.NET 5中en-AU区域性下DateTimeFormat.FirstDayOfWeek返回错误值的问题及解决方法

Great question! Let's break down why this discrepancy happens and how to fix it in .NET 5.

Why the Difference? #

The root cause lies in how globalization data is handled between .NET Framework and .NET 5+:

• .NET Framework (like 4.7.2) relies on Windows NLS (National Language Support) for regional data, which correctly sets Monday as the first day of the week for the en-AU culture.
• .NET 5+ uses ICU (International Components for Unicode) by default as its cross-platform globalization engine. Early versions of ICU had a mismatch in the FirstDayOfWeek value for en-AU, incorrectly returning Sunday instead of Monday.

How to Get the Correct Value in .NET 5 #

Here are three reliable solutions depending on your use case:

1. Switch Back to Windows NLS (Windows-only Apps)

If your application only runs on Windows, you can revert to using the same regional data as .NET Framework. Add this configuration to your .csproj file:

<PropertyGroup>
  <UseSystemResourceKeys>true</UseSystemResourceKeys>
</PropertyGroup>

Alternatively, set the environment variable DOTNET_SYSTEM_GLOBALIZATION_USENLS=true when launching your app. This will make .NET 5 use Windows' NLS data, so FirstDayOfWeek will return Monday for en-AU.

2. Manually Set the First Day of the Week

For cross-platform compatibility, explicitly set the correct FirstDayOfWeek for the en-AU culture:

var auCulture = new CultureInfo("en-AU");
auCulture.DateTimeFormat.FirstDayOfWeek = DayOfWeek.Monday;

// Use this culture instance for date formatting operations
var firstDay = auCulture.DateTimeFormat.FirstDayOfWeek; // Returns Monday

If you want this to apply globally across your app, set it as the default thread culture:

var auCulture = new CultureInfo("en-AU");
auCulture.DateTimeFormat.FirstDayOfWeek = DayOfWeek.Monday;

CultureInfo.DefaultThreadCurrentCulture = auCulture;
CultureInfo.DefaultThreadCurrentUICulture = auCulture;

3. Upgrade to a Newer .NET Version

Microsoft fixed this ICU data mismatch in later .NET releases (starting with .NET 6). Upgrading to .NET 6 or a newer LTS version will automatically resolve the issue, and you'll get the correct FirstDayOfWeek value for en-AU without any code changes.

内容的提问来源于stack exchange，提问作者Luca Bellavia