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Allow ApplicationLanguages.PrimaryLanguageOverride to be unset #5726

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d12d29b
Allow empty string for PrimaryLanguageOverride to unset value
whiskhub Aug 20, 2025
7881142
Add unit test to verify PrimaryLanguageOverride unset behavior
whiskhub Aug 20, 2025
076d803
Add test to verify OS value is properly set
whiskhub Sep 26, 2025
ad46f6d
Merge branch 'microsoft:main' into main
whiskhub Sep 26, 2025
fd9e16d
Move packaged-only tests
whiskhub Sep 26, 2025
1352029
Fix namespace usings
whiskhub Sep 30, 2025
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21 changes: 21 additions & 0 deletions dev/MRTCore/mrt/Microsoft.Windows.ApplicationModel.Resources/TestCommon/CommonTestCode.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,7 @@
using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Microsoft.Windows.ApplicationModel.Resources;
using Microsoft.Windows.Globalization;
#else
using System;
using System.ComponentModel;
Expand All @@ -18,6 +19,7 @@
using WEX.TestExecution;
using WEX.TestExecution.Markup;
using Microsoft.Windows.ApplicationModel.Resources;
using Microsoft.Windows.Globalization;
#endif

namespace CommonTestCode
Expand Down Expand Up @@ -585,4 +587,23 @@ public static void NoResourceFileWithContextTest()
Verify.IsNull(resourceCandidate);
}
}

public class ApplicationLanguagesTest
{
public static void PrimaryLanguageOverrideAcceptsEmptyStringTest()
{
ApplicationLanguages.PrimaryLanguageOverride = "fr-FR";
ApplicationLanguages.PrimaryLanguageOverride = "";

Verify.AreEqual(ApplicationLanguages.PrimaryLanguageOverride, "");
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consider adding a verification that winrt::Windows::Globalization::ApplicationLanguages::PrimaryLanguageOverride is reset for packaged process, as that's what matters.

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Thanks! I've added a test

}

public static void PrimaryLanguageOverrideAcceptsNullStringTest()
{
ApplicationLanguages.PrimaryLanguageOverride = "fr-FR";
ApplicationLanguages.PrimaryLanguageOverride = null;

Verify.AreEqual(ApplicationLanguages.PrimaryLanguageOverride, ""); // C# projection of null HSTRING is empty string
}
}
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The tests are added under CommonTestCode. But they don't work for unpackaged test. Ideally they need to be moved to unittests.cs, or add a packaged check.

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True, I moved the test code over

}
2 changes: 1 addition & 1 deletion dev/MRTCore/mrt/Microsoft.Windows.ApplicationModel.Resources/src/ApplicationLanguages.cpp
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Expand Up @@ -41,7 +41,7 @@ namespace winrt::Microsoft::Windows::Globalization::implementation

void ApplicationLanguages::PrimaryLanguageOverride(hstring const& language)
{
bool isValidLanguageTag = IsWellFormedTag(language.c_str());
bool isValidLanguageTag = language.empty() || IsWellFormedTag(language.c_str());
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my impression is that winrt::Windows::Globalization::ApplicationLanguages::PrimaryLanguageOverride(language) requires nullptr to reset PrimaryLanguageOverride. Have you verified that the test reached line 53?

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In WinRT, an empty HSTRING is equivalent to a nullptr (https://devblogs.microsoft.com/oldnewthing/20160615-00/?p=93675).

So, windows::Globalization::ApplicationLanguages::PrimaryLanguageOverride does accept both nullptr/null and empty string.

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Also, it doesn't seem possible for externals to build anything from the Windows App SDK due to missing permissions for the ProjectReunion internal NuGet source. So I was only able to test in my own app.

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I checked the implementation of winrt::Windows::Globalization::ApplicationLanguages::PrimaryLanguageOverride(language) in OS, and it will reset the override when null is passed in, and fail if empty string is passed in.

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In C++/WinRT it is impossible to compare a winrt::hstring to nullptr - the equality operator is explicitly deleted: bool operator==(hstring const& left, std::nullptr_t) = delete;
This is exactly because an empty HSTRING is always represented by a null pointer internally. Please see the blog post by Raymond Chen: https://devblogs.microsoft.com/oldnewthing/20220706-00/?p=106836

Maybe the OS code is not written in C++WinRT and is therefore able to compare with nullptr, but code with C++/WinRT cannot do that.

In C#, WG.ApplicationLanguages.PrimaryLanguageOverride = null; and WG.ApplicationLanguages.PrimaryLanguageOverride = ""; will also both work. You can try in a test app.

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OS implementation doesn't use C++/WinRT. I assume it works because eventually nullptr is passed to it.


THROW_HR_IF_MSG(E_INVALIDARG, !isValidLanguageTag, "The parameter is incorrect");

Expand Down
27 changes: 27 additions & 0 deletions dev/MRTCore/mrt/Microsoft.Windows.ApplicationModel.Resources/unittests/UnitTest.cs
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Expand Up @@ -3,6 +3,7 @@

using System;
using Microsoft.Windows.ApplicationModel.Resources;
using Microsoft.Windows.Globalization;
using WEX.Common.Managed;
using WEX.Logging.Interop;
using WEX.TestExecution;
Expand Down Expand Up @@ -165,4 +166,30 @@ public void NoResourceFileWithContextTest()
CommonTestCode.ResourceContextTest.NoResourceFileWithContextTest();
}
}

[TestClass]
public class ApplicationLanguagesTest
{
[TestMethod]
public void PrimaryLanguageOverrideAcceptsEmptyStringTest()
{
CommonTestCode.ApplicationLanguagesTest.PrimaryLanguageOverrideAcceptsEmptyStringTest();
}

[TestMethod]
public void PrimaryLanguageOverrideAcceptsNullStringTest()
{
CommonTestCode.ApplicationLanguagesTest.PrimaryLanguageOverrideAcceptsNullStringTest();
}

[TestMethod]
public void PrimaryLanguageOverrideSetsWindowsValueTest()
{
ApplicationLanguages.PrimaryLanguageOverride = "fr-FR";
Verify.AreEqual(Windows.Globalization.ApplicationLanguages.PrimaryLanguageOverride, "fr-FR");

ApplicationLanguages.PrimaryLanguageOverride = null;
Verify.AreEqual(Windows.Globalization.ApplicationLanguages.PrimaryLanguageOverride, ""); // C# projection of null HSTRING is empty string
}
}
}