问题
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?
回答1:
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. While this solution works because trying to check with FileAccess.Read will fail if the file has a Write or Read lock on it, however, this solution will not work if the file doesn't have a Write or Read lock on it, i.e. it has been opened (for reading or writing) with FileShare.Read or FileShare.Write access.
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;
}
回答2:
You can suffer from a thread race condition on this which there are documented examples of this being used as a security vulnerability. If you check that the file is available, but then try and use it you could throw at that point, which a malicious user could use to force and exploit in your code.
Your best bet is a try catch / finally which tries to get the file handle.
try
{
using (Stream stream = new FileStream("MyFilename.txt", FileMode.Open))
{
// File/Stream manipulating code here
}
} catch {
//check here why it failed and ask user to retry if the file is in use.
}
回答3:
Use this to check if a file is locked:
using System.IO;
using System.Runtime.InteropServices;
internal static class Helper
{
const int ERROR_SHARING_VIOLATION = 32;
const int ERROR_LOCK_VIOLATION = 33;
private static bool IsFileLocked(Exception exception)
{
int errorCode = Marshal.GetHRForException(exception) & ((1 << 16) - 1);
return errorCode == ERROR_SHARING_VIOLATION || errorCode == ERROR_LOCK_VIOLATION;
}
internal static bool CanReadFile(string filePath)
{
//Try-Catch so we dont crash the program and can check the exception
try {
//The "using" is important because FileStream implements IDisposable and
//"using" will avoid a heap exhaustion situation when too many handles
//are left undisposed.
using (FileStream fileStream = File.Open(filePath, FileMode.Open, FileAccess.ReadWrite, FileShare.None)) {
if (fileStream != null) fileStream.Close(); //This line is me being overly cautious, fileStream will never be null unless an exception occurs... and I know the "using" does it but its helpful to be explicit - especially when we encounter errors - at least for me anyway!
}
}
catch (IOException ex) {
//THE FUNKY MAGIC - TO SEE IF THIS FILE REALLY IS LOCKED!!!
if (IsFileLocked(ex)) {
// do something, eg File.Copy or present the user with a MsgBox - I do not recommend Killing the process that is locking the file
return false;
}
}
finally
{ }
return true;
}
}
For performance reasons I recommend you read the file content in the same operation. Here are some examples:
public static byte[] ReadFileBytes(string filePath)
{
byte[] buffer = null;
try
{
using (FileStream fileStream = File.Open(filePath, FileMode.Open, FileAccess.ReadWrite, FileShare.None))
{
int length = (int)fileStream.Length; // get file length
buffer = new byte[length]; // create buffer
int count; // actual number of bytes read
int sum = 0; // total number of bytes read
// read until Read method returns 0 (end of the stream has been reached)
while ((count = fileStream.Read(buffer, sum, length - sum)) > 0)
sum += count; // sum is a buffer offset for next reading
fileStream.Close(); //This is not needed, just me being paranoid and explicitly releasing resources ASAP
}
}
catch (IOException ex)
{
//THE FUNKY MAGIC - TO SEE IF THIS FILE REALLY IS LOCKED!!!
if (IsFileLocked(ex))
{
// do something?
}
}
catch (Exception ex)
{
}
finally
{
}
return buffer;
}
public static string ReadFileTextWithEncoding(string filePath)
{
string fileContents = string.Empty;
byte[] buffer;
try
{
using (FileStream fileStream = File.Open(filePath, FileMode.Open, FileAccess.ReadWrite, FileShare.None))
{
int length = (int)fileStream.Length; // get file length
buffer = new byte[length]; // create buffer
int count; // actual number of bytes read
int sum = 0; // total number of bytes read
// read until Read method returns 0 (end of the stream has been reached)
while ((count = fileStream.Read(buffer, sum, length - sum)) > 0)
{
sum += count; // sum is a buffer offset for next reading
}
fileStream.Close(); //Again - this is not needed, just me being paranoid and explicitly releasing resources ASAP
//Depending on the encoding you wish to use - I'll leave that up to you
fileContents = System.Text.Encoding.Default.GetString(buffer);
}
}
catch (IOException ex)
{
//THE FUNKY MAGIC - TO SEE IF THIS FILE REALLY IS LOCKED!!!
if (IsFileLocked(ex))
{
// do something?
