VB FTP 代码问题

[Amseen]

Public Class Form1
    Dim wc As New Net.WebClient
    Dim passwort As String = "密码"
    Dim benutzername As String = "用户名"

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Control.CheckForIllegalCrossThreadCalls = False
        BackgroundWorker1.RunWorkerAsync()
    End Sub

    Private Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
        Try
            wc.Credentials = New Net.NetworkCredential(benutzername, passwort)
            TextBox1.Text = wc.DownloadString("ftp://***.com/chat.txt")
        Catch ex As Exception
            MessageBox.Show("Error!")
        End Try
    End Sub

    Private Sub BackgroundWorker1_RunWorkerCompleted(ByVal sender As Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
        BackgroundWorker1.RunWorkerAsync()
    End Sub

    Private Sub sentannui_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles sentannui.Click
        BackgroundWorker2.RunWorkerAsync()
    End Sub

    Private Sub BackgroundWorker2_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker2.DoWork
        BackgroundWorker1.CancelAsync()

        wc.Credentials = New Net.NetworkCredential(benutzername, passwort)
        wc.UploadString("ftp://***.com/chat.txt", TextBox1.Text & "" & TextBox0.Text & ": " & TextBox2.Text & vbNewLine) '这一句出现的错误，用户代码未处理 NotSupportedException. WebClient does not support concurrent I/O operations.请问有什么解决方法吗？

        Try
            wc.Credentials = New Net.NetworkCredential(benutzername, passwort)
            TextBox1.Text = wc.DownloadString("ftp://***.com/chat.txt")
        Catch ex As Exception
            MessageBox.Show("Error!")
        End Try
    End Sub

    Private Sub BackgroundWorker2_RunWorkerCompleted(ByVal sender As Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker2.RunWorkerCompleted
        TextBox2.Clear()
        BackgroundWorker1.RunWorkerAsync()
    End Sub
End Class

[Amseen]

学校的作业制作简单的Ftp聊天，不太懂出错的意思，请问有什么解决方法吗？