From a brief look using Reflector, it looks like String.Substring() allocates memory for each substring. Am I correct that this is the case? I thought that wouldn't be necessary since strings are immutable.
My underlying goal was to create a IEnumerable<string> Split(this String, Char) extension method that allocates no additional memory.
One reason why most languages with immutable strings create new substrings rather than refer into existing strings is because this will interfere with garbage collecting those strings later.
What happens if a string is used for its substring, but then the larger string becomes unreachable (except through the substring). The larger string will be uncollectable, because that would invalidate the substring. What seemed like a good way to save memory in the short term becomes a memory leak in the long term.