Protobuf-net Extension interfaces - serialization

Question

I have a problem understanding protobuf-net extensions and how they exactly work. I need to serialize a 2D custom class array and so I decited to wrap it into a class something like:
class CustomData : IExtension
{
private CustomClass[,] data;
public CustomClass this[int index_X, int index_Y]
{
get
{
return data[index_X, index_Y];
}
set
{
data[index_X, index_Y] = value;
}
}
public Stream BeginQuery()
{
var stream = new MemoryStream();
int pos = 0;
byte[] packedData = SomeStaticClass.ConvertToByteArray(data)
using(var writer = new ProtoWriter(stream, null, null))
{
ProtoWriter.WriteFieldHeader(1, WireType.String, writer);
ProtoWriter.WriteBytes(packedData, writer);
}
return stream;
}
public void EndQuery(Stream stream)
{
stream.Close();
}
//... and the other 3 implemented funcs from IExtension
}
So this ofcourse is in a custom IExtensible which has GetExtensionObject() returning an instance of the CustomData object. The problem comes when I try to append extension data to an instance. Here is what I'm doing:
public void DoAppend()
{
var stream = new MemoryStream();
using (ProtoWriter writer = new ProtoWriter(stream, null, null))
{
var test = new CustomExtensibleClass(300, 300);
ProtoWriter.AppendExtensionData(test, writer);
}
var result = stream.ToArray();
}
The problem is that "result" contains no data. I expected that the data I appended and written via BeginQuery() will be transfered to the stream of the ProtoWriter but I suppose this is not the case.
Can somebody explaing what I am doing wrong or at least how can use the appended data?