how can i invoke MAPI32.DLL, and waht's wrong with my code

Question

i write a application that can read mail,i use MAPI32.DLL,but there have a System.NullReferenceException Exception.what's wrong with my code,can you help me? using System; using System.Windows.Forms; using System.Runtime.InteropServices; using System.Text; namespace test { public class Mail { public Mail() { } [ DllImport( "MAPI32.DLL", SetLastError=true)] public static extern long MAPILogon(long UIParam, string User, string Password, long Flags, long Reserved, out long Session); public const int MAPI_FORCE_DOWNLOAD = 4096; public static long LogOnMail(string User, string Password, out long Session) { return MAPILogon(0,User, Password,2,0, out Session); } } }