Win32 File Name Comparison

Question

Does anyone know what culture settings Win32 uses when dealing with case-insensitive files names?

Is this something that varies based on the user's culture, or are the casing rules that Win32 uses culture invariant?

Answer 1

An approximate answer is at Comparing Unicode file names the right way.

Basically, the recommendation is to uppercase both strings (using CharUpper, CharUpperBuff, or LCMapString), then compare using a binary comparison (i.e. memcmp or wmemcmp, not CompareString with an invariant locale). The file system doesn't do Unicode normalization, and the case rules are not dependent on locale settings.

There are unfortunate ambiguous cases when dealing with characters whose casing rules have changed across different versions of Unicode, but it's about as good as you can do.

Answer 2

Comparing file names in native code and Don't compare filenames are a couple of good blog posts on this topic. The first has C/C++ code for OrdinalIgnoreCaseCompareStrings, and the second tells you how that doesn't always work for filenames and what to do to mitigate that.

Then there are the Unicode problems. While these new OrdinalIgnoreCase string comparison algorithms are great for your local NTFS drive, they might not yield the right answer on your FAT drive, or a network share.

So what's the answer? When possible, let the file system tell you. CreateFile can tell you if a given filename exists. Just pick the right creation disposition. If you need to compare to handles, you can often use GetFileInformationByHandle; look at dwVolumeSerialNumber/nFileIndexHigh/nFileIndexLow.

Answer 3

If you're using .NET, the official recommendation from Microsoft is to use StringComparison.OrdinalIgnoreCase for comparison and ToUpperInvariant for normalization (to be later compared using Ordinal comparison). This also applies to Registry keys and values, environment variables etc.

See New Recommendations for Using Strings in Microsoft .NET 2.0 for more details.

Note that while it's reliable on NTFS, it can fail with network shares, for example. See @SteveSteiner's answer and links in his post for solutions.