Does String.ToLower() always allocate memory?

Question

Does String.ToLower() return the same reference (e.g. without allocating any new memory) if all the characters are already lower-case?

Memory allocation is cheap, but running a quick check on zillions of short strings is even cheaper. Most of the time the input I'm working with is already lower-case, but I want to make it that way if it isn't.

I'm working with C# / .NET in particular, but my curiosity extends to other languages so feel free to answer for your favorite one!

NOTE: Strings are immutable but that does not mean a function always has to return a new one, rather it means nothing can change their character content.

Answer 1

String is an immutable. String.ToLower() will always return new instance thereby generating another instance on every ToLower() call.

Answer 2

I expect so, yes. A quick test agrees (but this is not evidence):

string a = "abc", b = a.ToLower();
bool areSame = ReferenceEquals(a, b); // false

In general, try to work with comparers that do what you want. For example, if you want a case-insensitive dictionary, use one:

var lookup = new Dictionary<string, int>(
    StringComparer.InvariantCultureIgnoreCase);

Likewise:

bool ciEqual = string.Equals("abc", "ABC",
    StringComparison.InvariantCultureIgnoreCase);

Answer 3

Java implementation of String.toLowerCase() from Sun actually doesn't always allocate new String. It checks if all chars are lowercase, and if so, it returns original string.

Answer 4

[edit]
Interning doesn't help -- see the comments to this answer.