Replace dead #if _WINDOWS test guards with runtime OS checks

[paulmedynski]

Summary

The test projects (UnitTests, TestCommon) never define _WINDOWS/_UNIX — those constants are only set by the main src csproj based on TargetOs. This made all #if NET && _WINDOWS blocks around UseManagedNetworking in the test code permanently dead, so the switch's default was never actually exercised.

This PR replaces the compile-time _WINDOWS guards with runtime RuntimeInformation.IsOSPlatform(OSPlatform.Windows) checks, kept under #if NET. The s_useManagedNetworking field / UseManagedNetworking property only exist in the SqlClient assembly when it is built for .NET on Windows (#if NET && _WINDOWS); on Unix .NET it is a constant => true (no field) and on .NET Framework => false (no field). Since the repo builds per-OS (build OS == run OS), the runtime Windows check is an accurate proxy for the old _WINDOWS constant, and #if NET still excludes .NET Framework where the field never exists.

Changes

• tests/Common/LocalAppContextSwitchesHelper.cs
  • Added using System.Runtime.InteropServices;.
  • Changed the _useManagedNetworkingOriginal field and UseManagedNetworking property guards from #if NET && _WINDOWS to #if NET.
  • Wrapped capture (constructor) and restore (Dispose) of s_useManagedNetworking in if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows)).
• tests/UnitTests/.../LocalAppContextSwitchesTest.cs
  • Added using System.Runtime.InteropServices;.
  • Replaced both #if NET && _WINDOWS blocks (the reset and the Assert.False(...UseManagedNetworking)) with runtime Windows checks inside the existing #if NET.

Effect

• On Windows / .NET: the UseManagedNetworking default (false) is now actually tested — previously the assertion was dead code.
• On Unix / .NET and .NET Framework: behavior unchanged (branch skipped / excluded), and the nonexistent field is never touched via reflection.

Testing

• Tests added or updated (re-enabled the previously-dead UseManagedNetworking coverage)
• Public API changes documented (n/a)
• No breaking changes introduced

Verified on Linux / net9.0:

• UnitTests build: succeeded, 0 warnings, 0 errors.
• LocalAppContextSwitchesTest: passed (Windows branch correctly skipped at runtime).

Note: the net462 / Windows path could not be executed from the Linux dev host, but that path is unchanged in behavior (still excluded via #if NET).

Work item: AB#45771

[Copilot]

Pull request overview

This PR fixes dead test code paths by replacing compile-time #if NET && _WINDOWS guards (which aren’t defined in the test projects) with runtime OS checks, so the UseManagedNetworking AppContext switch behavior is actually exercised where it exists.

Changes:

• Updated test and helper code to use RuntimeInformation.IsOSPlatform(OSPlatform.Windows) under #if NET instead of _WINDOWS.
• Ensured reflection-based capture/restore of s_useManagedNetworking only happens at runtime on Windows.
• Re-enabled effective coverage of the Windows/.NET default for UseManagedNetworking (previously skipped at compile time).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Microsoft.Data.SqlClient/tests/Common/LocalAppContextSwitchesHelper.cs	Switch capture/restore now uses runtime Windows checks under #if NET to avoid reflecting missing fields on non-Windows.
src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/LocalAppContextSwitchesTest.cs	Replaced dead _WINDOWS guarded assertions/reset with runtime Windows checks inside the existing #if NET.

Comments suppressed due to low confidence (1)

src/Microsoft.Data.SqlClient/tests/Common/LocalAppContextSwitchesHelper.cs:386

• UseManagedNetworking is now compiled for all .NET test runs, but its setter always reflects the private field s_useManagedNetworking, which only exists in the product assembly when built for .NET on Windows. If a test accidentally sets this property on non-Windows, it will fail with a generic "Field not found" exception. Consider guarding the setter with a Windows runtime check and throwing a clearer PlatformNotSupportedException (and update the remarks accordingly).

    /// <remarks>
    /// The underlying s_useManagedNetworking field only exists in the SqlClient
    /// assembly when it is built for .NET on Windows. Callers must only use this
    /// property when running on Windows (see
    /// RuntimeInformation.IsOSPlatform(OSPlatform.Windows)); otherwise the
    /// reflection lookup of the field will fail.
    /// </remarks>
    public bool? UseManagedNetworking
    {
        get => GetSwitchPropertyValue(nameof(UseManagedNetworking));
        set => SetSwitchValue("s_useManagedNetworking", value);
    }

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.94%. Comparing base (c30bbbc) to head (cb1f654).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4423      +/-   ##
==========================================
- Coverage   65.78%   63.94%   -1.85%     
==========================================
  Files         286      282       -4     
  Lines       43696    66658   +22962     
==========================================
+ Hits        28745    42622   +13877     
- Misses      14951    24036    +9085     

Flag	Coverage Δ
CI-SqlClient	?
PR-SqlClient-Project	63.94% <ø> (?)

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.