我无法循环将数据写入二进制文件

Question

我的所有数据（名称/卷号/类）都是固定长度（14 字节），我希望程序继续从数组中获取一个随机名称，然后生成一个随机卷号并继续将其写入文件直到我按“A”。

在我运行完程序后，前 14 个字节总是空的，然后我的数据在接下来的 14 个字节中。不多不少。不知道为什么只写了一条数据记录。

这是我的代码：

    Dim randomvalue As Integer

    'Making this array so that each data entry has 14 bytes to it

    Dim array1(91) As Integer

    array1(1) = 14
    For x = 2 To 91
        array1(x) = array1(x) + 14
    Next

    Dim n1, n2, n3, n4 As Integer

    'Array with names of fixed lenght, I want to take a name from only these 9:

    Dim array2(9) As String
    array2(1) = "aleen"
    array2(2) = "talha"
    array2(3) = "waddi"
    array2(4) = "hasna"
    array2(5) = "hassa"
    array2(6) = "zainn"
    array2(7) = "faqeh"
    array2(8) = "furru"
    array2(9) = "ibrah"

  'There is a structure above submain with studentname/rollnumber/class all declared as string 

    Dim newstudent As student
    Dim studentstream As New FileStream("C:\Users\Students\Desktop\A2Filing.dat", FileMode.OpenOrCreate)
    Dim bw As New BinaryWriter(studentstream)

    While Console.ReadLine <> "A"
        Console.WriteLine("Press any key other than 'A' if you want to make another entry into the file")


        'CInt(Math.Floor((upperlimit - lowerlimit + 1) * Rnd())) + lowerlimit
        'Randomize a number b/w 1-9 to get a studentname from the array

        randomvalue = CInt(Math.Floor((9 - 1 + 1) * Rnd())) + 1
        newstudent.studentname = array2(randomvalue)

        n1 = Rnd() + (Rnd() * 50)
        n2 = Rnd() * 10
        n3 = Rnd() * 255
        n4 = Rnd() * Rnd()
        newstudent.rollnumber = Convert.ToString(Left(n1, 1) + Left(n2, 1) + Left(n3, 1) + Left(n4, 1))

        newstudent.studentclass = "A2"

        'Randomize a number between 91 and 1 to place the data in

        Dim randomvalue2 As Integer
        randomvalue2 = CInt(Math.Floor((91 - 1 + 1) * Rnd())) + 1
        studentstream.Position = array1(randomvalue2)


        bw.Write(newstudent.studentname)
        bw.Write(newstudent.rollnumber)
        bw.Write(newstudent.studentclass)
    End While

Answer 1

这里有一些注释：

• 您初始化时的array1 项目将全部为14，您必须将其更改为跟随，但正如我稍后所说，不需要定义这样的数组！

  For x = 2 To 91
    array1(x) = array1(x - 1) + 14   'modified to array1(x - 1)
  Next
  

• 在bw.Write 之前，你有这个：studentstream.Position = array1(randomvalue2)，它将覆盖文件位置！ array1 中的所有项目也等于 14。这就是为什么你总是得到一个小文件大小（不多不少）。您不需要以这种方式设置位置。 只需从您的代码中删除该行即可逐个附加数据。

• 根据文档，如果您使用BinaryWriter 编写字符串，它将以字符串长度为前缀！ see this question。

  Dim bw As New BinaryWriter(studentstream, System.Text.Encoding.ASCII)
  
  bw.Write("A")  'writes: 1 A  => where 1 is length of string "A"
  bw.Write("BC") 'writes: 2 B  => where 2 is length of string "BC" 
  
  'note that for Unicode Encoding, it will be 2 bytes per char
  Dim bw As New BinaryWriter(studentstream, System.Text.Encoding.Unicode)
  bw.Write("A")  '2 A 0        where 2 is length of unicde string [A 0]
  bw.Write("BC") '4 B 0 C 0    where 4 is length of unicde string [B 0 C 0]
  

• 如果要写不带长度前缀的字符串，可以使用System.Text.Encoding.ASCII.GetBytes(str)将其转为字节数组：

  bw.Write(System.Text.Encoding.ASCII.GetBytes(newstudent.studentname))
  

• 另一个选项也是使用StreamWriter 而不是BinaryWriter，因为您似乎不需要它。 see this link

• 请注意，最好使用 Using 块打开文件。否则，完成后不要忘记调用Close 方法（studentstream.Close()）。如果您不这样做，您可能会丢失最后一块或全部数据！

  Using studentstream As New FileStream("C:\...\Desktop\A2Filing.dat", FileMode.Create)
  

• 如果文件已存在，请使用FileMode.Create 覆盖该文件