Speed up File.Exists for non existing network shares

Question

I have to check if a set of file paths represent an existing file.

It works fine except when the path contains a network share on a machine that's not on the current network. In this case it takes a pretty long time (30 or 60 seconds) to timeout.

Questions

• Is there a way to shorten the timeout for non existing network shares? (I'm certain that when they do exist they'll answer quickly, so a timeout of 1 sec would be fine)

• Is there any other way to solve this issue without starting to cache and making the algorithm more complex? (ie, I already know these X network shares don't exist, skip the rest of the matching paths)

UPDATE: Using Threads work, not particularly elegant, though

public bool pathExists(string path) 
{
    bool exists = true;
    Thread t = new Thread
    (
        new ThreadStart(delegate () 
        {
            exists = System.IO.File.Exists(path); 
        })
    );
    t.Start();
    bool completed = t.Join(500); //half a sec of timeout
    if (!completed) { exists = false; t.Abort(); }
    return exists;
}

This solution avoids the need for a thread per attempt, first check which drives are reachable and store that somewhere.

---

Experts exchange solution:

First of all, there is a "timeout" value that you can set in the IsDriveReady function. I have it set for 5 seconds, but set it for whatever works for you.

3 methods are used below:

1. The first is the WNetGetConnection API function that gets the UNC (\servername\share) of the drive
2. The second is our main method: The Button1_Click event
3. The third is the IsDriveReady function that pings the server.

This worked great for me! Here you go:

'This API Function will be used to get the UNC of the drive
Private Declare Function WNetGetConnection Lib "mpr.dll" Alias _
"WNetGetConnectionA" _
(ByVal lpszLocalName As String, _
ByVal lpszRemoteName As String, _
ByRef cbRemoteName As Int32) As Int32


'This is just a button click event - add code to your appropriate event
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

    Dim bIsReady As Boolean = False

    For Each dri As IO.DriveInfo In IO.DriveInfo.GetDrives()

        'If the drive is a Network drive only, then ping it to see if it's ready.
        If dri.DriveType = IO.DriveType.Network Then

            'Get the UNC (\\servername\share) for the 
            '    drive letter returned by dri.Name
            Dim UNC As String = Space(100)
            WNetGetConnection(dri.Name.Substring(0, 2), UNC, 100)

            'Presuming the drive is mapped \\servername\share
            '    Parse the servername out of the UNC
            Dim server As String = _
                 UNC.Trim().Substring(2, UNC.Trim().IndexOf("\", 2) - 2)

            'Ping the server to see if it is available
            bIsReady = IsDriveReady(server)

        Else
            bIsReady = dri.IsReady

        End If

        'Only process drives that are ready
        If bIsReady = True Then
            'Process your drive...
            MsgBox(dri.Name & " is ready:  " & bIsReady)

        End If

    Next

    MsgBox("All drives processed")

End Sub

Private Function IsDriveReady(ByVal serverName As String) As Boolean
    Dim bReturnStatus As Boolean = False

    '***  SET YOUR TIMEOUT HERE  ***
    Dim timeout As Integer = 5    '5 seconds

    Dim pingSender As New System.Net.NetworkInformation.Ping()
    Dim options As New System.Net.NetworkInformation.PingOptions()

    options.DontFragment = True

    'Enter a valid ip address
    Dim ipAddressOrHostName As String = serverName
    Dim data As String = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
    Dim buffer As Byte() = System.Text.Encoding.ASCII.GetBytes(data)
    Dim reply As System.Net.NetworkInformation.PingReply = _
                pingSender.Send(ipAddressOrHostName, timeout, buffer, options)

    If reply.Status = Net.NetworkInformation.IPStatus.Success Then
        bReturnStatus = True

    End If

    Return bReturnStatus
End Function

Answer 1

Use Threads to do the checks. I think that threads can be timed out.