What is the difference between Culture and UICulture?

Could someone give me a bit more information on the difference between Culture and UICulture within the .NET framework? What they do and when to use what?

Culture affects how culture-dependent data (dates, currencies, numbers and so on) is presented. Here are a few examples:

var date = new DateTime(2000, 1, 2);
var number = 12345.6789;

Thread.CurrentThread.CurrentCulture = new CultureInfo("de-DE");
Console.WriteLine(date); // 02.01.2000 00:00:00
Console.WriteLine(number.ToString("C")); // 12.345,68 €

Thread.CurrentThread.CurrentCulture = new CultureInfo("fr-CA");
Console.WriteLine(date); // 2000-01-02 00:00:00
Console.WriteLine(number.ToString("C")); // 12 345,68 $

Thread.CurrentThread.CurrentCulture = new CultureInfo("en-US");
Console.WriteLine(date); // 1/2/2000 12:00:00 AM
Console.WriteLine(number.ToString("C")); // $12,345.68

Culture also affects parsing of user input in the same way:

const string numberString = "12.345,68";
decimal money;

Thread.CurrentThread.CurrentCulture = new CultureInfo("de-DE");
money = decimal.Parse(numberString); // OK!

Thread.CurrentThread.CurrentCulture = new CultureInfo("en-US");
money = decimal.Parse(numberString); // FormatException is thrown, TryParse would return false

Beware of cases where the parsing succeeds but the result is not what you would expect it to be.

const string numberString = "12.345";
decimal money;

Thread.CurrentThread.CurrentCulture = new CultureInfo("de-DE");
money = decimal.Parse(numberString); // 12345

Thread.CurrentThread.CurrentCulture = new CultureInfo("en-US");
money = decimal.Parse(numberString); // 12.345, where the . is a decimal point

UICulture affects which resource file (Resources.lang.resx) is going to be loaded to by your application.

So to load German resources (presumably localized text) you would set UICulture to the German culture and to display German formatting (without any impact on which resources are loaded) you would set Culture.

Culture and UICulture

Values are pairs of two-letter strings, the first is for defining language and the second for defining the region. Example:

en-GB here en represents English and GB represents Great Briton

en-US here en represents English and US represents United States

Use Culture for Culture dependent functions like date, time. and UICulture is for correct resource file loading.

Just a small matter to consider in addition to @Vache's awesome explanation: You can set both UICulture and Culture at (page level and application level).

In order to set them at application level, simply add globalization session in web.config

e.g. <globalization uiCulture="es" culture="es-MX" />

And to set them at the page level, which is good to add on a specific (individual) page, set the Culture and UICulture attributes within @ page directive

e.g. <%@ Page UICulture="es" Culture="es-MX" %>

The UICulture property might change for each Web browser, whereas the Culture stays constant.

The Culture value can be set to specific cultures only, such as en-US or en-GB. This prevents the requirement to identify the correct currency symbol to use for en, where en-US and en-GB have different currency symbols. Users can set the UI culture and culture in their browsers.