If strings are immutable in .NET, then why does Substring take O(n) time?

Question

Given that strings are immutable in .NET, I\'m wondering why they have been designed such that string.Substring() takes O(substring.Length) time, i

Answer 1

Java used to reference larger strings, but:

Java changed its behavior to copying as well, to avoid leaking memory.

I feel like it can be improved though: why not just do the copying conditionally?

If the substring is at least half the size of the parent, one can reference the parent. Otherwise one can just make a copy. This avoids leaking a lot of memory while still providing a significant benefit.