从字符数组转换时的字符串长度 [英] String length when converting from a character array

问题描述

我在处理字符串方面遇到严重问题。
由于我的问题很难描述，因此我将以一些演示代码来再现它们：

  Dim s1 As String = hi 
 Dim c（30）As Char 
c（0）= h 
c（1）= i 
 Dim s2 As String = CStr（c ）
 s2 = s2.Trim（）
如果不是s1 = s2，则
 MsgBox（s1 +！= + s2 + Environment.NewLine + _ 
仍然无法打印... + Environment.NewLine + _ 
 s1.length： + s1.Length.ToString + Environment.NewLine + _ 
 s2.length： + s2。 Length.ToString + Environment.NewLine）
结束于
  

结果消息框如下所示：

此比较失败的原因是s2的长度为31（从原始数组大小开始），而s1的长度为2。

当从字节数组中读取字符串信息时，我经常会遇到这种问题，例如例如，当处理来自MP3的ID3Tag或具有预先指定长度的其他编码（ASCII，UTF8等）信息时。

是否有任何快速，干净的方法来防止这种情况问题？

最简单的方法是将s2修剪到调试器显示的字符串上？

解决方案

为清楚起见，我更改了变量名称：

  Dim myChars（30）As Char 
 myChars（0）= h c'无法将字符串转换为char 
 myChars（1）= i c'在选项严格（变窄）下
 Dim myStrA作为新字符串（myChars） 
 Dim myStrB As String = CStr（myChars）
  

简短的答案是这样的：

在内部，字符串是字符数组。最后两行都使用NET代码创建一个字符串，另一行使用VB函数。事实是，尽管数组有31个元素，但仅初始化了2个：

。

但是， String.Trim 具有 overload ，可让您指定要删除的字符：

  myStrA = myStrA.Trim（Convert.ToChar（0））
'使用VB命名空间常量
 myStrA = myStrA.Trim（Microsoft.VisualBasic.ControlChars .NullChar）
  

您可以指定多个字符：

 'nul和空格：
 myStrA = myStrA.Trim（Convert.ToChar（0）， c）
  

---

字符串 可以被索引/迭代为char数组：

 对于InAs32 = 0到myStrA.Length 
 Console.Write（ {0} is'{1}'， n，myStrA（n））'或myStrA.Chars（n）
下一个
  

0是'h'

1是'i'

2是'

（输出窗口将不会您甚至无法更改字符串的char数组来更改字符串数据，但是：

  myStrA（2） =！ c 
  

由于它们是只读的，因此不会编译。

另请参见：

ASCII表

I'm having serious problems with string-handling. As my problems are rather hard to describe, I will start with some demo code reproducing them:

Dim s1 As String = "hi"
Dim c(30) As Char
c(0) = "h"
c(1) = "i"
Dim s2 As String = CStr(c)
s2 = s2.Trim()
If not s1 = s2 Then
   MsgBox(s1 + " != " + s2 + Environment.NewLine + _
          "Anything here won't be printed anyway..." + Environment.NewLine + _ 
          "s1.length: " + s1.Length.ToString + Environment.NewLine + _
          "s2.length: " + s2.Length.ToString + Environment.NewLine)
End If                    

The result messagebox looks like this:

The reason that this comparison fails is that s2 has the length 31 (from the original array-size) while s1 has the length 2.

I stumble over this kind of problem quite often when reading string-information out of byte-arrays, for example when handling ID3Tags from MP3s or other encoded (ASCII, UTF8, ...) information with pre-specified length.

Is there any fast and clean way to prevent this problem?

What is the easiest way to "trim" s2 to the string shown by the debugger?

解决方案

I changed the variable names for clarity:

Dim myChars(30) As Char
myChars(0) = "h"c           ' cannot convert string to char
myChars(1) = "i"c           ' under option strict (narrowing)
Dim myStrA As New String(myChars)
Dim myStrB As String = CStr(myChars)

The short answer is this:

Under the hood, strings are character arrays. The last 2 lines both create a string one using NET code, the other a VB function. The thing is that, although the array has 31 elements, only 2 were initialized:

The rest are null/Nothing, which for a Char means Chr(0) or NUL. Since NUL is used to mark the end of a String, only the characters up to that NUL will print in the Console, MessageBox etc. Text appended to the string will not display either.

---

Concepts

Since the strings above are created directly from a char array, the length is that of the original array. The Nul is a valid char so they get added to the string:

Console.WriteLine(myStrA.Length)     ' == 31

So, why doesn't Trim remove the nul characters? MSDN (and Intellisense) tells us:

[Trim] Removes all leading and trailing white-space characters from the current String object.

The trailing null/Chr(0) characters are not white-space like Tab, Lf, Cr or Space, but is a control character.

However, String.Trim has an overload which allows you to specify the characters to remove:

myStrA = myStrA.Trim(Convert.ToChar(0))
' using VB namespace constant
myStrA = myStrA.Trim( Microsoft.VisualBasic.ControlChars.NullChar)

You can specify multiple chars:

' nuls and spaces:
myStrA = myStrA.Trim(Convert.ToChar(0), " "c)

---

Strings can be indexed / iterated as a char array:

    For n As Int32 = 0 To myStrA.Length
        Console.Write("{0} is '{1}'", n, myStrA(n))  ' or myStrA.Chars(n)
    Next

0 is 'h'
1 is 'i'
2 is '

(The output window will not even print the trailing CRLF.) You cannot change the string's char array to change the string data however:

   myStrA(2) = "!"c

This will not compile because they are read-only.

See also:

ASCII table

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