[RESOLVED] Why won't Convert.ToByte convert 128?

[Maylar]

This is odd...

I have a function that breaks a string into individual characters then converts the chars to a byte array. It's used for sending serial data to an external device.

Dim msg As String = Chr(241) + Chr(128) + Chr(0) + Chr(129) ' character string
SndMsg32(msg)

 Function SndMsg32(ByRef MsgData As String) As Boolean

        Dim bChar As Byte
        Dim i As Short
        Dim Z As Char
        Dim SndBuf(256) As Byte 'array of bytes to hold message


        For i = 1 To Len(MsgData)
            Z = Mid(MsgData, i, 1)  'Mid is 1 based
            bChar = System.Convert.ToByte(Z)  ' convert char back to byte
            SndBuf(i-1) = bChar  'byte array is zero based
        Next i

        SndMsg32 = True
    End Function

VB2008 and 2010 throw an exception if the value of one of those chars is 128. Zero is OK, and so are 127 and 129, but 128 gets caught with a message - "Value was either too large or too small for an unsigned byte."

So, what's up with the number 128?

[Inferrd]

The Chr method works with the ANSI Code Page that is in effect for the current thread that you code is running on. In the Western hemisphere, it is most often Windows-1252. Windows-1252 places the Euro sign at character code 128.

VB stores your string as a sequence of Char objects. In VB, Char objects correspond to Unicode code points in the Unicode BMP. They are 16 bit values. The Euro sign is mapped to Unicode code point U+20AC, and that is the value that Chr(128) is stored as internally in your string.

The System.Convert.ToByte Method works with the 16 bit Char values, so you are basically trying to stuff the value 0x20AC into an unsigned Byte, and it won't fit.

Try using bChar = Asc(Z) instead.

[dday9]

I do not have this error when I compile this:

Imports System
Public Module Module1
	Public Sub Main()
		Console.WriteLine(Convert.ToByte(128))
	End Sub
End Module

[Inferrd]

That's because the overload of the Convert.ToByte method you are using there is the one that takes a 32 bit Integer as an argument (not a Char), and 128 as an Integer will happily fit into an unsigned Byte.

[dday9]

Ah, ok. That makes sense then.

[Maylar]

The Chr method works with the ANSI Code Page that is in effect for the current thread that you code is running on. In the Western hemisphere, it is most often Windows-1252. Windows-1252 places the Euro sign at character code 128.

VB stores your string as a sequence of Char objects. In VB, Char objects correspond to Unicode code points in the Unicode BMP. They are 16 bit values. The Euro sign is mapped to Unicode code point U+20AC, and that is the value that Chr(128) is stored as internally in your string.

The System.Convert.ToByte Method works with the 16 bit Char values, so you are basically trying to stuff the value 0x20AC into an unsigned Byte, and it won't fit.

Try using bChar = Asc(Z) instead.

Thanks. That worked.