I've found that My.Computer.FileSystem.WriteAllText in .Net 2.0 writes three 
additional, non-text bytes in front of the string it's supposed to be 
writing. These bytes are not in the normal text range of values, and don't 
show up if you load the file into Notepad or other text viewers, but can be 
easily seen with Visual Studio 2005's Binary editor. Additionally, if you use 
this method to write out an HTML file to be embedded in other HTML (like an 
Outlook signature), these bytes get rendered. Finding this bug was a pain, to 
say the least. I have an example VS2005 solution folder which shows the bug, 
and would be happy to send it to someone at MS if anyone is interested, but 
you can verify in about 2 minutes with a simple "Hello World!" output to a 
new text file using the above method.

Bo McIlvain wrote:

> I've found that My.Computer.FileSystem.WriteAllText in .Net 2.0 writes three 
> additional, non-text bytes in front of the string it's supposed to be 
> writing. [...]


My guess is that you are seeing the text-encoding signature (the "byte 
order mark") for the text file.  It's not a bug, it's by design.  If you 
don't want the bytes, you need to make sure that the encoding is a plain 
ASCII file.

 From http://msdn2.microsoft.com/en-us/library/27t17sxs.aspx

"When no encoding is specified, UTF-8 is used. The byte order mark (BOM) 
for the encoding is written to the file unless you specify 
System.Text.Encoding.Default, which uses the system's current ANSI code 
page."

Pete

Missed that, thanks. Too bad about the effect on things like Outlook, if my 
opinion counted the straightforward option (you know, text + standard 
whitespace in a text file) would be the default. Thanks for the response, it 
helps to know to look out for these things. If I'd seen it I'd have saved 
myself a lot of time.

Bo.

"Peter Duniho" wrote:


> Bo McIlvain wrote:
> > I've found that My.Computer.FileSystem.WriteAllText in .Net 2.0 writes three 
> > additional, non-text bytes in front of the string it's supposed to be 
> > writing. [...]
> 
> My guess is that you are seeing the text-encoding signature (the "byte 
> order mark") for the text file.  It's not a bug, it's by design.  If you 
> don't want the bytes, you need to make sure that the encoding is a plain 
> ASCII file.
> 
>  From http://msdn2.microsoft.com/en-us/library/27t17sxs.aspx
> 
> "When no encoding is specified, UTF-8 is used. The byte order mark (BOM) 
> for the encoding is written to the file unless you specify 
> System.Text.Encoding.Default, which uses the system's current ANSI code 
> page."
> 
> Pete
> 

Hi Bo,

The My.Computer.FileSystem.WriteAllText has two variations:

Public Sub WriteAllText( _
   ByVal file As String, _
   ByVal text As String, _
   ByVal append As Boolean _
)
' -or-
Public Sub WriteAllText( _
   ByVal file As String, _
   ByVal text As String, _
   ByVal append As Boolean, _
   ByVal encoding As System.Text.Encoding _
)

The first one is using System.Text.Encoding.UTF8 as the default encoding.

The System.Text.Encoding.UTF8 is an instance of 
System.Text.Encoding.UTF8Encoding.

The System.Text.Encoding.UTF8Encoding has a constructor which accepts a 
boolean parameter named "encoderShouldEmitUTF8Identifier" (see 
http://msdn2.microsoft.com/en-us/library/s064f8w2(VS.80).aspx): when it's 
true (which is the case for the instance returned by 
System.Text.Encoding.UTF8), it means that a Unicode byte order mark (short 
for BOM) is provided. 

Basically a BOM at the beginning of a file will help the applications that 
are reading the file to determine what's the correct encoding of the file.

You may find more information about BOM here:

#Byte-order mark - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Byte_Order_Mark

#FAQ - UTF-8, UTF-16, UTF-32 & BOM
http://unicode.org/faq/utf_bom.html

#Byte Order Mark
http://msdn2.microsoft.com/en-us/library/ms776429.aspx

In .NET, when a encoding has specified BOM, the IO classes will output 
those extra bytes in the beginning of a file when writing the file. 

For notepad, it will try to detect the encoding from the BOM:

#The Old New Thing : Some files come up strange in Notepad
http://blogs.msdn.com/oldnewthing/archive/2004/03/24/95235.aspx

Hope this helps.


Sincerely,
Walter Wang (wawang@online.microsoft.com, remove 'online.')
Microsoft Online Community Support

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