Why is Android.Runtime.JavaObject internal?

[cyraid]

Android framework version

net9.0-android

Affected platform version

VS 2022

Description

So I get an item from AdapterView using GetItemAtPosition and I get a JavaObject with an instance field in there that I need but I cannot access unless with Reflection as per some Stackoverflow posts on how to get the C# instance object out of a JavaObject. Why is this not available to us? Could someone make it available? :)

Steps to Reproduce

1. Make new Android Project
2. Use ListView
3. Have an ArrayAdapter (for example)
4. Use a class that is not Java.Lang.Object for the items in ArrayAdapter.
5. Add an Item Click
6. Try using AdapterView to GetItemAtPosition and be surprised that you can only get a Java.Lang.Object without being able to access the instance.

Did you find any workaround?

No response

Relevant log output

[grendello]

Hi @cyraid, the short answer to "why is it internal" is that it's "always" been like this, ever since the class got added 15 years ago. Here's the commit message (it's not in the public repo, it predates open-sourcing of Xamarin.Android by a few years):

Date:   Tue Oct 12 21:49:21 2010 +0000

    Add handling for non-IJavaObjects in the generic type converter.  This
    uses a new internal JavaObject proxy which hooks into all the virtual
    methods on JLO and forwards to the proxied System.Object.  This might be
    robust enough to use to pursue some more general conversion of JLOs in
    the API.

This is internal API without anything that can be of value to code external to Mono.Android.dll. The only field in the class is object inst;, which is an instance of some Java.Lang.Object derivative class and since, as you describe, your class is not derived from Java.Lang.Object there's not much value there for you.

[cyraid]

According to Yonglun Liu Microsoft External Staff and other answers on StackOverflow (like here) have the following:

public static class ObjectTypeHelper
{
    public static T Cast<T>(this Java.Lang.Object obj) where T : class
    {
        var propertyInfo = obj.GetType().GetProperty("Instance");
        return propertyInfo == null ? null : propertyInfo.GetValue(obj, null) as T;
    }
}

Let me re-iterate: If you call GetItem from an AdapterView you get a Android.Runtime.JavaObject, not your object you put in. Inside that object is the Instance property, which gets the inst field, according to:

public object Instance {
	get { return inst; }
}

https://github.com/dotnet/android/blob/main/src/Mono.Android/Android.Runtime/JavaObject.cs#L90-L92

But since the entire class is internal, we have no access to it, yet people are needing that instance just getting it with reflection. But then if you want trimming, you have to go through other hoops.

Wouldn't it be better to just expose it? You don't even need to expose it, you could add it to src/Mono.Android/Java.Interop/JavaObjectExtensions.cs and add something like (untested):

public static bool TryCast<TResult>(this Android.Runtime.JavaObject obj, [MaybeNullWhen(false)] out TResult dest) where TResult : class
{

  if (obj.Instance is TResult destObj)
  {
    dest = destObj;
    return true;
  }

  dest = default(TResult);

  return false;

}

Edit: That inst is not an instance of another Java Object. I'm currently using the GetProperty method listed above, and getting my original Instance this way (I'm using ArrayAdapter and the objects going into it are not derived from Java.Lang.Object) but are being converted internally to JavaObject it seems.

[grendello]

To get your original object you can use the JavaCast<T> extension method on it. Since the object is Android.Runtime.JavaObject, it is also a Java.Lang.Object and therefore also an IJavaObject.

[jonpryor]

@cyraid asked:

Why is Android.Runtime.JavaObject internal?

Because we were attempting to reduce cognitive load. If it were public, you would be expected to know about it, and thus special-case it. As it's internal, it's something we were expected to know about and "unwrap" when appropriate.

The problem is that it then became a leaky abstraction…

Regardless, Yonglun Liu Microsoft External Staff et al never asked us how to properly unwrap a value; the code they provided is wrong. Please don't use Reflection to lookup the Instance property.

