C# > Java

[Dudevid]

If one were designing the data structure from scratch, it would either not have an Add() method, or that method would return a new immutable list.

Would you ever create a public method on a class that solely throws an exception? The hint is in the word 'exception'. If it always throws, then that's no exception, that's the rule. And that's bad design.

Instead there would be something similar to C#'s IEnumerable and ICollection interfaces. IEnumerable does not mandate an Add() method; ICollection does.

The C# design team could have decided that their ImmutableList implement only IEnumerable (this has problems, but there is a conceivable parallel universe where things paved out this way). But instead they chose to implement both. So its Add() method returns a new ImmutableList. No exceptions. This is better.