Question

我在Visual Basic的Windows Application Form项目中运行此代码。这段代码应该给我一个字节数组的CRC-32字节，但无论我做什么我都不能输出任何字节。对不起，如果有一些非常明显我在这里丢失的东西，但我已经尝试了＃34; MsgBox＆＃34;，＆＃34;立即屏幕＆＃34;使用＆＃34; debug.print＆＃34;我也尝试在文本框中打印字节，但没有结果。如果你能帮助我，我感激不尽。

Public Class Form1

Public Sub Main()

    Dim lCrc32Value As Long

    On Error Resume Next
    lCrc32Value = InitCrc32()
    lCrc32Value = AddCrc32("This is the original message!", _
       lCrc32Value)
    'Debug.Print(Hex$(GetCrc32(lCrc32Value)))

    TextBox1.Text = (Hex$(GetCrc32(lCrc32Value))).ToString
End Sub
'// Then declare this array variable Crc32Table
Private Crc32Table(255) As Long
'// Then all we have to do is write public functions like
'these...
Public Function InitCrc32(Optional ByVal Seed As Long = _
   &HEDB88320, Optional ByVal Precondition As _
   Long = &HFFFFFFFF) As Long

    '// Declare counter variable iBytes, 
    'counter variable iBits, 
    'value variables lCrc32 and lTempCrc32

    Dim iBytes As Integer, iBits As Integer, lCrc32 As Long
    Dim lTempCrc32 As Long

    '// Turn on error trapping
    On Error Resume Next

    '// Iterate 256 times
    For iBytes = 0 To 255

        '// Initiate lCrc32 to counter variable
        lCrc32 = iBytes

        '// Now iterate through each bit in counter byte
        For iBits = 0 To 7
            '// Right shift unsigned long 1 bit
            lTempCrc32 = lCrc32 And &HFFFFFFFE
            lTempCrc32 = lTempCrc32 \ &H2
            lTempCrc32 = lTempCrc32 And &H7FFFFFFF

            '// Now check if temporary is less than zero and then 
            'mix Crc32 checksum with Seed value
            If (lCrc32 And &H1) <> 0 Then
                lCrc32 = lTempCrc32 Xor Seed
            Else
                lCrc32 = lTempCrc32
            End If
        Next

        '// Put Crc32 checksum value in the holding array
        Crc32Table(iBytes) = lCrc32
    Next

    '// After this is done, set function value to the 
    'precondition value
    InitCrc32 = Precondition

End Function

'// The function above is the initializing function, now 
'we have to write the computation function
Public Function AddCrc32(ByVal Item As String, _
  ByVal Crc32 As Long) As Long

    '// Declare following variables
    Dim bCharValue As Byte, iCounter As Integer, lIndex As Long
    Dim lAccValue As Long, lTableValue As Long

    '// Turn on error trapping
    On Error Resume Next

    '// Iterate through the string that is to be checksum-computed
    For iCounter = 1 To Len(Item)

        '// Get ASCII value for the current character
        bCharValue = Asc(Mid$(Item, iCounter, 1))

        '// Right shift an Unsigned Long 8 bits
        lAccValue = Crc32 And &HFFFFFF00
        lAccValue = lAccValue \ &H100
        lAccValue = lAccValue And &HFFFFFF

        '// Now select the right adding value from the 
        'holding table
        lIndex = Crc32 And &HFF
        lIndex = lIndex Xor bCharValue
        lTableValue = Crc32Table(lIndex)

        '// Then mix new Crc32 value with previous 
        'accumulated Crc32 value
        Crc32 = lAccValue Xor lTableValue
    Next

    '// Set function value the the new Crc32 checksum
    AddCrc32 = Crc32

End Function

'// At last, we have to write a function so that we 
'can get the Crc32 checksum value at any time
Public Function GetCrc32(ByVal Crc32 As Long) As Long
    '// Turn on error trapping
    On Error Resume Next

    '// Set function to the current Crc32 value
    GetCrc32 = Crc32 Xor &HFFFFFFFF

End Function
'// And for testing the routines above...



End Class

Answer 1

有几个问题。首先，所有CRC代码都应该在不与表单混合的类中。其次，Option Strict没有打开，因为有几个隐式转换可能会破坏结果：

' converting Integer to Byte
bCharValue = Asc(Mid$(Item, iCounter, 1))

' see note 7
DIm lIndex As Long
' converting integer to Long
lIndex = Crc32 And &HFF

' here too:
lTableValue = Crc32Table(lIndex)

＃3。摆脱On Error Resume next。这不是错误处理 - 它隐藏了你的错误。

＃4。你永远不会调用Main函数。 不要从表单加载调用它将从那里调试太困难。添加一个按钮并从点击事件中调用Main。

＃5强烈建议从旧的VB函数迁移到.NET中的等效函数，这些函数更加健壮。

＃6。对AddCRC32的更改：

Public Function AddCrc32(ByVal Item As String, ByVal Crc32 As Long) As Long
    '// Declare following variables
    Dim bCharValue As Byte, iCounter As Integer, lIndex As Long
    Dim lAccValue As Long, lTableValue As Long

    ' convert text to char array all at once
    Dim strChars = Item.ToCharArray

    '// Iterate through the string that is to be checksum-computed
    For iCounter = 0 To Item.Length - 1

        '// Get ASCII value for the current character
        'bCharValue = Asc(Mid$(Item, iCounter, 1))

        ' NET version
        bCharValue = Convert.ToByte(strChars(iCounter))

        '// Right shift an Unsigned Long 8 bits
        lAccValue = Crc32 And &HFFFFFF00
        lAccValue = lAccValue \ &H100
        lAccValue = lAccValue And &HFFFFFF

        '// Now select the right adding value from the 
        'holding table
        lIndex = Crc32 And &HFF
        lIndex = lIndex Xor bCharValue
        lTableValue = Crc32Table(CInt(lIndex))

        '// Then mix new Crc32 value with previous 
        'accumulated Crc32 value
        Crc32 = lAccValue Xor lTableValue
    Next

    '// Set function value the the new Crc32 checksum
    ' a REALLY big indicator that this came from VB6:
    AddCrc32 = Crc32

    ' NET is usually:
    ' Return Crc32

End Function

＃7：上面的lIndex修复应该是我记得的CRC32。另一种方法是反过来：将其更改为Integer并将Crc32 And &HFF转换为整数。

注意，我没有检查任何CRC32内容（比如表初始化程序），只检查OPTION Strict问题并将其挂起，并检查初始异常。

附录

我找到了自己的旧CRC32代码，在比较时，我发现了一些与CRC相关的问题：

Buildtable/InitCrc32过程中的几乎所有内容（包括返回值都应该是Integer（Int32）不长（Int64））。看起来您从VB6源代码复制了代码并且没有正确更新（在任何地方都不需要Long）。

对于文本编码，您可能需要这样：

Dim encoding As New System.Text.UTF8Encoding()
Dim byt As Byte() = encoding.GetBytes(s)

而不是上面给出的Convert.ToByte。正如我所说，我主要考虑的是以某种形式或方式使其工作，而不是对算法进行认证。

为什么这个Visual基本代码不会产生任何结果？

1 个答案: