ContentSharingModality ContentAdded event is not getting triggered (Lync silverlight application) - silverlight-4.0

Question

My ultimate aim is to transfer a file from one Lync client to another. I have following code.
First of all I have following 2 events registered
1.
((Modality)_conversation.Modalities[ModalityTypes.ContentSharing]).ModalityStateChanged += Modality_ModalityStateChanged;
2.
((ContentSharingModality)_conversation.Modalities[ModalityTypes.ContentSharing]).ContentAdded += _sharingModality_ContentAdded;
code for those event is
void _sharingModality_ContentAdded(object sender, ContentCollectionChangedEventArgs e)
{
MessageBox.Show("content added\n"+e.Item);
}
void Modality_ModalityStateChanged(object sender, ModalityStateChangedEventArgs e)
{
if (e.NewState == ModalityState.Connected)
{
textBox1.Text += "\nconnected";
send_file();
}
if (e.NewState == ModalityState.Connecting)
{
textBox1.Text += "\nconnecting";
}
}
Then I have a method which creates a file in isolated storage named "abc.txt".
Next there is a code which connects the content sharing modality.
private void button4_Click(object sender, RoutedEventArgs e)
{
if (_conversation.State == ConversationState.Active)
{
((Modality)_conversation.Modalities[ModalityTypes.ContentSharing])
.BeginConnect((ar) =>{((Modality)_conversation.Modalities[ModalityTypes.ContentSharing]).EndConnect(ar); }
, null);
else { MessageBox.Show("conversation not active"); }
}
After this there is 'send_file' method which actually upload the file. (this method id previously called when modality state changes to 'connected' but there (I think) conversation changes to multiparty and method returns false at 'canInvoke' statement. So Im calling it again and this time it succeeds. It is as below
void send_file()
{
if (((ContentSharingModality)_conversation.Modalities[ModalityTypes.ContentSharing]).State == ModalityState.Connected)
{
try
{
if (((ContentSharingModality)_conversation.Modalities[ModalityTypes.ContentSharing]).CanInvoke(ModalityAction.CreateShareableNativeFileOnlyContent))
{
ContentSharingModality contentSharingModality = (ContentSharingModality)_conversation.Modalities[ModalityTypes.ContentSharing];
contentSharingModality.BeginCreateContentFromFile(ShareableContentType.NativeFile, "samplefile.txt", fileNameFromIsolatedStorage, true,
(ar) =>
{
ShareableContent sContent = contentSharingModality.EndCreateContentFromFile(ar);
//_NativeFileNameAndPath = string.Empty;
sContent.Upload();
}
, null);
MessageBox.Show("upload done");
}
else { MessageBox.Show("u cannot invoke"); }
}
catch (Exception e1) { MessageBox.Show(e1.Message); }
}
else { MessageBox.Show("modality inactive"); }
}
Finally this is all I'm trying to do. The same code will lie on both sender & receiver machines. I'm new to lync development and very confused about what is going wrong. Please help. Thanks!