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Commit9a27140

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[Mono.Android] JavaCast<T>() should throw on unsupported types (#9145)
Context:dotnet/java-interop#910Context:dotnet/java-interop@1adb796Context:8928f11Context:35f41dcContext:ab412a5The `.JavaCast<T>()` extension method is for checking type castsacross the JNI boundary. As such, it can be expected to fail.Failure is indicated by throwing an exception.var n = new Java.Lang.Integer(42);var a = n.JavaCast<Java.Lang.IAppendable>(); // this should throw, right?The last line should throw because, at this point anyway,[`java.lang.Integer`][0] doesn't implement the[`java.lang.Appendable`][1] interface.What *actually* happens as ofab412a5 is that`n.JavaCast<IAppendable>()` ***does not throw***;instead, it returns `null`!This is a ***BREAK*** from .NET 8, in which an exception *is* thrown:System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.InvalidCastException: Unable to convert instance of type 'crc64382c01b66da5f22f/MainActivity' to type 'java.lang.Appendable'. at Java.Lang.IAppendableInvoker.Validate(IntPtr handle) at Java.Lang.IAppendableInvoker..ctor(IntPtr handle, JniHandleOwnership transfer) at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Constructor(Object obj, IntPtr* args) at System.Reflection.MethodBaseInvoker.InvokeDirectByRefWithFewArgs(Object obj, Span`1 copyOfArgs, BindingFlags invokeAttr) --- End of inner exception stack trace --- at System.Reflection.MethodBaseInvoker.InvokeDirectByRefWithFewArgs(Object obj, Span`1 copyOfArgs, BindingFlags invokeAttr) at System.Reflection.MethodBaseInvoker.InvokeWithFewArgs(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) at System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) at System.Reflection.ConstructorInfo.Invoke(Object[] parameters) at Java.Interop.TypeManager.CreateProxy(Type type, IntPtr handle, JniHandleOwnership transfer) at Java.Interop.TypeManager.CreateInstance(IntPtr handle, JniHandleOwnership transfer, Type targetType) at Java.Lang.Object.GetObject(IntPtr handle, JniHandleOwnership transfer, Type type) at Java.Interop.JavaObjectExtensions._JavaCast[IAppendable](IJavaObject instance)What happened™?!What happened are (at least) two things:First isdotnet/java-interop@1adb7964 and8928f11, which optimizedinterface invokers (dotnet/java-interop#910). Part of thisoptimization involved *removing* the `Validate(IntPtr)` method fromthe interface invoker classes. (Was this part of the optimization?No. It's just that@jonpryor didn't think that `Validate()` wasneeded anymore, and that the checks it performed was already doneelsewhere. Turns Out™, that wasn't true!)The result of8928f11 is that `.JavaCast<T>()` would *always* returna non-`null` value for the requested type. Worse, any methodinvocations on that value would ***CRASH THE PROCESS***:var n = new Java.Lang.Integer(42);var a = n.JavaCast<Java.Lang.IAppendable>(); // does NOT throwa.Append('a');// app dies`adb logcat` contains:JNI DETECTED ERROR IN APPLICATION: can't call java.lang.Appendable java.lang.Appendable.append(char) on instance of java.lang.Integer in call to CallObjectMethodAOops.Secondly, commit35f41dc actually added the checks that@jonpryorthought were already there, but as part of these checks`TypeManager.CreateInstance()` now returns `null` if the Java-sidetype checks fail. This means that `.JavaCast<T>()` now returns`null`, which prevents the above `JNI DETECTED ERROR IN APPLICATION`crash (good), but means `.JavaCast<T>()` returns `null` which willlikely result in `NullReferenceException`s if the value is used.Fix `.JavaCast<T>()` by updating `JavaObjectExtensions.JavaCast<T>()`to throw an `InvalidCastException` if `TypeManager.CreateInstance()`returns `null`.Additionally, massively simplify the `.JavaCast<T>()` code paths byremoving the semantic distinction between interface and class types.This in turn introduces *new* (intentional!) semantic changes.Suppose we have an `ArrayList` instance:var list = new Java.Util.ArrayList();and to ensure "appropriate magic" happens, *alias* `list`:var alias = new Java.Lang.Object(list.Handle, JniHandleOwnership.DoNotTransfer);We now have two managed instances referring to the same Java instance.Next, we want to use `alias` as an `AbstractList`:var abstractList = alias.JavaCast<Java.Util.AbstractList>();var newAlias = !object.ReferenceEquals(list, abstractList);As `java.util.ArrayList` inherits `java.util.AbstractList`, this willreturn a non-`null` value. *However*, what *type and instance* will`abstractList` be?In .NET 8 and .NET 9 *prior to this commit*, `newAlias` will be trueand `abstractList` will be an `AbstractListInvoker`, meaning therewill be *three* managed instances referring to the same Java-side`ArrayList`.Starting with this commit, `abstractList` be the same value as `list`,have a runtime type of `ArrayList`, and `newAlias` will be false.[0]:https://developer.android.com/reference/java/lang/Integer[1]:https://developer.android.com/reference/java/lang/Appendable
1 parentea7d8c2 commit9a27140

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3 files changed

+32
-44
lines changed

3 files changed

+32
-44
lines changed

‎src/Mono.Android/Java.Interop/JavaObjectExtensions.cs‎

Lines changed: 6 additions & 43 deletions
Original file line numberDiff line numberDiff line change
@@ -81,41 +81,9 @@ internal static TResult? _JavaCast<
8181
if(instance.Handle==IntPtr.Zero)
8282
thrownewObjectDisposedException(instance.GetType().FullName);
8383

