尝试通过FTP上传文件时出现空引用异常

Question

我制作了一个FTP上传应用程序，它将FileDialog中选择的文件名更改为4个ComboBox中的一组变量。但每当我尝试上传文件时，它说：
Path Cannot be Null Paramenter name: path

在即时窗口中说： A first chance exception of type 'System.ArgumentNullException' occurred in mscorlib.dll A first chance exception of type 'System.NullReferenceException' occurred in Microsoft.VisualBasic.dll

和此：

我已多次查看代码，但我不明白有什么问题，因为我对VB很陌生。

这是我使用的代码：

Dim Filename As String
    Dim originalFile As String

    Private Function enumerateCheckboxes(ByVal path As String)
        originalFile = path
        Dim fName As String
        For Each Control In Me.Controls
            If (TypeOf Control Is ComboBox AndAlso DirectCast(Control, ComboBox).SelectedIndex > -1) Then
                fName += CStr(Control.SelectedItem.Key) + "_"
            End If
        Next
        Try
            fName = path + fName.Substring(0, fName.Length - 1) + ".pdf"
        Catch ex As Exception
            MsgBox(ex.Message)
            MsgBox("Stack Trace: " & vbCrLf & ex.StackTrace)
        End Try
        Return fName
    End Function

    Public Function OpenDialog()
        Dim FD As OpenFileDialog = New OpenFileDialog()

        FD.Title = "Selecteer een bestand"
        FD.InitialDirectory = "C:\"
        FD.Filter = "All files (*.*)|*.*|All files (*.*)|*.*"
        FD.FilterIndex = 2
        FD.RestoreDirectory = True

        If FD.ShowDialog() = DialogResult.OK Then
            Dim Filename As String = FD.FileName
            Filename = StrReverse(Filename)
            Filename = Mid(Filename, InStr(Filename, "\"), Len(Filename))
            Filename = StrReverse(Filename)
            MsgBox(enumerateCheckboxes(Filename))
        End If
    End Function

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim request As System.Net.FtpWebRequest = DirectCast(System.Net.WebRequest.Create("ip"), System.Net.FtpWebRequest)
        request.Credentials = New System.Net.NetworkCredential("username", "password")
        request.Method = System.Net.WebRequestMethods.Ftp.UploadFile

        Dim file() As Byte

        Try
            Filename = OpenDialog()
            System.IO.File.ReadAllBytes(Filename)
        Catch ex As Exception
            MessageBox.Show(ex.Message)
            MessageBox.Show("Stack Trace: " & vbCrLf & ex.StackTrace)
        End Try

        FileSystem.Rename(originalFile, Filename)
        Dim strz As System.IO.Stream = request.GetRequestStream()
        strz.Write(file, 0, file.Length)
        strz.Close()
        strz.Dispose()
        FileSystem.Rename(Filename, originalFile)
    End Sub

提前谢谢

Answer 1

ArgumentNullException不是NullReferenceException。它告诉你其中一个函数的参数是Nothing。

这部分是由这两行引起的：

Filename = OpenDialog()
System.IO.File.ReadAllBytes(Filename)

在这种情况下，您的Filename似乎是Nothing，因为OpenDialog没有返回值。您需要返回从OpenFileDialog。

中选择的文件