Description

When cross-compiling for Apple mobile (iOS/tvOS) with EnableAggressiveTrimming=true using the Mono runtime, the Mono IL trimmer incorrectly evaluates PlatformDetection.IsWindows as true. The call chain passes through RuntimeInformation.IsOSPlatform(OSPlatform.Windows) → OperatingSystem.IsOSPlatform("WINDOWS"), which performs a runtime string comparison against the OSPlatformName constant.

Repro

Discovered via the FileVersionInfo_CustomManagedAssembly test (PR #126872). The test uses PlatformDetection.IsWindows to select the expected file extension:

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

When cross-compiled targeting iOS (Mono runtime, EnableAggressiveTrimming=true), the field gets the Windows value (.dll) despite running on iOS, causing:

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

Call Chain

1. PlatformDetection.IsWindows → RuntimeInformation.IsOSPlatform(OSPlatform.Windows) (PlatformDetection.Windows.cs:20)
2. RuntimeInformation.IsOSPlatform(OSPlatform) → OperatingSystem.IsOSPlatform(osPlatform.Name) (RuntimeInformation.cs:26)
3. OperatingSystem.IsOSPlatform(string) → compares against OSPlatformName, a compile-time constant set via #if TARGET_* (OperatingSystem.cs:14-47, 127-138)

Context

• PR with workaround: Fix FileVersionInfo test failure on Apple mobile with trimming #126872

Build Information

Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1378235&view=results
Build error leg or test failing: FileVersionInfo_CustomManagedAssembly
Pull request: #126872

Error Message

Fill the error message using step by step known issues guidance.

{
  "ErrorMessage": "[FAIL] System.Diagnostics.Tests.FileVersionInfoTest.FileVersionInfo_CustomManagedAssembly",
  "BuildRetry": false,
  "ExcludeConsoleLog": false
}

Known issue validation

Build: 🔎 https://dev.azure.com/dnceng-public/public/_build/results?buildId=1378235
Error message validated: [[FAIL] System.Diagnostics.Tests.FileVersionInfoTest.FileVersionInfo_CustomManagedAssembly]
Result validation: ✅ Known issue matched with the provided build.
Validation performed at: 4/15/2026 9:56:28 AM UTC

Report

Summary