vb.net - 为什么我的在线文本文件在 VB.NET 中下载为空白？

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(public)文件夹中的一个文本文件，其中包含我的应用程序的版本号 (1.0.0.0)。我使用的下载链接在手动打开时工作正常，但是当我的 VB 代码执行时，它确实正确下载了文本文件，但是当我打开它时文件是空白的，我无法弄清楚哪里出了问题。

在模块 1 中，我有一个用于下载的子程序和一个用于随后读取文件的子程序:

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

它们是从主代码中调用的:

    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，一切似乎都是正确的，没有错误或异常，但我的 VB 代码下载的文件是空白的，但手动单击代码中的下载链接会下载内容完整的文件。谁能看出为什么会这样？

Answer 1

该代码将下载该位置的任何内容，但该链接似乎会将您重定向到另一个地方。它下载从链接获得的响应，该响应什么也没有，因为那里什么也没有。

您需要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，您不需要使用 Call keyword .它的存在只是为了向后兼容 VB6 和旧版本。

---

编辑:

这是使用 HttpWebRequest class 下载文件的示例.通过设置其 AllowAutoRedirect和 MaximumAutomaticRedirections您允许它在尝试下载文件之前重定向的属性。

''' <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

示例用法:

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