Return IntPtr with array to c++

Question

I have c# method ReadBytesInTask, which is called from c++ code, using function pointer acquired with Marshal.GetFunctionPointerForDelegate method.

ReadBytesInTask gets ptr to unmanaged short[] array populates it and sends back to unmanaged code. Array is unmarshalled to unmanaged memory correctly inside method, but c++ gets only zero populated array.
What should I do?

[UnmanagedFunctionPointer(CallingConvention.Cdecl, CharSet = CharSet.Auto)]
private delegate int BytesReader([In,Out] IntPtr buffer, int samplesCount, string taskId);

private static readonly BytesReader _readBytesInTask = ReadBytesInTask;

private static int ReadBytesInTask(IntPtr buffer, int samplesCount)
{
    var bufferSize = samplesCount;              
    var samplesToRead = bufferSize <= task.Buffer.Length - task.Offset ? bufferSize : task.Buffer.Length - task.Offset;

    buffer = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(short))*samplesToRead);
    Marshal.Copy(task.Buffer, task.Offset, buffer, samplesToRead);

    task.Offset += samplesToRead;
    if (samplesToRead < bufferSize) task.Offset = 0;
    return samplesToRead;
}

Answer 1

You are changing the pointer and not what it points to. So you will need a pointer to pointer. So it would be something like this:

private static int ReadBytesInTask(IntPtr* buffer, int samplesCount)