Fix GCHandle leak in NetworkReachability.SetNotification

[Copilot]

NetworkReachability.SetNotification allocates a GCHandle to keep the object alive during native callbacks but never frees it, causing a memory leak.

Changes

• Track allocation state: Added gchAllocated bool field
• Free on removal: Free GCHandle when SetNotification(null) is called
• Free on dispose: Override Dispose to free handle if object is disposed without explicitly removing notification
• Add tests: Added 4 tests covering set/remove, dispose, multiple cycles, and callback updates

Example

// Before: GCHandle leaks when notification is removed or object is disposed
using var nr = new NetworkReachability(IPAddress.Loopback);
nr.SetNotification(flags => Console.WriteLine(flags));
nr.SetNotification(null);  // GCHandle leaked here
// GCHandle also leaked on dispose

// After: GCHandle properly freed in both cases

Original prompt

---

This section details on the original issue you should resolve

<issue_title>NetworkReachability.SetNotification leaks a GCHandle</issue_title>
<issue_description>The GCHandle allocated here:

https://github.com/xamarin/xamarin-macios/blob/2930c052b32318af63c1f3b90f6447f6907548b8/src/SystemConfiguration/NetworkReachability.cs#L252

is never freed.

Also we don't have tests for NetworkReachability.SetNotification.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes NetworkReachability.SetNotification leaks a GCHandle #12962

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[vs-mobiletools-engineering-service2]

✅ [CI Build #4834577] Build passed (Build packages) ✅

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[vs-mobiletools-engineering-service2]

✅ [PR Build #4834577] Build passed (Detect API changes) ✅

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Hash: 48345778f6685c49978be336dc77c74cde1ecc19 [PR build]

[vs-mobiletools-engineering-service2]

✅ API diff for current PR / commit

NET (empty diffs)

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

✅ API diff vs stable

NET (empty diffs)

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

ℹ️ Generator diff

Generator Diff: vsdrops (html) vsdrops (raw diff) gist (raw diff) - Please review changes)

Pipeline on Agent
Hash: 48345778f6685c49978be336dc77c74cde1ecc19 [PR build]

[vs-mobiletools-engineering-service2]

🔥 [CI Build #4834577] Build failed (Build macOS tests) 🔥

Build failed for the job 'Build macOS tests' (with job status 'Failed')

Pipeline on Agent
Hash: 48345778f6685c49978be336dc77c74cde1ecc19 [PR build]

[rolfbjarne]

Closing in favor of #24147.