ByteBuffer extension ToByteArray makes temporary memory allocations

[alex3696]

https://github.com/anotherlab/UsbSerialForAndroid/blob/main/UsbSerialForAndroid/Extensions/BufferExtensions.cs

        public static byte[] ToByteArray(this ByteBuffer buffer)
        {
            IntPtr classHandle = JNIEnv.FindClass("java/nio/ByteBuffer");
            IntPtr methodId = JNIEnv.GetMethodID(classHandle, "array", "()[B");
            IntPtr resultHandle = JNIEnv.CallObjectMethod(buffer.Handle, methodId);

            byte[] result = JNIEnv.GetArray<byte>(resultHandle);

            JNIEnv.DeleteLocalRef(resultHandle);

            return result;
        }

byte[] result = JNIEnv.GetArray(resultHandle);
this line still makes temporary array allocations, not wrapping existing array
look to https://github.com/dotnet/android/blob/main/src/Mono.Android/Android.Runtime/JNIEnv.cs

		public static T[]? GetArray<T> (IntPtr array_ptr)
		{
			if (array_ptr == IntPtr.Zero)
				return null;

			if (typeof (T).IsValueType)
				AssertCompatibleArrayTypes (array_ptr, typeof (T[]));

			int cnt = _GetArrayLength (array_ptr);
			T[] ret = new T [cnt];
			CopyArray<T> (array_ptr, ret);
			return ret;
		}

1. I didn't find a way to "display" arrays from java to .net
2. I did not find a way to copy an array from a java array to .net array without using JNIEnv.GetArray (Java.Nio.ByteBuffer.Get this method does it in a similar way? didn't find implementation)
   if you know how to do this - this would be the basis of the solution.

But... android 9 introduce GetDirectBufferAddress
https://github.com/dotnet/android/blob/main/src/Mono.Android/Java.Nio/Buffer.cs
Now we can copy from the native raw pointer to .NET array, directly without memory allocations

        public static void CopyTo(this ByteBuffer buffer, int srcOffset, byte[] dstArr, int offset, int count)
        {
            ArgumentOutOfRangeException.ThrowIfGreaterThan(count, buffer.Capacity() - srcOffset);
            nint srcPtr = buffer.GetDirectBufferAddress();
            ArgumentOutOfRangeException.ThrowIfZero(srcPtr);
            srcPtr += srcOffset;
            Marshal.Copy(srcPtr, dstArr, offset, count);
        }

To do this, you need to initialize the buffer like this:
ByteBuffer bb = ByteBuffer.AllocateDirect(10);

what do you say?

[alex3696]

I found a solution: how to make a shared byte[] buffer between NET and Java

        NetBuffer = new byte[capacity];
        _handle = GCHandle.Alloc(NetBuffer, GCHandleType.Pinned);
        IntPtr ndb = JNIEnv.NewDirectByteBuffer(_handle.AddrOfPinnedObject(), capacity);
        ByteBuffer? jdb = Java.Lang.Object.GetObject<ByteBuffer>(ndb, JniHandleOwnership.TransferLocalRef);

https://github.com/alex3696/UsbSerialForAndroid.Net/blob/MyMaster/UsbSerialForAndroid.Net/Helper/NetDirectByteBuffer.cs
I use it in developing my version of a serial asynchronous port with a read buffer.
I had to develop it because... I have not found a single working solution for high-load and high-speed use cases

[anotherlab]

This is an open-source project, and if you would like to submit a PR, we would be happy to review it.