of usage:

using System.Collections.Immutable;

var list = ImmutableList.Create<int>(1, 2, 3);
var newList = list.Add(4); // a new collection is created, the original remains unchanged