[c#] String comparison: InvariantCultureIgnoreCase vs OrdinalIgnoreCase?

Which would be better code:

int index = fileName.LastIndexOf(".", StringComparison.InvariantCultureIgnoreCase);

or

int index = fileName.LastIndexOf(".", StringComparison.OrdinalIgnoreCase);

Neither code is always better. They do different things, so they are good at different things.

InvariantCultureIgnoreCase uses comparison rules based on english, but without any regional variations. This is good for a neutral comparison that still takes into account some linguistic aspects.

OrdinalIgnoreCase compares the character codes without cultural aspects. This is good for exact comparisons, like login names, but not for sorting strings with unusual characters like é or ö. This is also faster because there are no extra rules to apply before comparing.

You seem to be doing file name comparisons, so I would just add that OrdinalIgnoreCase is closest to what NTFS does (it's not exactly the same, but it's closer than InvariantCultureIgnoreCase)

FXCop typically prefers OrdinalIgnoreCase. But your requirements may vary.

For English there is very little difference. It is when you wander into languages that have different written language constructs that this becomes an issue. I am not experienced enough to give you more than that.

OrdinalIgnoreCase

The StringComparer returned by the OrdinalIgnoreCase property treats the characters in the strings to compare as if they were converted to uppercase using the conventions of the invariant culture, and then performs a simple byte comparison that is independent of language. This is most appropriate when comparing strings that are generated programmatically or when comparing case-insensitive resources such as paths and filenames. http://msdn.microsoft.com/en-us/library/system.stringcomparer.ordinalignorecase.aspx

InvariantCultureIgnoreCase

The StringComparer returned by the InvariantCultureIgnoreCase property compares strings in a linguistically relevant manner that ignores case, but it is not suitable for display in any particular culture. Its major application is to order strings in a way that will be identical across cultures. http://msdn.microsoft.com/en-us/library/system.stringcomparer.invariantcultureignorecase.aspx

The invariant culture is the CultureInfo object returned by the InvariantCulture property.

The InvariantCultureIgnoreCase property actually returns an instance of an anonymous class derived from the StringComparer class.