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 :(

Answer 2

From Microsoft on MSDN:

Best Practices for Using Strings in the .NET Framework

Recommendations for String Usage

• Use the String.ToUpperInvariant method instead of the String.ToLowerInvariant method when you normalize strings for comparison.

Why? From Microsoft:

Normalize strings to uppercase

There is a small group of characters that when converted to lowercase cannot make a round trip.

What is example of such a character that cannot make a round trip?

• Start: Greek Rho Symbol (U+03f1) ϱ
• Uppercase: Capital Greek Rho (U+03a1) Ρ
• Lowercase: Small Greek Rho (U+03c1) ρ

ϱ , Ρ , ρ

.NET Fiddle

Original: ϱ
ToUpper: Ρ
ToLower: ρ

That is why, if your want to do case insensitive comparisons you convert the strings to uppercase, and not lowercase.

So if you have to choose one, choose Uppercase.

Answer 3

According to MSDN it is more efficient to pass in the strings and tell the comparison to ignore case:

String.Compare(strA, strB, StringComparison.OrdinalIgnoreCase) is equivalent to (but faster than) calling

String.Compare(ToUpperInvariant(strA), ToUpperInvariant(strB), StringComparison.Ordinal).

These comparisons are still very fast.

Of course, if you are comparing one string over and over again then this may not hold.