84-
TyperesultType=typeof(TResult);
85-
if(resultType.IsClass){
86-
return(TResult)CastClass(instance,resultType);
87-
}
88-
elseif(resultType.IsInterface){
89-
return(TResult?)Java.Lang.Object.GetObject(instance.Handle,JniHandleOwnership.DoNotTransfer,resultType);
90-
}
91-
else
92-
thrownewNotSupportedException(FormattableString.Invariant($"Unable to convert type '{instance.GetType().FullName}' to '{resultType.FullName}'."));
93-
}
94-
95-
staticIJavaObjectCastClass(
96-
IJavaObjectinstance,
97-
[DynamicallyAccessedMembers(Constructors)]
98-
TyperesultType)
99-
{
100-
varklass=JNIEnv.FindClass(resultType);
101-
try{
102-
if(klass==IntPtr.Zero)
103-
thrownewArgumentException("Unable to determine JNI class for '"+resultType.FullName+"'.","TResult");
104-
if(!JNIEnv.IsInstanceOf(instance.Handle,klass))
105-
thrownewInvalidCastException(
106-
FormattableString.Invariant($"Unable to convert instance of type '{instance.GetType().FullName}' to type '{resultType.FullName}'."));
107-
}finally{
108-
JNIEnv.DeleteGlobalRef(klass);
109-
}
110-
111-
if(resultType.IsAbstract){
112-
// TODO: keep in sync with TypeManager.CreateInstance() algorithm
113-
varinvokerType=GetInvokerType(resultType);
114-
if(invokerType==null)
115-
thrownewArgumentException("Unable to get Invoker for abstract type '"+resultType.FullName+"'.","TResult");
116-
resultType=invokerType;
117-
}
118-
return(IJavaObject)TypeManager.CreateProxy(resultType,instance.Handle,JniHandleOwnership.DoNotTransfer);
84+
return(TResult)Java.Lang.Object.GetObject(instance.Handle,JniHandleOwnership.DoNotTransfer,typeof(TResult))??
85+
thrownewInvalidCastException(
86+
FormattableString.Invariant($"Unable to convert instance of type '{instance.GetType().FullName}' to type '{typeof(TResult).FullName}'."));
11987
}
12088

12189
internalstaticIJavaObject?JavaCast(
@@ -132,14 +100,9 @@ static IJavaObject CastClass (
132100
if(resultType.IsAssignableFrom(instance.GetType()))
133101
returninstance;
134102

135-
if(resultType.IsClass){
136-
returnCastClass(instance,resultType);
137-
}
138-
elseif(resultType.IsInterface){
139-
return(IJavaObject?)Java.Lang.Object.GetObject(instance.Handle,JniHandleOwnership.DoNotTransfer,resultType);
140-
}
141-
else
142-
thrownewNotSupportedException(FormattableString.Invariant($"Unable to convert type '{instance.GetType().FullName}' to '{resultType.FullName}'."));
103+
return(IJavaObject?)Java.Lang.Object.GetObject(instance.Handle,JniHandleOwnership.DoNotTransfer,resultType)??
104+
thrownewInvalidCastException(
105+
FormattableString.Invariant($"Unable to convert instance of type '{instance.GetType().FullName}' to type '{resultType.FullName}'."));
143106
}
144107

145108
// typeof(Foo) -> FooInvoker

‎src/Mono.Android/Java.Interop/TypeManager.cs‎

Lines changed: 0 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -336,7 +336,6 @@ internal static IJavaPeerable CreateInstance (IntPtr handle, JniHandleOwnership
336336

337337
handleClass=JniEnvironment.Types.GetObjectClass(newJniObjectReference(handle));
338338
if(!JniEnvironment.Types.IsAssignableFrom(handleClass,typeClass)){
339-
Logger.Log(LogLevel.Info,"*jonp*",$"# jonp: can't assign `{GetClassName(handleClass.Handle)}` to `{GetClassName(typeClass.Handle)}`");
340339
returnnull;
341340
}
342341
}finally{

‎tests/Mono.Android-Tests/Java.Interop/JavaObjectExtensionsTests.cs‎

Lines changed: 26 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -40,6 +40,32 @@ public void JavaCast_InterfaceCast ()
4040
}
4141
}
4242

43+
[Test]
44+
publicvoidJavaCast_BadInterfaceCast()
45+
{
46+
usingvarn=newJava.Lang.Integer(42);
47+
vare=Assert.Catch<Exception>(()=>JavaObjectExtensions.JavaCast<Java.Lang.IAppendable>(n));
48+
if(eisSystem.Reflection.TargetInvocationExceptiontie){
49+
// .NET 8 behavior
50+
Assert.IsTrue(tie.InnerExceptionisInvalidCastException);
51+
}elseif(eisInvalidCastExceptionice){
52+
// Yay
53+
}else{
54+
Assert.Fail($"Unexpected exception type:{e.GetType()}:{e.ToString()}");
55+
}
56+
}
57+
58+
[Test]
59+
publicvoidJavaCast_ObtainOriginalInstance()
60+
{
61+
usingvarlist=newJava.Util.ArrayList();
62+
usingvarcopy=newJava.Lang.Object(list.Handle,JniHandleOwnership.DoNotTransfer);
63+
usingvaral=JavaObjectExtensions.JavaCast<Java.Util.AbstractList>(copy);
64+
65+
// Semantic change: in .NET 8, `al` is a new `AbstractListInvoker` instance, not `list`
66+
Assert.AreSame(list,al);
67+
}
68+
4369
[Test]
4470
publicvoidJavaCast_InvalidTypeCastThrows()
4571
{

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