Need small help in NFC app for Windows Phone

Question

i m working on windows phone 8 api NFC .

pls help me below code

this code is working fine

 NdefUriRecord rec = new NdefUriRecord { Uri = "http://www.nfcinteractor.com/" };
  NdefMessage msg = new NdefMessage { rec };
  _device.PublishBinaryMessage("NDEF", msg.ToByteArray().AsBuffer(), MessageWrittenHandler);
  LogStatusMessage("Publishing message ...");


but simler  to this code not working and not showing any exe.. also

  NdefUriRecord rec = new NdefUriRecord { Uri = "nfcshare:Hello+world" };
  NdefMessage msg = new NdefMessage { rec };
  _device.PublishBinaryMessage("NDEF:WriteTag", msg.ToByteArray().AsBuffer(), MessageWrittenHandler);
  LogStatusMessage("Publishing message ...");


i m using hare WriteTag feature ,both  sample syntax is almost same.but second sample of code is not working .
i need your help for second sample syntax . where i missed ..