> I have the need to write a byte of information to a specific location in a
> text file.
>
> eg. the file looks something like this.
>
> FYYNN
> Line 1
> Line 2
> <eof>
>
> The first character is a flag indicating the file has changed and the 2 -
> 4th characters are flags indicating the line of data that has changed ie Y or
> N.
>
> What I need to do is set the last four characters on the first line and then
> write a line coresponding to each of the 'Y' flags.  Finally and it must be
> the last thing I do is set the first character on the first line to a flag
> which is used by a second application to initate an action.
>
> I hope this makes sense.
>
> initial myfile.txt
> WNNNN
> <eof>
>
> Below is a sub that I am using to try and update the first flag.
>
> Sub WriteFile()
>
>         Dim fs As New System.IO.FileStream("C:\myFile.txt",
> System.IO.FileMode.Open, System.IO.FileAccess.ReadWrite)
>         Dim w As New System.IO.BinaryWriter(fs)
>
>         fs.Seek(1, IO.SeekOrigin.Begin)
>         w.Seek(1, SeekOrigin.Begin)
>         w.Write(CChar("X"))
>     End Sub 'Main
>
> Resulting myfile.txt
> ïX¿WNNNN
> <eof>
>

"Göran Andersson" wrote:

> If you want to write directly to the file you have to first determine
> what kind of file it is, then encode the characters according to this.
> That is far more complicated than your attempt.
>
>
> It looks like it's an 8-bit unicode file (UTF8), and that you have
> written the character in the middle of the preamble (which is an
> encoding identifier at the beginning of the file).
>
> The preamble for an UTF8 file is the three bytes EF BB BF. As you seek
> to the second byte in the file (as the index is zero based) you
> overwrite the second and third bytes of the preamble. The reason that
> you are writing two bytes to the file is that the Char data type in .NET
> is a 16 bit unicode character, and writing that using a BinaryWriter
> results in two bytes.
>
> After you written to the file, it will contain this data:
>
> EF 'X' 00 'W' 'N' 'N' 'N' 'N'
>
> As this is no longer a unicode file (as you destroyed the preamble), it
> will be read as a regular ANSI text file, and the EF and 00 bytes will
> show as "weird" characters.
>
>
> The easiest way to do this is to simply read the entire file, make the
> changes, and write it all back again.
>
>
> Craig wrote:
> > I have the need to write a byte of information to a specific location in a
> > text file.
> >
> > eg. the file looks something like this.
> >
> > FYYNN
> > Line 1
> > Line 2
> > <eof>
> >
> > The first character is a flag indicating the file has changed and the 2 -
> > 4th characters are flags indicating the line of data that has changed ie Y or
> > N.
> >
> > What I need to do is set the last four characters on the first line and then
> > write a line coresponding to each of the 'Y' flags.  Finally and it must be
> > the last thing I do is set the first character on the first line to a flag
> > which is used by a second application to initate an action.
> >
> > I hope this makes sense.
> >
> > initial myfile.txt
> > WNNNN
> > <eof>
> >
> > Below is a sub that I am using to try and update the first flag.
> >
> > Sub WriteFile()
> >
> >         Dim fs As New System.IO.FileStream("C:\myFile.txt",
> > System.IO.FileMode.Open, System.IO.FileAccess.ReadWrite)
> >         Dim w As New System.IO.BinaryWriter(fs)
> >
> >         fs.Seek(1, IO.SeekOrigin.Begin)
> >         w.Seek(1, SeekOrigin.Begin)
> >         w.Write(CChar("X"))
> >     End Sub 'Main
> >
> > Resulting myfile.txt
> > ïX¿WNNNN
> > <eof>
> >
>