mono.android.runtime.JavaObject cannot be cast to byte[] when using List<byte[]> #4098

BobbyShoe · 2020-01-08T16:27:51Z

Steps to Reproduce

Declare a List<byte[]> in C#. Pass this to a Java library and the mono.android.runtime.JavaObject cannot be cast to byte[] error is received when the object is accessed in Java.

Here is the sample call from the Java library. The call to csdBuffer.get throws the exception.

public static void setCsdBuffers(MediaFormat format, List<byte[]> csdBuffers) {
    for (int i = 0; i < csdBuffers.size(); i++) {
      format.setByteBuffer("csd-" + i, ByteBuffer.wrap(csdBuffers.get(i)));
    }
  }

Seems like a similar issue to the java.lang.ClassCastException information mentioned on this page
https://docs.microsoft.com/en-us/xamarin/android/troubleshooting/troubleshooting#javalangclasscastexception-monoandroidruntimejavaobject-cannot-be-cast-to

However the work around suggested there states to use the similar Java collection for the inner collection in the generic. However in this case a byte[] is the inner collection, so is there some comparable Java type that could be used?

Expected Behavior

A List<byte[]> can be passed between C# and Java code.

Actual Behavior

ClassCastException is thrown when the Java code attempts to access the byte array.

Version Information

Xamarin 16.4.000.307

Running on Android 7.1

Log File

Caused by: java.lang.ClassCastException: mono.android.runtime.JavaObject cannot be cast to byte[]

The text was updated successfully, but these errors were encountered:

jonpryor · 2020-01-10T00:26:18Z

The bug is that JavaConvert.LocalJniHandleConverters doesn't special-case array types: https://github.com/xamarin/xamarin-android/blob/38a10a17eaf16b052e3c5ffbee502565acc688f5/src/Mono.Android/Java.Interop/JavaConvert.cs#L310-L356

Consider this Java method:

public class Gxa4098 {
	public static int genericListMarshaling (List<byte[]> bytes) {
		int count = 0;
		for (byte[] value : bytes ) {
			count += value.length;
		}
		return count;
	}
}

Which results in the binding:

[Register ("genericListMarshaling", "(Ljava/util/List;)I", "")]
public static unsafe int GenericListMarshaling (global::System.Collections.Generic.IList<byte[]> p0)
{
	const string __id = "genericListMarshaling.(Ljava/util/List;)I";
	IntPtr native_p0 = global::Android.Runtime.JavaList<byte[]>.ToLocalJniHandle (p0);
	try {
		JniArgumentValue* __args = stackalloc JniArgumentValue [1];
		__args [0] = new JniArgumentValue (native_p0);
		var __rm = _members.StaticMethods.InvokeInt32Method (__id, __args);
		return __rm;
	} finally {
		JNIEnv.DeleteLocalRef (native_p0);
	}
}

Which calls Android.Runtime.JavaList<byte[]>.ToLocalJniHandle(): https://github.com/xamarin/xamarin-android/blob/38a10a17eaf16b052e3c5ffbee502565acc688f5/src/Mono.Android/Android.Runtime/JavaList.cs#L949-L961

Which hits the JavaList<T> constructor, which calls JavaList<T>.Add(): https://github.com/xamarin/xamarin-android/blob/38a10a17eaf16b052e3c5ffbee502565acc688f5/src/Mono.Android/Android.Runtime/JavaList.cs#L712

Which calls JavaConvert.WithLocalJniHandle(): https://github.com/xamarin/xamarin-android/blob/38a10a17eaf16b052e3c5ffbee502565acc688f5/src/Mono.Android/Android.Runtime/JavaList.cs#L800-L812

Because JavaConvert.LocalJniHandleConverters doesn't special-case arrays, we hit the "fallback" path, which wraps the array into a JavaObject: https://github.com/xamarin/xamarin-android/blob/38a10a17eaf16b052e3c5ffbee502565acc688f5/src/Mono.Android/Java.Interop/JavaConvert.cs#L353-L356

A fix to consider would be to add array types to JavaConvert.LocalJniHandleConverters, but we might want to instead consider using following the "recursive type" approach used around JNIEnv.NewArray(Array, Type).

jonpryor · 2020-01-10T00:30:06Z

Patch for possible unit test:

diff --git a/tests/CodeGen-Binding/Xamarin.Android.JcwGen-Tests/BindingTests.cs b/tests/CodeGen-Binding/Xamarin.Android.JcwGen-Tests/BindingTests.cs
index d6cb60db..c53be34a 100644
--- a/tests/CodeGen-Binding/Xamarin.Android.JcwGen-Tests/BindingTests.cs
+++ b/tests/CodeGen-Binding/Xamarin.Android.JcwGen-Tests/BindingTests.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 
 using NUnit.Framework;
 
@@ -201,6 +202,19 @@ namespace Xamarin.Android.JcwGenTests {
 			var m = e.Message;
 			Assert.IsTrue (e.GetMessageInvoked);
 		}
+
+		// Context: https://github.com/xamarin/xamarin-android/issues/4098
+		[Test]
+		public void GenericListMarshaling ()
+		{
+			var list = new List<byte[]> {
+				new byte[]{ 1, 2, 3 },
+				new byte[]{ 4, 5 },
+				new byte[]{ 6 },
+			};
+			int count = Com.Xamarin.Android.Gxa4098.GenericListMarshaling (list);
+			Assert.AreEqual (6, count);
+		}
 	}
 
 	class ConstructorTest : Com.Xamarin.Android.CallMethodFromCtor {
diff --git a/tests/CodeGen-Binding/Xamarin.Android.McwGen-Tests/Xamarin.Android.McwGen-Tests.csproj b/tests/CodeGen-Binding/Xamarin.Android.McwGen-Tests/Xamarin.Android.McwGen-Tests.csproj
index 044fe4bd..4826e56b 100644
--- a/tests/CodeGen-Binding/Xamarin.Android.McwGen-Tests/Xamarin.Android.McwGen-Tests.csproj
+++ b/tests/CodeGen-Binding/Xamarin.Android.McwGen-Tests/Xamarin.Android.McwGen-Tests.csproj
@@ -97,6 +97,9 @@
     <TestJarEntry Include="java\com\xamarin\android\DataListener.java">
       <OutputFile>Jars/xamarin-test.jar</OutputFile>
     </TestJarEntry>
+    <TestJarEntry Include="java\com\xamarin\android\Gxa4098.java">
+      <OutputFile>Jars/xamarin-test.jar</OutputFile>
+    </TestJarEntry>
     <TestJarEntry Include="java\com\xamarin\android\Logger.java">
       <OutputFile>Jars/xamarin-test.jar</OutputFile>
     </TestJarEntry>
diff --git a/tests/CodeGen-Binding/Xamarin.Android.McwGen-Tests/java/com/xamarin/android/Gxa4098.java b/tests/CodeGen-Binding/Xamarin.Android.McwGen-Tests/java/com/xamarin/android/Gxa4098.java
new file mode 100644
index 00000000..79ee5f06
--- /dev/null
+++ b/tests/CodeGen-Binding/Xamarin.Android.McwGen-Tests/java/com/xamarin/android/Gxa4098.java
@@ -0,0 +1,13 @@
+package com.xamarin.android;
+
+import java.util.List;
+
+public class Gxa4098 {
+	public static int genericListMarshaling (List<byte[]> bytes) {
+		int count = 0;
+		for (byte[] value : bytes ) {
+			count += value.length;
+		}
+		return count;
+	}
+}

BenBtg · 2020-06-08T15:57:32Z

I have a Android binding for the Pointr SDK which is impacted by this issue and causes the following exception to be thrown.
java.lang.ClassCastException: mono.android.runtime.JavaObject cannot be cast to float[] at com.qozix.tileview.paths.BasicPathView.onDraw(BasicPathView.java:131) at android.view.View.draw(View.java:21975)

