如何在 VB.NET 中正确读取随机访问文件 [英] How to correctly read a random access file in VB.NET

问题描述

我正在尝试读取随机访问文件，但在第一个文件中出现以下错误错误 5(无法读取流的末尾).我不确定我在这里做错了什么，我该如何解决这个问题?

I am attempting to read a random access file, but I am getting the following error on the first file Error 5 (unable to read beyond end of the stream). I am not sure what I am doing wrong here, how might I fix this issue?

Structure StdSections
    'UPGRADE_WARNING: Fixed-length string size must fit in the buffer. Click for more: 'ms-help://MS.VSCC.v90/dv_commoner/local/redirect.htm?keyword="3C1E4426-0B80-443E-B943-0627CD55D48B"'
    <VBFixedString(15), System.Runtime.InteropServices.MarshalAs(System.Runtime.InteropServices.UnmanagedType.ByValArray, SizeConst:=15)> Public A() As Char 'BEAM  --- complete beam designation          15
    'UPGRADE_WARNING: Fixed-length string size must fit in the buffer. Click for more: 'ms-help://MS.VSCC.v90/dv_commoner/local/redirect.htm?keyword="3C1E4426-0B80-443E-B943-0627CD55D48B"'
    <VBFixedString(2), System.Runtime.InteropServices.MarshalAs(System.Runtime.InteropServices.UnmanagedType.ByValArray, SizeConst:=2)> Public B() As Char 'DSG   --- shape  ie "W" or "C"                2
    Dim C As Single 'DN    --- nominal depth of section            4
    Dim d As Single 'WGT   --- weight                              4
    .
    .
    .
End structure
''Note 'File1'is the existing RAF and holds complete path!

        Dim i,ffr,fLength,lastmembNo as integer
        sectionFound = False
        Dim std As new StdSections 
        fLength = Len(std)
        If fLength = 0 Then fLength = 168 ' 177
        ffr = FreeFile()
        FileOpen(ffr, File1, OpenMode.Random, OpenAccess.Read, OpenShare.LockRead, fLength)
        lastmembNo = CInt(LOF(ffr)) \ fLength

        For i = 1 To lastmembNo
            FileGet(ffr, std, i)
            >>Error 5 (unable to read beyond end of the stream) <<<                  
            If Trim(memberID) = Trim(std.A) Then
                    sectionFound = True
                end if
        next i

推荐答案

Wow Freefile！那是过去的爆炸！

Wow Freefile! That's a blast from the past!

我还没有真正使用 VB.NET 中旧的 OpenFile 等文件访问方法，所以我只是推测，但在 .NET 中，许多变量类型的大小发生了变化.例如Integer 现在是 32 位(4 个字节)，我认为 Boolean 是不同的，尽管 Single 仍然是 4 个字节.

I haven't really used the old OpenFile etc. file access methods in VB.NET, so I'm just speculating, but in .NET many of the variable types got changed in size. e.g. an Integer is now 32-bits (4 bytes), I think a Boolean is different, though a Single is still 4 bytes.

此外，.NET 中的字符串默认为 Unicode，而不是 ASCII，因此您不能依赖 .NET 字符串变量中的 1 个字符 = 1 个字节.事实上，.NET 实际上是在运行之前在 PC 上JIT 编译"程序，所以你不能像过去那样轻松地在内存中布置结构.

Also, strings in .NET are by default in Unicode, not ASCII, so you cannot rely on 1 character=1 byte in a .NET String variable. In fact, .NET actualy "JIT compiles" programs on the PC before running, so you can't really lay out structures in memory easily like the old days.

如果您想切换到新的基于Stream"的对象，这里有一些代码可以帮助您入门:

If you want to switch to the new "Stream" based objects, here's some code to get you started:

    Dim strFilename As String = "C:\Junk\Junk.txt"
    Dim strTest As String = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    Call My.Computer.FileSystem.WriteAllText(strFilename, strTest, False)
    Dim byt(2) As Byte
    Using fs As New FileStream(strFilename, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite)
      fs.Seek(16, SeekOrigin.Begin)
      fs.Read(byt, 0, 3)
      Dim s As String = Chr(byt(0)) & Chr(byt(1)) & Chr(byt(2))
      MsgBox(s)
      fs.Seek(5, SeekOrigin.Begin)
      fs.Write(byt, 0, 3)
    End Using
    Dim strModded As String = My.Computer.FileSystem.ReadAllText(strFilename)
    MsgBox(strModded)

不过我不会怪你保留旧方法:使用新方法，你需要定义一个类，然后有一个自定义例程来从 Byte() 转换为类的属性.比简单地将文件中的字节加载到内存中需要更多的工作.

I wouldn't blame you for keeping the old method though: with the new method, you'll need to define a class, and then have a custom routine to convert from Byte() to the properties of the class. More work than simply loading bytes from the file into memory.

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