What is the difference between CurrentCulture and CurrentUICulture properties of CultureInfo in .NET?

CurrentCulture is the .NET representation of the default user locale of the system. This controls default number and date formatting and the like.

CurrentUICulture refers to the default user interface language, a setting introduced in Windows 2000. This is primarily regarding the UI localization/translation part of your app.

Whatever regional options the system is configured to have will be the "Current" values in your .NET app.

Often times they are both the same. But on my system they would be different: I prefer my numbers and dates in the German format, so the CurrentCulture would be German, but I also prefer all my applications in English, so the CurrentUICulture would be English.

There is a nice article on the topic: Sorting it all Out: Why we have both CurrentCulture and CurrentUICulture

This is a simple trick I use to remember which one to use:

(date, currency, double).tostring = CurrentCulture

resource.fr-CA.resx file = currentUICulture

A good way to make a difference in addition to the nice explanations done by fellow users, and an important aspect in web application development is the following:

CurrentCulture represents the setup of the web server. For example, if your ASP.NET web application is hosted in Germany, the value of CutlureInfo.CurrentCulture would most probably be de-DE. Thus, the default .ToString() formatting for IFormattable types would use the default German formattings, or the ones which have been set up on the server OS as defaults.

CurrentUICulture can be captured from the user agent, and may represent the user interface culture of the client connecting to the website. For example, if you load that website from Russia, your local settings are set to use Russian language, and your user agent sends your locale settings to the server (Opera and IE do this automatically, not sure for Chrome and FireFox), the CurrenUICulture would represent ru-RU. This will cause any resources like localized strings retrieved via ResourceManager or localization expressions in ASP.NET aspx/ascx files to be in Russian (if translations are available).

Differences:

CurrentCulture is for formatting of dates and currency while CurrentUICulture goes with language/translations. It will be used by ResourceManager to look up resources by culture. Namespace of CurrentCulture class is in System.Globalization while CurrentUICulture comes from System.Threading. CurrentCulture is persisted across different requests in the session while CurrentUICulture needs to be set with every request.

Likeness:

They both are System.Globalization.CultureInfo instances.

It is worth noting that the CurrentUICulture supports non-country specific locales such as 'en' (Neutral cultures) whereas CurrentCulture ONLY supports country specific locales such as 'en-GB'. Setting CurrentCulture to a neutral culture will throw an ArgumentException.

I assume that this is because formats such as dates and currency are more strongly linked to the country itself, but the language displayed is often interchangeable between countries.