why I am unable to insert text after insertion?

Question

I want to insert some characters after insertion,but the text inserted is before insertion.

the code segment :

......//ignore some codes and definitioans

  ITfContext *pContext=GetContext();//GetContext is custom-function.
  ITfRange* range(NULL);
  wchar_t *pTxBuf=m_ImeData->TxBuffer;

  hr=pContext->GetSelection(ec,TF_DEFAULT_SELECTION,1,&ts,&nSels);
  
  if( SUCCEEDED(hr) )
  {
   range=ts.range;
   TF_SELECTION ts={0};
   ULONG nSels(0); 
   ULONG len=wcslen(pTxBuf);
   LONG n(0);
   hr=range->SetGravity(ec,TF_GRAVITY_FORWARD,TF_GRAVITY_FORWARD);
   hr=range->SetText(ec,TF_ST_CORRECTION,pTxBuf,len);
      
  }...