我不能将数据写入循环中的二进制文件

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取一个随机名，然后制作一个随机卷号并继续将其写入文件，直到按“A”。

我完成程序运行后，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。没有什么比这更少。我不知道为什么它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

有一些注意事项这里：

• 的array1项目，你init将全部是14，你必须将其更改为以下，但由于我稍后会说，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 
  

• bw.Write之前，你有这样的：studentstream.Position = array1(randomvalue2)这将覆盖文件位置！ array1中的所有项目都等于14.这就是为什么你总是得到一个小文件大小（没有什么比这更少的了）。你不需要这样设置位置。 只需从代码中删除该行以便一个接一个追加数据。

• 根据文档，如果您使用BinaryWriter来编写字符串，它将以字符串长度作为前缀！ 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] 
  

• 如果你想要写串不长的前缀，你可以用System.Text.Encoding.ASCII.GetBytes(str)将其转换为字节数组：

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

• 另一种选择也将使用StreamWriter而不是BinaryWriter，因为它似乎你不需要它。 see this link

• 请注意，它最好用Using块打开文件。否则，当您完成时，请不要忘记拨打Close方法（studentstream.Close()）。如果你不这样做，你可能会丢失最后一块或整个数据！

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

• 使用FileMode.Create覆盖文件，如果它已经存在