Something I don't quite understand about "file in use by another process" exceptions.....

  • Thread starter Thread starter Robinson
  • Start date Start date
R

Robinson

Hi,

I was just playing around with my log files and tried to open a log file
programmatically that was considered "in use" (I got an in-use exception).
The file is being used by my debug writer and I wanted to read lines from
the file on one of my forms. It's a simple text file by the way. Now
obviously the exception I get from the code trying to open the file for
reading looks like this:

The process cannot access the file 'C:\Documents and
Settings\mymachine\Application Data\mycomapny\myproduct\log.txt' because it
is being used by another process.

What I don't understand is that I can double click the file and have it open
up in notepad.exe. I don't get an "in use" exception from Explorer, so what
is notepad.exe doing that I cannot being able to open the file? I notice
that you can File.Copy the file while it's in-use and this strikes me as one
way I could get to read a file already in use by the system (copy and then
open and read the copy). Can anyone shed any light on Windows inner
workings in this respect?

Thanks,



Robin
 
GS said:
what is your code for opening the file?
Did you open for read only?
Click to expand...

Hi,

Yes, I tried File.ReadAll (as a test), a StreamReader and File.Open ( for
reading ). All of these threw the exception, but File.Copy did not.


Robin
 
That was not the question.

The question was whether or not you specified that you are opening the file
in read-only mode when opening the file for reading.
 
Marina Levit said:
That was not the question.

The question was whether or not you specified that you are opening the
file in read-only mode when opening the file for reading.
Click to expand...

File.ReadAllLines does not have an overload to specify for read-only, just
path and encoding.

File.Open, has Create, Append, CreateNew, Open, OpenOrCreate, Truncate (I
chose Open)

Dim theStream As New StreamReader, has overloads for encoding and
auto-detection of encoding, but not a "read only" flag. However in the
latter case, I'm sure it's read only in any case because it's a
streamreader, not a streamreaderorwriter.

Did I miss something here?



Robin
 
Hello Robinson,

I've not validated your assertions, but the FileStream class does provide
an Open method that takes a (I believe) FileMode enum (flag) value.

-Boo
 
Robinson said:
File.ReadAllLines does not have an overload to specify for read-only, just
path and encoding.

File.Open, has Create, Append, CreateNew, Open, OpenOrCreate, Truncate (I
chose Open)

Dim theStream As New StreamReader, has overloads for encoding and
auto-detection of encoding, but not a "read only" flag. However in the
latter case, I'm sure it's read only in any case because it's a
streamreader, not a streamreaderorwriter.
Click to expand...

Hi Robin,

You mentioned trying to understand the inner workings. All of these
..NET framework methods are just indirect means to call the Windows API.

And when opening a file directly with the API, in addition to the
filename it expects three parameters:

FileMode: Create, Append, CreateNew, Open, OpenOrCreate, Truncate
FileAccess: Read, Write, ReadWrite
FileShare: Delete, Inheritable, None, Read, ReadWrite, Write

I don't know my way around the .NET framework yet as well as the API,
but it seems that the problem is that the methods you've tried so far
assume FileShare=None, and do not allow you to override it to
ReadWrite, which is what you really need. But I do know of one that
does (there may be more):

Dim f As New System.IO.FileStream("c:\log.txt", IO.FileMode.Open,
IO.FileAccess.Read, IO.FileShare.ReadWrite)
 
GhostInAK said:
Hello Robinson,

I've not validated your assertions, but the FileStream class does provide
an Open method that takes a (I believe) FileMode enum (flag) value.

-Boo
Click to expand...

Thanks for all above replies, I did indeed miss an overload when searching
the documentation. The correct way of doing it is:

theStream = File.Open(Application.UserAppDataPath + "\log.txt",
FileMode.Open, FileAccess.Read, FileShare.ReadWrite)
 
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