Use GC.KeepAlive() to prevent early collection of method parameters

[jonpryor]

@brendanzagaeski has diagnosed an issue wherein object references are collected when we think they shouldn't be.

Consider the following generator-emitted binding code:

 public unsafe bool HideSoftInputFromWindow (Android.OS.IBinder? windowToken, [global::Android.Runtime.GeneratedEnum] Android.Views.InputMethods.HideSoftInputFlags flags)
 {
 const string __id = "hideSoftInputFromWindow.(Landroid/os/IBinder;I)Z";
 try {
 JniArgumentValue* __args = stackalloc JniArgumentValue [2];
/* 1 */ __args [0] = new JniArgumentValue ((windowToken == null) ? IntPtr.Zero : ((global::Java.Lang.Object) windowToken).Handle);
/* 2 */ __args [1] = new JniArgumentValue ((int) flags);
/* 3 */ var __rm = _members.InstanceMethods.InvokeAbstractBooleanMethod (__id, this, __args);
 return __rm;
 } finally {
 }
 }

Depending on "various factors", it is possible that Mono's GC will decide that windowToken -- used on line (1) -- is eligible for collection on line (2), when it must be kept alive until after line (3) completes execution.

If windowToken isn't kept alive, then it could be finalized before/during the InvokeAbstractBooleanMethod() invocation, which could invalidate/change windowToken.Handle, and cause lots of head scratching, confusion, and app crashes.

The fix? We need to emit a GC.KeepAlive() for all reference types until after the Invoke* invocation has completed:

 public unsafe bool HideSoftInputFromWindow (Android.OS.IBinder? windowToken, [global::Android.Runtime.GeneratedEnum] Android.Views.InputMethods.HideSoftInputFlags flags)
 {
 const string __id = "hideSoftInputFromWindow.(Landroid/os/IBinder;I)Z";
 try {
 JniArgumentValue* __args = stackalloc JniArgumentValue [2];
 __args [0] = new JniArgumentValue ((windowToken == null) ? IntPtr.Zero : ((global::Java.Lang.Object) windowToken).Handle);
 __args [1] = new JniArgumentValue ((int) flags);
 var __rm = _members.InstanceMethods.InvokeAbstractBooleanMethod (__id, this, __args);
/* FIX */ global::System.GC.KeepAlive (windowToken);
 return __rm;
 } finally {
 }
 }

[brendanzagaeski]

Release status update

A new Preview version of Xamarin.Android has now been published that includes the fix for this item. The fix is not yet included in a Release version. I will update this item again when a Release version is available that includes the fix.

Fix included in Xamarin.Android SDK version 11.1.0.15.

Fix included on Windows in Visual Studio 2019 version 16.8 Preview 4. To try the Preview version that includes the fix, check for the latest updates in Visual Studio Preview.

Fix included on macOS in Visual Studio 2019 for Mac version 8.8 Preview 4. To try the Preview version that includes the fix, check for the latest updates on the Preview updater channel.