A question that arose in response to my previous post (about how string comparisons can produce unexpected results when done in a culture-sensitive way) was Which is right, StringComparison.Ordinal or StringComparison.InvariantCulture? The short answer: StringComparison.Ordinal.

There is a good article explaining the differences between the StringComparison enum values on MSDN. I'll summarize here, but without mentioning the "IgnoreCase" variations in the enum since those are generally understood (though there are subtleties with case conversion).

StringComparison.Ordinal is the best choice in most cases. When specified in comparison operations, these cause a character by character comparison based strictly on the numeric value of the characters. This has a couple of advantages:

1. It is very fast.
2. It is usually what you want.

The performance benefits include not needing to do any table lookups as well as the ability to fail fast if the two string lengths are not equal.

It generally only makes sense to use StringComparison.CurrentCulture if you are going to display the result of the operation to the user, such as in a list where the items are supposed to be sorted alphabetically according to the user's culture. Instead of a character by character comparison, it is a "linguistic" comparison.

This leaves StringComparison.InvariantCulture. The author of the MSDN article was hard pressed to come up with a good reason for using StringComparison.InvariantCulture from .NET 2.0 forward. It does a linguistic comparison, but in a way that is always the same regardless of the current culture. In other words, it isn't a character by character comparison, but it isn't necessarily correct for the current culture either.

An example clarifies things:

static void Main(string[] args)
{
  Console.WriteLine("Grüße".EndsWith("sse", StringComparison.Ordinal));
  Console.WriteLine("Grüße".EndsWith("sse", StringComparison.CurrentCulture));
  Console.WriteLine("Grüße".EndsWith("sse", StringComparison.InvariantCulture));
  Console.WriteLine("Grüße".EndsWith("sse"));
}

This example uses the German ess-szet or sharp s character. In German, ß and ss are linguistically equivalent, so the results of running the program, even using the English (United States) culture, are:

False
True
True
True

If a user searched a document for "Grüsse", it would be appropriate to show matches for both "Grüsse" and "Grüße", and so use CurrentCulture. But for most other day-to-day string manipulations within code, such as comparing file names for equivalence, you should use one of the Ordinal values.

Both are valid (and distinct) file names, after all, which is not discernible using either CurrentCulture or InvariantCulture.