Create a GUI for an FTP Client with VB.NET

This column presents an assortment of skills you might need to complete the Windows FTP solution or similar kinds of applications. It further explores the TcpClient namespace and using new controls in Visual Studio .NET 2005. It also demonstrates a couple of patterns that you should know. Although the example it presents is implemented with VS.NET 2005, you could implement it in VS.NET 2003 with simple control substitutions.

Add Get File List to Your FTP Client

Client is one of those words that can be a bit ambiguous. Client can be code that talks to a server, but it also can mean GUI. (A less ambiguous term for GUI client is presentation layer.) This section refers to the client code and not a GUI.

To implement NLST, you need to add three more methods to your FtpClient:

• GetFileList—directly implements the NLST command
• CreateDataSocket—implements the PASV command and creates a second data socket (The PASV command tells the server to wait for a client to connect to it on a port the server designated.)
• RequestResponse—returns the actual data that is returned from the server

RequestResponse is useful when you want to retrieve data like a list of files as opposed to server code. Listing 1 shows the implementation of GetFileList, CreateDataSocket, and RequestResponse.

Listing 1: GetFileList, CreateDataSocket, and RequestResponse (to be added to the FtpClient class)

Public Function GetFileList(ByVal mask As String) As String()
Const BUFFER_SIZE As Integer = 512
        Dim buffer(BUFFER_SIZE) As Byte

        Dim socket As Socket = Nothing
        Dim bytes As Integer
        Const separator As Char = "n"
        Dim messageChunks As String()
        Dim temp As String = ""

        ' Implement this
        socket = CreateDataSocket()
        Try
            SendCommand("NLST " + mask)

            While (True)
                Array.Clear(buffer, 0, buffer.Length)
                bytes = socket.Receive(buffer, buffer.Length, 0)
                temp += ASCII.GetString(buffer, 0, bytes)
                If (bytes < buffer.Length) Then Exit While

            End While

            messageChunks = temp.Split(separator)
        Finally
            socket.Close()
        End Try

        Return messageChunks
End Function

Private Function CreateDataSocket() As Socket
Dim index1, index2, len, port, partCount As Integer
        Dim ipData, buf, ipAddress As String
        Dim parts(6) As Integer
        Dim ch As Char

        Dim socket As Socket
        Dim ep As IPEndPoint

        Dim reply As String = RequestResponse("PASV")
        index1 = reply.IndexOf("(")
        index2 = reply.IndexOf(")")
        ipData = reply.Substring(index1 + 1, index2 - index1 - 1)
        len = ipData.Length
        partCount = 0
        buf = ""

        Dim I As Integer = 0
        While (I <= len - 1 And partCount <= 6)
            ch = Char.Parse(ipData.Substring(I, 1))
            If (Char.IsDigit(ch)) Then
                buf += ch
            ElseIf (ch <> ",") Then
                Throw New IOException("Malformed PASV reply: " + _
                                      reply)
            End If

            If ((ch = ",") Or (I + 1 = len)) Then
                Try
                    parts(partCount) = Int32.Parse(buf)
                    partCount += 1
                    buf = ""
                Catch
                    Throw New IOException("Malformed PASV reply: " _
                                          + reply)
                End Try
            End If
            I += 1
        End While

        ipAddress = String.Format("{0}.{1}.{2}.{3}", parts(0), _
                    parts(1), parts(2), parts(3))

        port = parts(4) << 8
        port = port + parts(5)

        socket = New Socket(AddressFamily.InterNetwork, _
                            SocketType.Stream, ProtocolType.Tcp)
        ep = New IPEndPoint(Dns.Resolve(ipAddress).AddressList(0), _
                            port)

        Try
            socket.Connect(ep)
        Catch ex As Exception
            Throw New IOException("Cannot connect to remote _
                                   server", ex)
        End Try

        Return socket

End Function


Private Function RequestResponse(ByVal command As String) As String
        command += Environment.NewLine
        Dim commandBytes() As Byte = ASCII.GetBytes(command)
        clientSocket.Send(commandBytes, commandBytes.Length, 0)
        Return ReadLine()
End Function

Add a MenuStrip to the Form

The next version of VS.NET replaces the MenuBar with a MenuStrip. The basic functionality is the same, but I find using the MenuStrip a bit clumsier. For example, to add a separator to menus in previous versions of VB, one only had to use the hyphen (-) as the text for the MenuItem. This was exceptionally easy. Whidbey (VS.NET 2005) objectifies as a ToolStripSeparator the indented line that separates regions of a menu. Instead of using a hyphen to create the separator, one needs to right-click over the menu in design mode and select Edit DropDownItems… from the context menu (see Figure 1).

