I have found out the reason, that's caused by the incorrect type of PostData. Construction of PostData ,as mentioned in my code, makes VT_BSTR VARIANT while the correct one is VT_ARRAY | VT_UI1
Here is the link for your reference hope it's helpful to you.
http://support.microsoft.com/default.aspx?scid=KB;en-us;q167658
One thing needs to be noted that the example of VS might has an defect and can not be used in unicode