How do I access a network drive through the usual System.IO classes?

Question

My software handles multiple operations on files, and I have now finished writing the related functions, using the System.IO classes.

I now need to add support for network drives. Using a mapping works very well (although Directory.GetFiles is a bit low, and I don't know why), but I'd now like to be able to deal directly with paths such as \\192.168.0.10\Shared Folder\MyDrive. Is there any way to handle this type of paths other than mounting the drive to an available drive letter, using the newly generated path, and then unmounting?

Answer 1

You can use the UNC path (which starts with \\) directly in your paths. However, you must account for the credentials for this connection, which can be the tricky part.

There are several approaches:

1. If the remote system is on the same domain or there is a trust relationship between the domains, and the user your program is running as has suitable access, it will "just work".

2. You can shell out and execute the net use command (through the Windows net.exe program) to make a connection with a specific username and password. Be aware that connection is usable by any program running in the user's session, not just your application. Use the /DELETE command to remove the connection when you are done. The typical syntax is: net use \\computername\sharename password /USER:domain\username.

3. You can P/Invoke WNetAddConnection2 to accomplish the same thing as net use without shelling out to net.exe. By passing NULL as lpLocalName, no drive letter is assigned, but the username and password will apply to subsequent accesses made through the UNC path. The WNetCancelConnection2 function can be used to disconnect.

4. You can P/Invoke LogonUser with the LOGON32_LOGON_NEW_CREDENTIALS flag followed by an impersonation to add additional remote credentials to your thread. Unlike #2 and #3, the effects on the user's entire session will be a little more limited. (In practice, this is rarely done in favor of the well-known WNetAddConnection2 solution.)

The following is a sample of how to call WNetAddConnection2 from VB.NET.

Private Sub Test()
    Dim nr As New NETRESOURCE
    nr.dwType = RESOURCETYPE_DISK
    nr.lpRemoteName = "\\computer\share"
    If WNetAddConnection2(nr, "password", "user", 0) <> NO_ERROR Then
        Throw New Exception("WNetAddConnection2 failed.")
    End If
    'Code to use connection here.'
    If WNetCancelConnection2("\\computer\share", 0, True) <> NO_ERROR Then
        Throw New Exception("WNetCancelConnection2 failed.")
    End If
End Sub

<StructLayout(LayoutKind.Sequential)> _
Private Structure NETRESOURCE
    Public dwScope As UInteger
    Public dwType As UInteger
    Public dwDisplayType As UInteger
    Public dwUsage As UInteger
    <MarshalAs(UnmanagedType.LPTStr)> _
    Public lpLocalName As String
    <MarshalAs(UnmanagedType.LPTStr)> _
    Public lpRemoteName As String
    <MarshalAs(UnmanagedType.LPTStr)> _
    Public lpComment As String
    <MarshalAs(UnmanagedType.LPTStr)> _
    Public lpProvider As String
End Structure

Private Const NO_ERROR As UInteger = 0
Private Const RESOURCETYPE_DISK As UInteger = 1

<DllImport("mpr.dll", CharSet:=CharSet.Auto)> _
Private Shared Function WNetAddConnection2(ByRef lpNetResource As NETRESOURCE, <[In](), MarshalAs(UnmanagedType.LPTStr)> ByVal lpPassword As String, <[In](), MarshalAs(UnmanagedType.LPTStr)> ByVal lpUserName As String, ByVal dwFlags As UInteger) As UInteger
End Function

<DllImport("mpr.dll", CharSet:=CharSet.Auto)> _
Private Shared Function WNetCancelConnection2(<[In](), MarshalAs(UnmanagedType.LPTStr)> ByVal lpName As String, ByVal dwFlags As UInteger, <MarshalAs(UnmanagedType.Bool)> ByVal fForce As Boolean) As UInteger
End Function

Answer 2

Using normal UNC paths such as the one you mentioned works perfectly for me. For example:

string[] dirs = Directory.GetDirectories(@"\\192.168.1.116\");

Works just fine. If it doesn't, you probably have a security issue or something. In which case, you'll have to look into impersonation to get around that. Check this for more on impersonation.