Figure 1: Modifying Menus Is a Bit More Confusing in Whidbey

Selecting the Edit DropDownItems menu opens an Items Collection Editor (see Figure 2) where you can have more precise manipulation of MenuStrip elements. (Beta software is subject to change, so hopefully Microsoft will simplify the MenuStrip behavior.)

Figure 2: Adding a Separator Requires Use and Knowledge of the Items Collection Editor.

MenuStrip behavior—once the menu is created—is similar to MenuBar behavior. Define the StripMenuItem and double-click to generate an event handler. For this article, I added File, View, and Help StripMenuItems. File supports connecting, disconnecting, and exiting. View supports requesting a remote files list, and Help has an About sub-item. For each of these menu items, I created an event handler in the usual way and invoked the related capabilities.

File|Connect invokes the FtpClient.Connect and FtpClient.Login behaviors. File|Disconnect invokes the FtpClient.Disconnect behavior. View|Get Remote Files invokes the FtpClient.GetFileList behavior, and Help|About uses MessageBox.Show (MsgBox) behavior to display a simple About dialog box.

The Form_Load method is used to create an instance of the FtpClient object and a FileListUserControl dynamically. (Find the complete listing for the form at the end of this article.) Whidbey presently separates designer-generated code from user-generated code. It will add designer-generated code to a file named filename.Designer.vb. For example, if you have a form named Form1, Whidbey will create Form1.vb, Form1.Designer.vb, and Form1.resx. Generally, you will add your code to Form1.vb in this scenario.

Create a File List User Control

To support listing files on the client and server, you can implement one user control and ultimately use it twice: once for the client-side files and once for the server-side files. In this example, you instantiate one user control for server-side files.

The FileListUserControl is comprised of a Label, a Panel with a Button and ComboBox, and a TreeView. You use the Label to indicate what the control contains. You can ultimately use the Button and ComboBox to change folders or drives where applicable and the TreeView to display the files discovered on the client or the server. For practical purposes, you can surface the Text value of the Label to permit consumers to change it, and hide the ComboBox when the FileListControl represents the remote server and show it when the FileListUserControl represents the client. Figure 3 shows the design-time view of the FileListUserControl.

Figure 3: The FileListUserControl Is a Generic User Control That Accepts a String Array But Is Generally Unaware of the Origin or Meaning of the Data.

The FileListUserControl is straightforward: Pass an array of strings and the SetFiles method calls an Add method for each string in the array, adding the string to the TreeView control. Listing 2 contains the code for the FileListUserControl. (The designer-generated code is contained in a file named *.designer.vb in Whidbey.)

Listing 2: The Implementation of the FileListUserControl

Imports System
Imports System.Collections
Imports System.ComponentModel
Imports System.Drawing
Imports System.Data
Imports System.IO
Imports System.Windows.Forms

Public Class FileListUserControl

    Public Property Caption() As String
        Get
            Return Label1.Text
        End Get
        Set(ByVal value As String)
            Label1.Text = value
        End Set
    End Property

    Public Sub SetFiles(ByVal files() As String)
        treeViewFiles.Nodes.Clear()
        Add(".")
        Add("..")

        Dim file As String
        For Each file In files
            If (file.Trim() <> String.Empty) Then
                Add(file.Trim())
            End If
        Next
    End Sub

    Private Sub Add(ByVal file As String)
        Broadcaster.Broadcast(String.Format("Adding {0}", file))
        treeViewFiles.Nodes.Add(New TreeNode(file))
    End Sub

    Private Sub FileListUserControl_Resize(ByVal sender _
    As System.Object, _
    ByVal e As System.EventArgs) Handles MyBase.Resize
        treeViewFiles.SetBounds(0, 80, Width, Height - 80)
        Label1.SetBounds(0, 0, Width, 40)
        Panel1.SetBounds(0, 40, Width, 40)
    End Sub
End Class

Attach the FTP Client to the GUI

Next, you need to attach the FtpClient class library to your GUI. You can accomplish this easily from an architectural perspective by using the Observer pattern. From a practical point of view, the Observer pattern is manifested as events.

