C# – way to check if a file is in use

cfilefile-iofile-lockingnet

I'm writing a program in C# that needs to repeatedly access 1 image file. Most of the time it works, but if my computer's running fast, it will try to access the file before it's been saved back to the filesystem and throw an error:

"File in use by another process"

I would like to find a way around this, but all my Googling has only yielded creating checks by using exception handling. This is against my religion, so I was wondering if anyone has a better way of doing it?

Best Answer

Updated NOTE on this solution: Checking with FileAccess.ReadWrite will fail for Read-Only files so the solution has been modified to check with FileAccess.Read.

ORIGINAL: I've used this code for the past several years, and I haven't had any issues with it.

Understand your hesitation about using exceptions, but you can't avoid them all of the time:

protected virtual bool IsFileLocked(FileInfo file)
{
    try
    {
        using(FileStream stream = file.Open(FileMode.Open, FileAccess.Read, FileShare.None))
        {
            stream.Close();
        }
    }
    catch (IOException)
    {
        //the file is unavailable because it is:
        //still being written to
        //or being processed by another thread
        //or does not exist (has already been processed)
        return true;
    }

    //file is not locked
    return false;
}

Related Solutions

Python – How to check whether a file exists without exceptions

If the reason you're checking is so you can do something like if file_exists: open_it(), it's safer to use a try around the attempt to open it. Checking and then opening risks the file being deleted or moved or something between when you check and when you try to open it.

If you're not planning to open the file immediately, you can use os.path.isfile

Return True if path is an existing regular file. This follows symbolic links, so both islink() and isfile() can be true for the same path.

import os.path
os.path.isfile(fname)

if you need to be sure it's a file.

Starting with Python 3.4, the pathlib module offers an object-oriented approach (backported to pathlib2 in Python 2.7):

from pathlib import Path

my_file = Path("/path/to/file")
if my_file.is_file():
    # file exists

To check a directory, do:

if my_file.is_dir():
    # directory exists

To check whether a Path object exists independently of whether is it a file or directory, use exists():

if my_file.exists():
    # path exists

You can also use resolve(strict=True) in a try block:

try:
    my_abs_path = my_file.resolve(strict=True)
except FileNotFoundError:
    # doesn't exist
else:
    # exists

Python – How to copy a file in Python

shutil has many methods you can use. One of which is:

from shutil import copyfile
copyfile(src, dst)

# 2nd option
copy(src, dst)  # dst can be a folder; use copy2() to preserve timestamp

Copy the contents of the file named src to a file named dst. Both src and dst need to be the entire filename of the files, including path.
The destination location must be writable; otherwise, an IOError exception will be raised.
If dst already exists, it will be replaced.
Special files such as character or block devices and pipes cannot be copied with this function.
With copy, src and dst are path names given as strs.

Another shutil method to look at is shutil.copy2(). It's similar but preserves more metadata (e.g. time stamps).

If you use os.path operations, use copy rather than copyfile. copyfile will only accept strings.

Best Answer

Related Solutions

Python – How to check whether a file exists without exceptions

Python – How to copy a file in Python

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