Instead, use Java.Interop.JniRuntime.JniValueManager.PeekValue(JniObjectReference), a'la:

android/src/Microsoft.Android.Runtime.NativeAOT/UncaughtExceptionMarshaler.cs

Line 12 in 288f107

var e = (JniEnvironment.Runtime.ValueManager.PeekValue (exception.PeerReference) as System.Exception)

Alternatively, perhaps we need a variation on #9038 and have a IJavaPeerable.TryConvert<T>() extension method, which also handles unwrapping in addition to TryJavaCast<T>() semantics…

---

@cyraid wrote:

If you call GetItem from an AdapterView you get a Android.Runtime.JavaObject, not your object you put in.

I wonder how you got there, though; how are you adding an item to the AdapterView with a type that isn't a Java.Lang.Object subclass and also is not through AdapterView<T>? It's likely an oversight/"bug".

For example, ArrayAdapter<T>.GetItem(int) will unwrap:

android/src/Mono.Android/Android.Widget/ArrayAdapter.cs

Lines 192 to 199 in 288f107

	public T? GetItem (int position)
	{
	if (id_getItem_I == IntPtr.Zero)
	id_getItem_I = JNIEnv.GetMethodID (class_ref, "getItem", "(I)Ljava/lang/Object;");
	return JavaConvert.FromJniHandle<T>(
	JNIEnv.CallNonvirtualObjectMethod (Handle, class_ref, id_getItem_I, new JValue (position)),
	JniHandleOwnership.TransferLocalRef);
	}

It looks like we should have considering doing a similar thing on AdapterView<T>.

If you have a codepath where you can obtain wrapped values, that's probably a bug.

[cyraid]

Hi @jonpryor ! Thank you for the time and detail looking into my issue.

I assume you mean:

JniRuntime.CurrentRuntime.ValueManager.PeekValue(obj)

As PeekValue does not seem to be a static method. I also don't seem to have a JniObjectReference from a Java.Lang.Object ?

I'm overriding ArrayAdapter<SomeClass>, but in an ListView.OnItemClickListener, I get:

public void OnItemClick(AdapterView? parent, View? view, int position, long id)

I suppose one could always make SomeClass a wrapper class if you can't extend Java.Lang.Object. But since the object was already marshalled I figure I would've taken advantage of it.

I would definitely advocate for adding a static cast method somewhere to unwrap in some cases like these where you have gotten an object somehow, and still need to produce something before any patches can be made.

Edit: As an aside, how do you refer to another github project and have the lines show up in the markdown? :) (like you did)

[jonpryor]

@cyraid asked:

I assume you mean:

JniRuntime.CurrentRuntime.ValueManager.PeekValue(obj)

As PeekValue does not seem to be a static method. I also don't seem to have a JniObjectReference from a Java.Lang.Object ?

You use the .PeerReference property to get a JniObjectReference from a Java.Lang.Object:

android/src/Mono.Android/Java.Lang/Object.cs

Line 59 in 5c5854f

public new JniObjectReference PeerReference => base.PeerReference;

It's [EditorBrowsable(.Never)], so code completion doesn't show it, but it exists.

I provided a link in my original reply:

android/src/Microsoft.Android.Runtime.NativeAOT/UncaughtExceptionMarshaler.cs

Line 12 in 288f107

var e = (JniEnvironment.Runtime.ValueManager.PeekValue (exception.PeerReference) as System.Exception)

JniEnvironment.Runtime.ValueManager.PeekValue(obj.PeerReference) 

@cyraid also wrote:

I'm overriding ArrayAdapter, but in an ListView.OnItemClickListener, I get:

public void OnItemClick(AdapterView? parent, View? view, int position, long id)

Presumably you're then calling AdapterView.GetItemAtPosition(int position)?

Yeah, it would likely be good if we added a GetItemAtPosition<T>(int position) overload so you can specify the type. Other similar "overloads" would likely be desired elsewhere as well.

In the meantime, JniValueManager.PeekValue() should work:

public void OnItemClick(AdapterView? parent, View? view, int position, long id)
{
    Java.Lang.Object? obj = parent.GetItemAtPosition(position);
    object? value = JniEnvironment.Runtime.ValueManager.PeekValue(obj.PeerReference)
    // …
}