string - Upper vs Lower Case

Question

When doing case-insensitive comparisons, is it more efficient to convert the string to upper case or lower case? Does it even matter?

It is suggested in this SO post that C# is more efficient with ToUpper because "Microsoft optimized it that way." But I've also read this argument that converting ToLower vs. ToUpper depends on what your strings contain more of, and that typically strings contain more lower case characters which makes ToLower more efficient.

In particular, I would like to know:

• Is there a way to optimize ToUpper or ToLower such that one is faster than the other?
• Is it faster to do a case-insensitive comparison between upper or lower case strings, and why?
• Are there any programming environments (eg. C, C#, Python, whatever) where one case is clearly better than the other, and why?

Answer 1

Converting to either upper case or lower case in order to do case-insensitive comparisons is incorrect due to "interesting" features of some cultures, particularly Turkey. Instead, use a StringComparer with the appropriate options.

MSDN has some great guidelines on string handling. You might also want to check that your code passes the Turkey test.

EDIT: Note Neil's comment around ordinal case-insensitive comparisons. This whole realm is pretty murky :(