) Fixes: #4098 Consider the following Java method: // Java public static int count (List<int[]> values) { int c = 0; for (int[] v : values) { c += v.length; } return c; } This is bound as: // C# [Register ("count", "(Ljava/util/List;)I", "")] public static unsafe int Count (IList<int[]> p0) { const string __id = "count.(Ljava/util/List;)I"; IntPtr native_p0 = global::Android.Runtime.JavaList<int[]>.ToLocalJniHandle (p0); try { JniArgumentValue* __args = stackalloc JniArgumentValue [1]; __args [0] = new JniArgumentValue (native_p0); var __rm = _members.StaticMethods.InvokeInt32Method (__id, __args); return __rm; } finally { JNIEnv.DeleteLocalRef (native_p0); } } The problem is that when the `InvokeInt32Method()` statement is hit, *Java* throws an exception: mono.android.runtime.JavaObject cannot be cast to int[] The cause of the problem is that while `JavaList<T>.ToLocalJniHandle()` created a `java.util.ArrayList` instance, the *contents* of the created `ArrayList` were `JavaObject` instances, *not* `int[]` instances. Thus, when the Java code attempts to cast an element from the list to `int[]`, the cast fails. The underlying problem is that e.g. `JavaConvert.WithLocalJniHandle<TValue, TReturn>(…)` didn't handle array types, so if `TValue` were an array, the array would be wrapped into a `JavaObject` instance, instead of being "deep marshaled" to a Java-side array. Add support for array marshaling to `JavaConvert`, so that `JavaLists<int[]>.ToLocalJniHandle()` properly creates a Java-side `ArrayList` which contains `int[]` values. This fixes the cast. Note that I looked at also adding cases for Kotlin's unsigned primitive arrays, however Kotlin only allows their use in very limited circumstances, and `List<uint[]>` is not allowed: // Kotlin UnsignedMethods.kt:39:61: error: expecting a '>' public open fun uintListInstanceMethod (value: List<UInt[]>) : List<UInt[]> { return value; } ^

brendanzagaeski · 2020-08-05T23:15:07Z

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 11.0.99.9.

Fix included on Windows in Visual Studio 2019 version 16.8 Preview 1. 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 1. To try the Preview version that includes the fix, check for the latest updates on the Preview updater channel

brendanzagaeski · 2020-11-10T19:55:19Z

Release status update

A new Release version of Xamarin.Android has now been published that includes the fix for this item.

Fix included in Xamarin.Android SDK version 11.1.0.17.

Fix included on Windows in Visual Studio 2019 version 16.8. To get the new version that includes the fix, check for the latest updates or install the most recent release from https://visualstudio.microsoft.com/downloads/.

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

dellis1972 assigned jpobst Jan 9, 2020

dellis1972 added this to the Under Consideration milestone Jan 9, 2020

jonpryor modified the milestones: Under Consideration, d16-6 Jan 10, 2020

jonpryor added the Area: Bindings Issues in Java Library Binding projects. label Jan 10, 2020

jpobst modified the milestones: d16-6, Under Consideration Apr 1, 2020

jpobst mentioned this issue Jun 9, 2020

[Mono.Android] Properly marshal generic lists of primitive arrays. #4788

Merged

jonpryor closed this as completed in #4788 Jun 10, 2020

brendanzagaeski modified the milestones: Under Consideration, In Preview: 11.1 (d16-8) Aug 5, 2020

ghost locked as resolved and limited conversation to collaborators Jun 5, 2022

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mono.android.runtime.JavaObject cannot be cast to byte[] when using List<byte[]> #4098

mono.android.runtime.JavaObject cannot be cast to byte[] when using List<byte[]> #4098

BobbyShoe commented Jan 8, 2020 •

edited by jpobst

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jonpryor commented Jan 10, 2020

jonpryor commented Jan 10, 2020

BenBtg commented Jun 8, 2020

brendanzagaeski commented Aug 5, 2020

brendanzagaeski commented Nov 10, 2020

mono.android.runtime.JavaObject cannot be cast to byte[] when using List<byte[]> #4098

mono.android.runtime.JavaObject cannot be cast to byte[] when using List<byte[]> #4098

Comments

BobbyShoe commented Jan 8, 2020 • edited by jpobst Loading

Steps to Reproduce

Expected Behavior

Actual Behavior

Version Information

Log File

jonpryor commented Jan 10, 2020

jonpryor commented Jan 10, 2020

BenBtg commented Jun 8, 2020

brendanzagaeski commented Aug 5, 2020

brendanzagaeski commented Nov 10, 2020

BobbyShoe commented Jan 8, 2020 •

edited by jpobst

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