BinaryReader.PeekChar ArgumentException: Conversion Buffer Overflow

[Tim]

Hi,

I'm experiencing some problem with the following code:

st = File.Open(sFilename, FileMode.Open, FileAccess.ReadWrite)
br = New BinaryReader(st)

Do Until br.PeekChar = -1
Dim buffer() As Byte = br.ReadBytes(1024)
...
Loop

Basically, PeekChar would sometimes cause the System.ArgumentException in
mscorlib.dll with the message "Conversion buffer overflow." However, when I
examine the value after inserting breakpoints, br.PeekChar returns normal
values without error. Furthermore, the number of bytes read before the
error occurs is always the same, assuming I am using the same file.

This problem disappears after I change the code to the following:

Dim len As Integer = 1

Do Until len = 0
Dim buffer(1024) As Byte
len = br.Read(buffer, 0, 1024)
...
Loop

This makes me believe that the problem _is_ indeed caused by PeekChar.

I've tried searching for people expericing similar problems online, but to
no avail. Anyone has any idea?

Thanks.


Tim

 

[Fergus Cooney]

Hi Tim,

I've got no idea what the problem is but I'm intrigued by 'Furthermore,
the number of bytes read before the error occurs is always the same, assuming
I am using the same file.' which suggests that there's something in the file
at that point which is causing the error. This makes sense if it's a
conversion error.
The questions must be, therefore:
What is it converting? and
What is it trying to convert it to?

Time to go get some bytes out of that file. And make sure that you have
the right bytes! ;-)

Regards,
Fergus

 

[Jay B. Harlow [MVP - Outlook]]

Tim,
Remember a Char is 2 bytes. Depending on the encoding on the underlying
stream if you have an odd number of bytes in the file I would expect the
error!

I would use a variation of your second loop, as PeekChar is NOT the EOF
function.

Dim len As Integer
Do
Dim buffer(1023) As Byte
len = br.Read(buffer, 0, 1024)
Loop Until len = 0

Although your second loop effectively does the same thing.

Dim buffer(1024) As Byte

BTW: You have an array of 1025 bytes, as the 1024 is the high index, the low
index is 0.

Hope this helps
Jay

 

[Tim]

Fergus,

I've looked into the content of the file at each point where the exception
occurs, and there appears to be absolutely no correlation between the bytes.
I'm not sure if VB is trying to do any kind of underlying conversion, but I
certainly am not


Tim

Hi Tim,

I've got no idea what the problem is but I'm intrigued by 'Furthermore,
the number of bytes read before the error occurs is always the same,
assuming
I am using the same file.' which suggests that there's something in the file
at that point which is causing the error. This makes sense if it's a
conversion error.
The questions must be, therefore:
What is it converting? and
What is it trying to convert it to?

Time to go get some bytes out of that file. And make sure that you have
the right bytes! ;-)

Regards,
Fergus

 

[Tim]

Jay,

The exceptions are occurring in the middle of the file (almost never at the
end), so I'm going to assume that sizeof(char) is not the issue.

I guess I was mislead by the book "Programming Microsoft Visual Basic .NET
Core Reference" by Francesco Balena, in which the following example is used:

Dim st2 as Stream = File.Open("c:\value.dat", FileMode.Open,
FileAccess.Read)
Dim br2 as New BinaryReader(st2)

Do Until br2.PeekChar = -1
Console.WriteLine(br2.ReadDouble)
Loop

br2.Close()
st2.Close()

But then again, since sizeof(double) % sizeof(char) == 0, I guess that's why
it works.

Thanks a lot for the help. I'll go ahead and use the second version.


Tim

 

[Jay B. Harlow [MVP - Outlook]]

Tim,
And what about Encoding itself?

Even if you are not at the end of the file, I understand that PeekChar needs
to decode the bytes into a Char. Based on the Encoding given (or not given)
when you open the Stream & BinaryReader that Encoding will attempt to
convert one or more bytes into a Char, if you just happen to be at a byte
that 'requires' four bytes to decode, but only have two bytes that make
sense... I would expect an Exception.

Hence I would not use PeekChar to check for EOF.

Remember the default encoding used on BinaryReader is UTF8Encoding.

Hope this helps
Jay