string.Contains returns wrong result on net-ios with .net 9

[markuspalme]

Description

string.Contains returns unexpected results when called on an empty string on iOS. This is the code in question:

"".Contains("ö", StringComparison.CurrentCultureIgnoreCase);

Results:

.net 8:

.net 9:

Project to reproduce the issue:
https://github.com/markuspalme/dotnetruntime-contains

Reproduction Steps

To reproduce the issue, call string.Containson an empty string and run the code on net9-ios:

"".Contains("ö", StringComparison.CurrentCultureIgnoreCase);

Expected behavior

The result should be false, an empty string does certainly not contain a character.

Actual behavior

On .net 9 the call returns true.

Regression?

This is a regression, the same code returns false on .net 8.

Known Workarounds

No response

Configuration

.net SDK 9.0.100 running on iOS 18.1 ARM64.

Other information

No response

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-globalization
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[markuspalme]

What's notable is that the culture name changed from .net 8 to .net 9 (same simulator device with the same settings).

I have tried to set the current culture before executing the test, but the result stays the same:

CultureInfo.CurrentCulture = new CultureInfo("en-DE");

[dotnet-policy-service]

Tagging subscribers to 'os-ios': @vitek-karas, @kotlarmilos, @ivanpovazan, @steveisok, @akoeplinger
See info in area-owners.md if you want to be subscribed.

[tarekgh]

@matouskozak could you please have a look? It looks like the used ICU on iOS does not have all collation data.

[matouskozak]

Thanks @markuspalme for filling the issue and @tarekgh for tagging.

I can confirm the issue reproduces. Looks like it works correctly with ASCII search values but breaks with special character. We will have to investigate where the issue is coming from. fyi: @vitek-karas

[markuspalme]

This bug not only applies with special characters, but also in this case with ASCII characters:

"".Contains("H") // returns true

Given how fundamental string.Contains is, can the priority of this bug be bumped up? It is causing a lot of issues with filtering down lists etc.