[.net] What does CultureInfo.InvariantCulture mean?

I have a string of text like so:

var foo = "FooBar";

I want to declare a second string called bar and make this equal to first and fourth character of my first foo, so I do this like so:

var bar = foo[0].ToString() + foo[3].ToString();

This works as expected, but ReSharper is advising me to put Culture.InvariantCulture inside my brackets, so this line ends up like so:

var bar = foo[0].ToString(CultureInfo.InvariantCulture)
        + foo[3].ToString(CultureInfo.InvariantCulture);

What does this mean, and will it affect how my program runs?

For things like numbers (decimal points, commas in amounts), they are usually preferred in the specific culture.

A appropriate way to do this would be set it at the culture level (for German) like this:

Thread.CurrentThread.CurrentCulture.NumberFormat = new CultureInfo("de").NumberFormat;

JetBrains offer a reasonable explanation,

"Ad-hoc conversion of data structures to text is largely dependent on the current culture, and may lead to unintended results when the code is executed on a machine whose locale differs from that of the original developer. To prevent ambiguities, ReSharper warns you of any instances in code where such a problem may occur."

but if I am working on a site I know will be in English only, I just ignore the suggestion.

According to Microsoft:

The CultureInfo.InvariantCulture property is neither a neutral nor a specific culture. It is the third type of culture that is culture-insensitive. It is associated with the English language but not with a country or region.

(from http://msdn.microsoft.com/en-us/library/4c5zdc6a(vs.71).aspx)

So InvariantCulture is similair to culture "en-US" but not exactly the same. If you write:

var d = DateTime.Now;
var s1 = d.ToString(CultureInfo.InvariantCulture);   // "05/21/2014 22:09:28"
var s2 = d.ToString(new CultureInfo("en-US"));       // "5/21/2014 10:09:28 PM"

then s1 and s2 will have a similar format but InvariantCulture adds leading zeroes and "en-US" uses AM or PM.

So InvariantCulture is better for internal usage when you e.g save a date to a text-file or parses data. And a specified CultureInfo is better when you present data (date, currency...) to the end-user.

When numbers, dates and times are formatted into strings or parsed from strings a culture is used to determine how it is done. E.g. in the dominant en-US culture you have these string representations:

• 1,000,000.00 - one million with a two digit fraction
• 1/29/2013 - date of this posting

In my culture (da-DK) the values have this string representation:

• 1.000.000,00 - one million with a two digit fraction
• 29-01-2013 - date of this posting

In the Windows operating system the user may even customize how numbers and date/times are formatted and may also choose another culture than the culture of his operating system. The formatting used is the choice of the user which is how it should be.

So when you format a value to be displayed to the user using for instance ToString or String.Format or parsed from a string using DateTime.Parse or Decimal.Parse the default is to use the CultureInfo.CurrentCulture. This allows the user to control the formatting.

However, a lot of string formatting and parsing is actually not strings exchanged between the application and the user but between the application and some data format (e.g. an XML or CSV file). In that case you don't want to use CultureInfo.CurrentCulture because if formatting and parsing is done with different cultures it can break. In that case you want to use CultureInfo.InvariantCulture (which is based on the en-US culture). This ensures that the values can roundtrip without problems.

The reason that ReSharper gives you the warning is that some application writers are unaware of this distinction which may lead to unintended results but they never discover this because their CultureInfo.CurrentCulture is en-US which has the same behavior as CultureInfo.InvariantCulture. However, as soon as the application is used in another culture where there is a chance of using one culture for formatting and another for parsing the application may break.

So to sum it up:

• Use CultureInfo.CurrentCulture (the default) if you are formatting or parsing a user string.
• Use CultureInfo.InvariantCulture if you are formatting or parsing a string that should be parseable by a piece of software.
• Rarely use a specific national culture because the user is unable to control how formatting and parsing is done.