c# - Could string comparisons really differ based on culture when the string is guaranteed not to change?

Question

I'm reading encrypted credentials/connection strings from a config file. Resharper tells me, "String.IndexOf(string) is culture-specific here" on this line:

if (line.Contains("host=")) {
    _host = line.Substring(line.IndexOf(
        "host=") + "host=".Length, line.Length - "host=".Length);

...and so wants to change it to:

if (line.Contains("host=")) {
    _host = line.Substring(line.IndexOf("host=", System.StringComparison.Ordinal) + "host=".Length, line.Length -   "host=".Length);

The value I'm reading will always be "host=" regardless of where the app may be deployed. Is it really sensible to add this "System.StringComparison.Ordinal" bit?

More importantly, could it hurt anything (to use it)?

Answer 1

Absolutely. Per MSDN (http://msdn.microsoft.com/en-us/library/d93tkzah.aspx),

This method performs a word (case-sensitive and culture-sensitive) search using the current culture.

So you may get different results if you run it under a different culture (via regional and language settings in Control Panel).

In this particular case, you probably won't have a problem, but throw an i in the search string and run it in Turkey and it will probably ruin your day.

See MSDN: http://msdn.microsoft.com/en-us/library/ms973919.aspx

These new recommendations and APIs exist to alleviate misguided assumptions about the behavior of default string APIs. The canonical example of bugs emerging where non-linguistic string data is interpreted linguistically is the "Turkish-I" problem.

For nearly all Latin alphabets, including U.S. English, the character i (u0069) is the lowercase version of the character I (u0049). This casing rule quickly becomes the default for someone programming in such a culture. However, in Turkish ("tr-TR"), there exists a capital "i with a dot," character (u0130), which is the capital version of i. Similarly, in Turkish, there is a lowercase "i without a dot," or (u0131), which capitalizes to I. This behavior occurs in the Azeri culture ("az") as well.

Therefore, assumptions normally made about capitalizing i or lowercasing I are not valid among all cultures. If the default overloads for string comparison routines are used, they will be subject to variance between cultures. For non-linguistic data, as in the following example, this can produce undesired results:

    Thread.CurrentThread.CurrentCulture = new CultureInfo("en-US")
Console.WriteLine("Culture = {0}",
   Thread.CurrentThread.CurrentCulture.DisplayName);
Console.WriteLine("(file == FILE) = {0}", 
   (String.Compare("file", "FILE", true) == 0));

Thread.CurrentThread.CurrentCulture = new CultureInfo("tr-TR");
Console.WriteLine("Culture = {0}",
   Thread.CurrentThread.CurrentCulture.DisplayName);
Console.WriteLine("(file == FILE) = {0}", 
   (String.Compare("file", "FILE", true) == 0));

Because of the difference of the comparison of I, results of the comparisons change when the thread culture is changed. This is the output:

Culture = English (United States)
(file == FILE) = True
Culture = Turkish (Turkey)
(file == FILE) = False

Here is an example without case:

var s1 = "é"; //é as one character (ALT+0233)
var s2 = "e?"; //'e', plus combining acute accent U+301 (two characters)

Console.WriteLine(s1.IndexOf(s2, StringComparison.Ordinal)); //-1
Console.WriteLine(s1.IndexOf(s2, StringComparison.InvariantCulture)); //0
Console.WriteLine(s1.IndexOf(s2, StringComparison.CurrentCulture)); //0