}
}
catch (Exception ex)
{
}
finally
{ }
return fileContents;
}
public static string ReadFileTextNoEncoding(string filePath)
{
string fileContents = string.Empty;
byte[] buffer;
try
{
using (FileStream fileStream = File.Open(filePath, FileMode.Open, FileAccess.ReadWrite, FileShare.None))
{
int length = (int)fileStream.Length; // get file length
buffer = new byte[length]; // create buffer
int count; // actual number of bytes read
int sum = 0; // total number of bytes read
// read until Read method returns 0 (end of the stream has been reached)
while ((count = fileStream.Read(buffer, sum, length - sum)) > 0)
{
sum += count; // sum is a buffer offset for next reading
}
fileStream.Close(); //Again - this is not needed, just me being paranoid and explicitly releasing resources ASAP
char[] chars = new char[buffer.Length / sizeof(char) + 1];
System.Buffer.BlockCopy(buffer, 0, chars, 0, buffer.Length);
fileContents = new string(chars);
}
}
catch (IOException ex)
{
//THE FUNKY MAGIC - TO SEE IF THIS FILE REALLY IS LOCKED!!!
if (IsFileLocked(ex))
{
// do something?
}
}
catch (Exception ex)
{
}
finally
{
}
return fileContents;
}
Try it out yourself:
byte[] output1 = Helper.ReadFileBytes(@"c:\temp\test.txt");
string output2 = Helper.ReadFileTextWithEncoding(@"c:\temp\test.txt");
string output3 = Helper.ReadFileTextNoEncoding(@"c:\temp\test.txt");
回答4:
Perhaps you could use a FileSystemWatcher and watch for the Changed event.
I haven't used this myself, but it might be worth a shot. If the filesystemwatcher turns out to be a bit heavy for this case, I would go for the try/catch/sleep loop.
回答5:
Just use the exception as intended. Accept that the file is in use and try again, repeatedly until your action is completed. This is also the most efficient because you do not waste any cycles checking the state before acting.
Use the function below, for example
TimeoutFileAction(() => { System.IO.File.etc...; return null; } );
Reusable method that times out after 2 seconds
private T TimeoutFileAction<T>(Func<T> func)
{
var started = DateTime.UtcNow;
while ((DateTime.UtcNow - started).TotalMilliseconds < 2000)
{
try
{
return func();
}
catch (System.IO.IOException exception)
{
//ignore, or log somewhere if you want to
}
}
return default(T);
}
回答6:
You can return a task which gives you a stream as soon as it becomes available. It's a simplified solution, but it is a good starting point. It's thread safe.
private async Task<Stream> GetStreamAsync()
{
try
{
return new FileStream("sample.mp3", FileMode.Open, FileAccess.Write);
}
catch (IOException)
{
await Task.Delay(TimeSpan.FromSeconds(1));
return await GetStreamAsync();
}
}
You can use this stream as usual:
using (var stream = await FileStreamGetter.GetStreamAsync())
{
Console.WriteLine(stream.Length);
}
回答7:
the only way I know of is to use the Win32 exclusive lock API which isn't too speedy, but examples exist.
Most people, for a simple solution to this, simply to try/catch/sleep loops.
回答8:
static bool FileInUse(string path)
{
try
{
using (FileStream fs = new FileStream(path, FileMode.OpenOrCreate))
{
fs.CanWrite
}
return false;
}
catch (IOException ex)
{
return true;
}
}
string filePath = "C:\\Documents And Settings\\yourfilename";
bool isFileInUse;
isFileInUse = FileInUse(filePath);
// Then you can do some checking
if (isFileInUse)
Console.WriteLine("File is in use");
else
Console.WriteLine("File is not in use");
Hope this helps!
