Question

我在Visual Studio Community 2015中下载文本文件时遇到问题。它是我的OneDrive公共文件夹中的文本文件，其中包含我的应用程序的版本号（1.0.0.0）。我手动打开时使用的下载链接工作正常，但是当我的VB代码执行时，它会正确下载文本文件，但是当我打开它时文件是空白的，我无法弄清楚它出错的地方。

在Module1中，我有一个用于下载的子文件和一个用于读取该文件的子文件：

Module Module1

    ' Public variables

    Public tempPath As String = Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData) & "\Temp"

    Sub DownloadToTemp(filePath As String)
        ' Download a text file, ready to read contents

        If Not IO.Directory.Exists(tempPath & "\Temp") Then
            IO.Directory.CreateDirectory(tempPath & "\Temp")
        End If

        Try
            My.Computer.Network.DownloadFile _
                (address:=filePath,
                destinationFileName:=tempPath & "\TempText.txt",
                userName:=String.Empty,
                password:=String.Empty,
                showUI:=False,
                connectionTimeout:=10000,
                overwrite:=True)

        Catch ex As Exception
            MsgBox("Can't read file" & vbCrLf & ex.Message)
        End Try

    End Sub

    Sub ReadFile()

        Dim fStream As New IO.FileStream(tempPath & "\TempText.txt", IO.FileMode.Open)
        Dim sReader As New System.IO.StreamReader(fStream)
        Dim sArray As String() = Nothing
        Dim index As Integer = 0

        Do While sReader.Peek >= 0
            ReDim Preserve sArray(index)
            sArray(index) = sReader.ReadLine
            index += 1
        Loop

        fStream.Close()
        sReader.Close()

        ' Test
        Dim fileContents As String = Nothing
        If sArray Is Nothing Then
            MsgBox("No Data Found")
            Exit Sub
        End If
        For index = 0 To UBound(sArray)
            fileContents = fileContents & sArray(index) & vbCrLf
        Next

        MessageBox.Show(fileContents)

    End Sub

End Module

他们从主要代码中调用：

    Private Sub frmSplash_Load(sender As Object, e As EventArgs) Handles MyBase.Load

    lblVersion.Text = "v" & Application.ProductVersion

    ' Check available version online

    Call DownloadToTemp("https://onedrive.live.com/download?resid=DE2331D1649390C1!16974&authkey=!AH2cr1S1SHs9Epk&ithint=file%2ctxt")
    Call ReadFile()

End Sub

所以，一切似乎都是正确和有效的，没有错误或异常，但是我的VB代码下载的文件是空白的，但是手动点击代码中的下载链接会将内容完整地下载。谁能明白为什么会这样呢？

Answer 1

代码将下载该位置的所有内容，但链接似乎会将您重定向到另一个位置。它会下载从链接获得的响应，因为没有任何内容，所以没有任何内容。

您需要指向该文件的直接链接才能正常工作。试试这个：

DownloadToTemp("https://gwhcha-dm2306.files.1drv.com/y4mwlpYyvyCFDPp3NyPM6WqOz8-Ocfn-W0_4RbdQBtNMATYn2jNgWMRgpl_gXdTBteipIevz07_oUjCkeNoJGUxNO9jC9IdXz60NNEvzx2cU9fYJU_oRgqBFyA8KkBs8VGc8gDbs2xz7d3FyFnkgRfq77A2guoosQkO4pVMDiEYRoJRCWOtQk2etsMXyT8nSEnPoGV6ZG0JWc6qt55Mhi_zeA/Hotshot_Version.txt?download&psid=1")

此外，您无需使用Call keyword。它仅用于向后兼容VB6和旧版本。

修改

以下是使用HttpWebRequest class下载文件的示例。通过设置其AllowAutoRedirect和MaximumAutomaticRedirections属性，您可以在尝试下载文件之前重定向它。

''' <summary> ''' Downloads a file from an URL and allows the page to redirect you. ''' </summary> ''' <param name="Url">The URL to the file to download.</param> ''' <param name="TargetPath">The path and file name to download the file to.</param> ''' <param name="AllowedRedirections">The maximum allowed amount of redirections (default = 32).</param> ''' <param name="DownloadBufferSize">The amount of bytes of the download buffer (default = 4096 = 4 KB).</param> ''' <remarks></remarks> Private Sub DownloadFileWithRedirect(ByVal Url As String, _ ByVal TargetPath As String, _ Optional ByVal AllowedRedirections As Integer = 32, _ Optional ByVal DownloadBufferSize As Integer = 4096) 'Create the request. Dim Request As HttpWebRequest = DirectCast(WebRequest.Create(Url), HttpWebRequest) Request.Timeout = 10000 '10 second timeout. Request.MaximumAutomaticRedirections = AllowedRedirections Request.AllowAutoRedirect = True 'Get the response from the server. Using Response As HttpWebResponse = DirectCast(Request.GetResponse(), HttpWebResponse) 'Get the stream to read the response. Using ResponseStream As Stream = Response.GetResponseStream() 'Declare a download buffer. Dim Buffer() As Byte = New Byte(DownloadBufferSize - 1) {} Dim ReadBytes As Integer = 0 'Create the target file and open a file stream to it. Using TargetFileStream As New FileStream(TargetPath, FileMode.Create, FileAccess.Write, FileShare.None) 'Start reading into the buffer. ReadBytes = ResponseStream.Read(Buffer, 0, Buffer.Length) 'Loop while there's something to read. While ReadBytes > 0 'Write the read bytes to the file. TargetFileStream.Write(Buffer, 0, ReadBytes) 'Read more into the buffer. ReadBytes = ResponseStream.Read(Buffer, 0, Buffer.Length) End While End Using End Using End Using End Sub

使用示例：

Try DownloadFileWithRedirect("https://onedrive.live.com/download?resid=DE2331D1649390C1!16974&authkey=!AH2cr1S1SHs9Epk&ithint=file%2ctxt", Path.Combine(tempPath, "TempText.txt")) Catch ex As Exception MessageBox.Show("An error occurred:" & Environment.NewLine & ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error) End Try

为什么我的在线文本文件在VB.NET中下载空白？

1 个答案: