关于FtpOpenFile函数

我在调用FtpOpenFile函数时，为什么函数返回值一直是0？
InternetOpen，InternetConnect都执行成功了，这是什么情况？
源码如下:
Option Explicit
Public Declare Function InternetOpen Lib "wininet.dll" Alias "InternetOpenA" (ByVal sAgent As String, ByVal LAccessType As Long, ByVal sProxyName As String, _
ByVal SProxyBypass As String, ByVal lFlags As Long) As Long
Public Declare Function InternetConnect Lib "wininet.dll" Alias "InternetConnectA" (ByVal hInternetSession As Long, ByVal sServerName As String, _
ByVal nServerPort As Integer, ByVal sUsername As String, ByVal sPassword As String, ByVal lService As Long, ByVal lFlags As Long, ByVal lContext As Long) As Long
Public Declare Function FtpGetFile Lib "wininet.dll" Alias "FtpGetFileA" (ByVal hFtpSession As Long, ByVal lpszRemoteFile As String, ByVal lpszNewFile As String, ByVal fFailIfExists As Boolean, _
ByVal dwFlagsAndAttributes As Long, ByVal dwFlags As Long, ByVal dwContext As Long) As Boolean
Public Declare Function InternetCloseHandle Lib "wininet.dll" (ByVal hInet As Long) As Integer
Public Declare Function FtpPutFile Lib "wininet.dll" Alias "FtpPutFileA" (ByVal hFtpSession As Long, ByVal lpszLocalFile As String, ByVal lpszRemoteFile As String, ByVal dwFlags As Long, ByVal dwContext As Long) As Boolean
Private Declare Function FtpCreateDirectory Lib "wininet.dll" Alias "FtpCreateDirectoryA" (ByVal hFtpSession As Long, ByVal lpszDirectory As String) As Boolean
Private Declare Function FtpFindFirstFile Lib "wininet.dll" Alias "FtpFindFirstFileA" (ByVal hFtpSession As Long, ByVal lpszSearchFile As String, lpFindFileData As WIN32_FIND_DATA, ByVal dwFlags As Long, ByVal dwContent As Long) As Long
Private Declare Function InternetFindNextFile Lib "wininet.dll" Alias "InternetFindNextFileA" (ByVal hFind As Long, lpvFindData As WIN32_FIND_DATA) As Long
Public Declare Function FtpOpenFile Lib "wininet.dll" Alias "FtpOpenFileA" (ByRef hConnect As Long, ByVal lpszFileName As String, ByVal dwAccess As Long, ByVal dwFlags As Long, ByRef dwContext As Long) As Boolean
Public Declare Function InternetWriteFile Lib "wininet.dll" (ByRef Hfile As Long, lpBuffer As String, ByVal dwNumberOfBytesToWrite As Long, ByRef lpdwNumberOfBytesWritten As Long) As Boolean
Public Const GENERIC_READ = &H80000000
Public Const GENERIC_WRITE = &H40000000
Public Const FTP_TRANSFER_TYPE_BINARY = &H2
Public Const FTP_TRANSFER_TYPE_ASCII = &H1
Function C_OpenFtpFile() 'FTPOpenFile
Dim lnginet As Long
Dim lnginetconn As Long
Dim Hfile As Long
lnginet = InternetOpen(vbNullString, 0, vbNullString, vbNullString, 0&)
If lnginet <> 0 Then
lnginetconn = InternetConnect(lnginet, "172.19.77.3", 0, "OQA_USER", "PQC123456", 1, 0, 0)
If lnginetconn = 0 Then
MsgBox "无法连接FTP服务器，请检验网络", vbInformation, "网络故障提示"
End If
End If
Hfile = FtpOpenFile(lnginetconn, "/FTT4-FTT3.txt", GENERIC_READ, FTP_TRANSFER_TYPE_ASCII, 0)'Hfile返回为0？？？
InternetCloseHandle lnginetconn
InternetCloseHandle lnginet
End Function