回答9:
The accepted answers above suffer an issue where if file has been opened for writing with a FileShare.Read mode or if the file has a Read-Only attribute the code will not work. This modified solution works most reliably, with two things to keep in mind (as true for the accepted solution also):
- It will not work for files that has been opened with a write share mode
- This does not take into account threading issues so you will need to lock it down or handle threading issues separately.
Keeping the above in mind, this checks if the file is either locked for writing or locked to prevent reading:
public static bool FileLocked(string FileName)
{
FileStream fs = null;
try
{
// NOTE: This doesn't handle situations where file is opened for writing by another process but put into write shared mode, it will not throw an exception and won't show it as write locked
fs = File.Open(FileName, FileMode.Open, FileAccess.ReadWrite, FileShare.None); // If we can't open file for reading and writing then it's locked by another process for writing
}
catch (UnauthorizedAccessException) // https://msdn.microsoft.com/en-us/library/y973b725(v=vs.110).aspx
{
// This is because the file is Read-Only and we tried to open in ReadWrite mode, now try to open in Read only mode
try
{
fs = File.Open(FileName, FileMode.Open, FileAccess.Read, FileShare.None);
}
catch (Exception)
{
return true; // This file has been locked, we can't even open it to read
}
}
catch (Exception)
{
return true; // This file has been locked
}
finally
{
if (fs != null)
fs.Close();
}
return false;
}
回答10:
Aside from working 3-liners and just for reference: If you want the full blown information - there is a little project on Microsoft Dev Center:
https://code.msdn.microsoft.com/windowsapps/How-to-know-the-process-704839f4
From the Introduction:
The C# sample code developed in .NET Framework 4.0 would help in finding out which is the process that is having a lock on a file. RmStartSession function which is included in rstrtmgr.dll has been used to create a restart manager session and according to the return result a new instance of Win32Exception object is created. After registering the resources to a Restart Manager session via RmRegisterRescources function, RmGetList function is invoked to check what are the applications are using a particular file by enumerating the RM_PROCESS_INFO array.
It works by connecting to the "Restart Manager Session".
The Restart Manager uses the list of resources registered with the session to determine which applications and services must be shut down and restarted. Resources can be identified by filenames, service short names, or RM_UNIQUE_PROCESS structures that describe running applications.
It might be a little overengineered for your particular needs... But if that is what you want, go ahead and grab the vs-project.
回答11:
In my experience, you usually want to do this, then 'protect' your files to do something fancy and then use the 'protected' files. If you have just one file you want to use like this, you can use the trick that's explained in the answer by Jeremy Thompson. However, if you attempt to do this on lots of files (say, for example when you're writing an installer), you're in for quite a bit of hurt.
A very elegant way this can be solved is by using the fact that your file system will not allow you to change a folder name if one of the files there it's being used. Keep the folder in the same file system and it'll work like a charm.
Do note that you should be aware of the obvious ways this can be exploited. After all, the files won't be locked. Also, be aware that there are other reasons that can result in your Move operation to fail. Obviously proper error handling (MSDN) can help out here.
var originalFolder = @"c:\myHugeCollectionOfFiles"; // your folder name here
var someFolder = Path.Combine(originalFolder, "..", Guid.NewGuid().ToString("N"));
try
{
Directory.Move(originalFolder, someFolder);
// Use files
}
catch // TODO: proper exception handling
{
// Inform user, take action
}
finally
{
Directory.Move(someFolder, originalFolder);
}
For individual files I'd stick with the locking suggestion posted by Jeremy Thompson.
回答12:
Here is some code that as far as I can best tell does the same thing as the accepted answer but with less code:
public static bool IsFileLocked(string file)
{
try
{
using (var stream = File.OpenRead(file))
return false;
}
catch (IOException)
{
return true;
}
}
However I think it is more robust to do it in the following manner:
public static void TryToDoWithFileStream(string file, Action<FileStream> action,
int count, int msecTimeOut)
{
FileStream stream = null;
for (var i = 0; i < count; ++i)
{
try
{
stream = File.OpenRead(file);
break;
}
catch (IOException)
{
Thread.Sleep(msecTimeOut);
}
}
action(stream);
}
回答13:
You can use my library for accessing files from multiple apps.
You can install it from nuget: Install-Package Xabe.FileLock
If you want more information about it check https://github.com/tomaszzmuda/Xabe.FileLock
ILock fileLock = new FileLock(file);
if(fileLock.Acquire(TimeSpan.FromSeconds(15), true))
{
using(fileLock)
{
// file operations here
}
}
fileLock.Acquire method will return true only if can lock file exclusive for this object. But app which uploading file must do it in file lock too. If object is inaccessible metod returns false.
回答14:
I once needed to upload PDFs to an online backup archive. But the backup would fail if the user had the file open in another program (such as PDF reader). In my haste, I attempted a few of the top answers in this thread but could not get them to work. What did work for me was trying to move the PDF file to its own directory. I found that this would fail if the file was open in another program, and if the move were successful there would be no restore-operation required as there would be if it were moved to a separate directory. I want to post my basic solution in case it may be useful for others' specific use cases.
string str_path_and_name = str_path + '\\' + str_filename;
FileInfo fInfo = new FileInfo(str_path_and_name);
bool open_elsewhere = false;
try
{
fInfo.MoveTo(str_path_and_name);
}
catch (Exception ex)
{
open_elsewhere = true;
}
if (open_elsewhere)
{
//handle case
}
回答15:
I'm interested to see if this triggers any WTF reflexes. I have a process which creates and subsequently launches a PDF document from a console app. However, I was dealing with a frailty where if the user were to run the process multiple times, generating the same file without first closing the previously generated file, the app would throw an exception and die. This was a rather frequent occurrence because file names are based on sales quote numbers.
Rather than failing in such an ungraceful manner, I decided to rely on auto-incremented file versioning:
private static string WriteFileToDisk(byte[] data, string fileName, int version = 0)
{
try
{
var versionExtension = version > 0 ? $"_{version:000}" : string.Empty;
var filePath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, $"{fileName}{versionExtension}.pdf");
using (var writer = new FileStream(filePath, FileMode.Create))
{
writer.Write(data, 0, data.Length);
}
return filePath;
}
catch (IOException)
{
return WriteFileToDisk(data, fileName, ++version);
}
}
Probably some more care can be given to the catch block to ensure I'm catching the correct IOException(s). I'll probably also clear out the app storage on startup since these files are intended to be temporary anyways.
I realize this goes beyond the scope of the OP's question of simply checking if the file is in use but this was indeed the problem I was looking to solve when I arrived here so perhaps it will be useful to someone else.
回答16:
Would something like this help?
var fileWasWrittenSuccessfully = false;
while (fileWasWrittenSuccessfully == false)
{
try
{
lock (new Object())
{
using (StreamWriter streamWriter = new StreamWriter(filepath.txt"), true))
{
streamWriter.WriteLine("text");
}
}
fileWasWrittenSuccessfully = true;
}
catch (Exception)
{
}
}
回答17:
Try and move/copy the file to a temp dir. If you can, it has no lock and you can safely work in the temp dir without getting locks. Else just try to move it again in x seconds.
回答18:
I use this workaround, but i have a timespan between when i check the file locking with IsFileLocked function and when i open the file. In this timespan some other thread can open the file, so i will get IOException.
So, i added extra code for this. In my case i want load XDocument:
XDocument xDoc = null;
while (xDoc == null)
{
while (IsFileBeingUsed(_interactionXMLPath))
{
Logger.WriteMessage(Logger.LogPrioritet.Warning, "Deserialize can not open XML file. is being used by another process. wait...");
Thread.Sleep(100);
}
try
{
xDoc = XDocument.Load(_interactionXMLPath);
}
catch
{
Logger.WriteMessage(Logger.LogPrioritet.Error, "Load working!!!!!");
}
}
What do you think? Can i change some thing? Maybe i did not have to use IsFileBeingUsed function at all?
Thanks
来源：https://stackoverflow.com/questions/876473/is-there-a-way-to-check-if-a-file-is-in-use