Globalization APIs use ICU libraries on Windows 10

.NET 5 and later versions use International Components for Unicode (ICU) libraries for globalization functionality when running on Windows 10 May 2019 Update or later.

Change description

In .NET Core 1.0 - 3.1 and .NET Framework 4 and later, .NET libraries use National Language Support (NLS) APIs for globalization functionality on Windows. For example, NLS functions were used to compare strings, get culture information, and perform string casing in the appropriate culture.

Starting in .NET 5, if an app is running on Windows 10 May 2019 Update or later, .NET libraries use ICU globalization APIs, by default.


Windows 10 May 2019 Update and later versions ship with the ICU native library. If the .NET runtime can't load ICU, it uses NLS instead.

Behavioral differences

You might see changes in your app even if you don't realize you're using globalization facilities. This section lists a couple of the behavioral changes you might see, but there are others too.


Consider the following code that calls String.IndexOf(String) to find the index of the newline character in a string.

string s = "Hello\r\nworld!";
int idx = s.IndexOf("\n");
  • In .NET Core 3.1 and earlier versions on Windows, the snippet prints 6.
  • In .NET 5 and on Windows 10 May 2019 Update and later versions, the snippet prints -1.
  • In .NET 6 and later versions, the snippet prints 6, however, ICU libraries are still used.

To fix this code by conducting an ordinal search instead of a culture-sensitive search, call the IndexOf(String, StringComparison) overload and pass in StringComparison.Ordinal as an argument.

You can run code analysis rules CA1307: Specify StringComparison for clarity and CA1309: Use ordinal StringComparison to find these call sites in your code.

For more information, see Behavior changes when comparing strings on .NET 5+.

Currency symbol

Consider the following code that formats a string using the currency format specifier C. The current thread's culture is set to a culture that includes only the language and not the country or region.

System.Threading.Thread.CurrentThread.CurrentCulture = new System.Globalization.CultureInfo("de");
string text = string.Format("{0:C}", 100);
  • In .NET Core 3.1 and earlier versions on Windows, the value of text is "100,00 €".
  • In .NET 5 and later versions on Windows 19H1 and later versions, the value of text is "100,00 ¤", which uses the international currency symbol instead of the euro. In ICU, the design is that a currency is a property of a country or region, not a language.

Reason for change

This change was introduced to unify .NET's globalization behavior across all supported operating systems. It also provides the ability for applications to bundle their own globalization libraries rather than depend on the operating system's built-in libraries.

Version introduced

.NET 5.0

No action is required on the part of the developer. However, if you wish to continue using NLS globalization APIs, you can set a run-time switch to revert to that behavior. For more information about the available switches, see the .NET globalization and ICU article.

Affected APIs

See also