To attach the FtpClient to the GUI, declare and create an instance of the FtpClient and then define and attach event handlers to the FtpClient using the AddHandler statement. The code that demonstrates how to manually add event handlers is shown in the Form1_Load event handler in Listing 3 and in context in the Form’s code listing at the end of this article.

Listing 3: The Form1_Load Method Demonstrates How to Manually Attach Event Handlers.

    Private ftp As FtpClient = Nothing
    Private FileControl As FileListUserControl

    Private Sub Form1_Load(ByVal sender As System.Object, _
        ByVal e As System.EventArgs) Handles MyBase.Load

        Broadcaster.Add(Me)

        ftp = New FtpClient("localhost", ".", "anonymous", _
            "pkimmel@softconcepts.com", 21)

        AddHandler ftp.OnTcpEvent, AddressOf OnTcpEvent

        FileControl = New FileListUserControl
        FileControl.Caption = "Remote Files"
        FileControl.Parent = Panel1
        Panel1.Controls.Add(FileControl)
        FileControl.Dock = DockStyle.Fill
    End Sub

(Rather than remove the code for the FileListUserControl, I show the code for dynamically creating the UserControl in Listing 3 too.) The AddHandler statement requires an object and its event field, and the second parameter is the AddressOf the event handler. Implicitly, AddressOf creates an instance of a delegate object, which is a class that encapsulates event handlers.

Update a StatusStrip

Whidbey has replaced the StatusBar with the StatusStrip. Although it is a little more clumsy to use than the StatusBar, it is more OOPy but will feel a tad foreign. Basically, the StatusStrip is a status bar that has more advanced designers similar to the MenuStrip’s designers. Instead of repeating that information here, let’s talk about a proper way to update status information on your presentation layer.

Status information could literally come from anywhere. The FtpClient’s state will change, and you might elect to report that status information. The FileListUserControl’s or Form1’s status might change, and you might elect to report that status information. Obviously, you could define a method in the Form that permits updating status, but then every status reporting object would need to have an instance of the Form. That would be a spaghetti code nightmare. If you asked yourself why, consider an example using just one reporting object, the Form. Suppose the FtpClient is told about the main form in the Windows client. This means the Windows client must have a reference to the FtpClient to invoke behaviors and the FtpClient must have a reference to the Windows Form to update status. In addition to being a circular reference, the FtpClient class also breaks if Form1 goes away, and you can’t reuse the FtpClient in some other application without dragging the Windows Forms client along with it. This is called tightly coupled code and it is a bad thing.

A better alternative would be to define an OnStatusChange method in the FtpClient, attach the Windows Form to that event, and raise the event when status changes. While this represents looser coupling and better cohesion, it still means that other status-reporting objects would need to follow suit with events and event handlers.

The best solution can be had by again using the Observer pattern. However, in this context, think of the Observer pattern by its alter ego publish-subscribe or what I call broadcaster and listeners. (Think radio station and radio listeners.) Defining a broadcaster and listener means that you can broadcast status messages (from anywhere) and any listener can tune it. By combining the Singleton (to ensure one instance of the broadcaster) and the Observer patterns, you can send status messages from anywhere without any knowledge of who or what is listening. (If you need some validation for this solution, keep in mind that it is precisely how Microsoft implemented the System.Diagnostics.Trace class and TraceListeners to send Trace information to the Output window in VS.NET itself.)

Implement the Listener

Listening is comprised of two parts: an interface named IListener and a typed collection of IListeners. Using an interface means that any class that implements IListener can tune in to broadcasts. You might think inheritance would work here, but remember an obvious listener is a Form and it already inherits from the System.Windows.Forms.Form and multiple inheritance is not allowed in VB.NET.