Fix FileVersionInfo test failure on Apple mobile with trimming

[kotlarmilos]

Note

PR description was AI/Copilot-generated.

Summary

Fixes FileVersionInfo_CustomManagedAssembly test failure on Apple mobile (iOS/tvOS) when running with EnableAggressiveTrimming=true.

Problem

The test expected InternalName with .dll extension on iOS, but the actual value was .exe. The field initializer used PlatformDetection.IsWindows to determine the expected extension:

private readonly string OriginalTestAssemblyInternalName = PlatformDetection.IsWindows ?
    "System.Diagnostics.FileVersionInfo.TestAssembly.dll" :
    "System.Diagnostics.FileVersionInfo.TestAssembly.exe";

PlatformDetection.IsWindows chains through RuntimeInformation.IsOSPlatform(OSPlatform.Windows), which appears to be incorrectly evaluated by the IL trimmer during cross-compilation (Windows host → iOS target), causing the field to be initialized with the Windows value (.dll) on iOS.

Fix

Replace PlatformDetection.IsWindows with OperatingSystem.IsWindows(), which is a compile-time constant (#if TARGET_WINDOWS) in the BCL and is immune to trimmer mis-evaluation.

Test Failure

Assert.Equal() Failure: Strings differ
Expected: ···"ystem.Diagnostics.FileVersionInfo.TestAssembly.dll"
Actual:   ···"ystem.Diagnostics.FileVersionInfo.TestAssembly.exe"

[Copilot]

Pull request overview

Fixes a trimming-related test expectation bug for Apple mobile (iOS/tvOS) by avoiding a runtime OS check (PlatformDetection.IsWindows) that can be mis-evaluated during cross-compilation + aggressive trimming, and instead using OperatingSystem.IsWindows() to select the expected internal-name extension.

Changes:

• Update FileVersionInfo_CustomManagedAssembly expected InternalName/OriginalFilename extension selection to use OperatingSystem.IsWindows().

[dotnet-policy-service]

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

[kotlarmilos]

/azp run runtime-ioslike

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[kotlarmilos]

The test is passing in runtime-ioslike pipeline: https://helix.dot.net/api/jobs/5906f052-6013-4ecd-96eb-7c3d494e21ab/workitems?api-version=2019-06-17

[jkotas]

PlatformDetection.IsWindows chains through RuntimeInformation.IsOSPlatform(OSPlatform.Windows), which appears to be incorrectly evaluated by the IL trimmer during cross-compilation (Windows host → iOS target), causing the field to be initialized with the Windows value (.dll) on iOS.

We should fix the IL trimmer evaluation during cross-compilation instead of this workaround.

[kotlarmilos]

Created a tracking issue and closing this PR: #126882