string.ToLower(), ToUpper, IndexOf, LastIndexOf, and Compare are all culture-dependent, as are some (floating point number and DateTime-related) calls to ToString. Fortunately, all have variants which accept an argument specifying the culture or formatter to use. Leave that argument off and the call will use the system default culture, possibly creating problems with international characters.

string.CompareTo() is also culture specific, but has no overload that takes a culture information, so instead it’s better to use CompareOrdinal, or Compare with culture.

Calls without a culture may work fine in the system’s "home" environment, but break in ways that are extremely difficult to diagnose for customers who use different encodings. Such bugs can be nearly, if not completely, impossible to reproduce when it’s time to fix them.

Noncompliant Code Example

var lowered = someString.ToLower(); //Noncompliant

Compliant Solution

var lowered = someString.ToLower(CultureInfo.InvariantCulture);

or

var lowered = someString.ToLowerInvariant();