[ObjCRuntime] Provide a way to remove an object from the object map.

[rolfbjarne]

Problem: empty NSStrings (@"") are singletons, and trying to create one always returns the same handle.

This causes problems for the following sequence of events:

• Such a singleton is exposed to managed code, and a corresponding managed wrapper is created.
• The managed wrapper is manually disposed.
• The singleton surfaces again, and now we can end up returning the existing (disposed) wrapper from the object map instead of a new managed instance.

For empty NSStrings in particular:

1. There's a managed static NSString.Empty field, with a managed wrapper for a native @"" singleton.
2. Tests can create managed wrappers for @"", and if those managed wrappers are disposed, the managed NSString.Empty field now points to a disposed managed wrapper.

The MonoTouchFixtures.Foundation.BundleTest.GetLocalizedString test in particular disposes of strings, and can dispose NSString.Empty. This is from a modified runtime where an exception is thrown if NSString.Empty is disposed:

MonoTouchFixtures.Foundation.BundleTest
    [FAIL] GetLocalizedString : System.InvalidOperationException : This Foundation.NSString instance was unexpectedly disposed
       at Foundation.NSObject.Dispose(Boolean disposing) in /Users/rolf/work/dotnet/macios/main/macios/src/Foundation/NSObject2.cs:line 1108
       at Foundation.NSObject.Dispose() in /Users/rolf/work/dotnet/macios/main/macios/src/Foundation/NSObject2.cs:line 365
       at MonoTouchFixtures.Foundation.BundleTest.GetLocalizedString()
       at MonoTouchFixtures.Foundation.BundleTest.GetLocalizedString()
       at System.RuntimeMethodHandle.InvokeMethod(ObjectHandleOnStack target, Void** arguments, ObjectHandleOnStack sig, BOOL isConstructor, ObjectHandleOnStack result)
       at System.RuntimeMethodHandle.InvokeMethod(ObjectHandleOnStack target, Void** arguments, ObjectHandleOnStack sig, BOOL isConstructor, ObjectHandleOnStack result)
       at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
       at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)

Fixes this test failure:

MonoTouchFixtures.Foundation.StringTest
    [FAIL] ReleaseEmptyString :   RetainCount
    Expected: 18446744073709551615
    But was:  0
        at MonoTouchFixtures.Foundation.StringTest.ReleaseEmptyString()

because this test calls release a number of times on the static NSString.Empty field, and expects a certain retainCount afterwards, but that fails because NSString.Empty has been disposed, and then RetainCount returns 0 because it's being called on a null native object.

[Copilot]

Pull request overview

Adds an internal mechanism to remove specific managed wrappers from the ObjCRuntime object map to avoid reusing disposed wrappers for native singleton/constant handles (notably the @"" empty NSString singleton).

Changes:

• Add Runtime.RemoveFromObjectMap (NSObject obj) to remove a specific wrapper from the runtime object map.
• Add an internal NSObject.RemoveFromObjectMap setter hook to invoke the runtime removal from object-initializer syntax.
• Mark NSString.Empty to be removed from the object map immediately after creation to prevent it from being returned/disposed via handle lookups.

Reviewed changes

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

File	Description
src/ObjCRuntime/Runtime.cs	Introduces RemoveFromObjectMap API and related documentation near object-map lookup logic.
src/Foundation/NSObject2.cs	Adds an internal setter-only hook to trigger object-map removal on an instance.
src/Foundation/NSString.cs	Updates NSString.Empty initialization to remove it from the object map.

[dalexsoto]

Whoa really interesting issue 👍

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✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (MacCatalyst): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (macOS): All 1 tests passed. Html Report (VSDrops) Download
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✅ dotnettests (tvOS): All 1 tests passed. Html Report (VSDrops) Download
✅ framework: All 2 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 4 tests passed. Html Report (VSDrops) Download
✅ generator: All 5 tests passed. Html Report (VSDrops) Download
✅ interdependent-binding-projects: All 4 tests passed. Html Report (VSDrops) Download
✅ introspection: All 6 tests passed. Html Report (VSDrops) Download
✅ linker: All 44 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 11 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 15 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 12 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 11 tests passed. Html Report (VSDrops) Download
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✅ xcframework: All 4 tests passed. Html Report (VSDrops) Download
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✅ Tests on macOS Monterey (12): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Ventura (13): All 5 tests passed. Html Report (VSDrops) Download
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