提问



我正在用C#编写一个需要重复访问1个图像文件的程序。大部分时间都可以工作,但是如果我的计算机运行得很快,它会在将文件保存回文件系统之前尝试访问该文件并抛出错误:另一个进程正在使用的文件。


我想找到解决这个问题的方法,但是我所有的谷歌搜索都只是通过使用异常处理来创建检查。这违背了我的宗教信仰,所以我想知道是否有人有更好的方法呢?

最佳参考


此解决方案的更新说明:使用FileAccess.ReadWrite检查对于只读文件将失败,因此已修改解决方案以使用FileAccess.Read进行检查。虽然此解决方案有效,因为如果文件上有写入或读取锁定,尝试使用FileAccess.Read检查将失败,但是,如果文件没有写入或读取锁定,此解决方案将无效,即已使用FileShare.Read或FileShare.Write访问打开(用于读取或写入)。


ORIGINAL:
我在过去的几年里一直使用这个代码,但我没有遇到任何问题。


理解你对使用异常的犹豫,但你不能一直避免它们:


protected virtual bool IsFileLocked(FileInfo file)
{
    FileStream stream = null;

    try
    {
        stream = file.Open(FileMode.Open, FileAccess.Read, FileShare.None);
    }
    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;
    }
    finally
    {
        if (stream != null)
            stream.Close();
    }

    //file is not locked
    return false;
}

其它参考1


您可能会遇到线程争用情况,其中有文档示例将此用作安全漏洞。如果您检查该文件是否可用,但是然后尝试使用它,那么您可能会抛出这一点,恶意用户可以使用该文件来强制和利用您的代码。


你最好的选择是尝试catch/finally,它试图获取文件句柄。


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.
}

其它参考2


用它来检查文件是否被锁定:


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;
}
}


出于性能原因,我建议您在同一操作中读取文件内容。这里有些例子:


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;
}


自己尝试一下:


byte[] output1 = Helper.ReadFileBytes(@"c:\temp\test.txt");
string output2 = Helper.ReadFileTextWithEncoding(@"c:\temp\test.txt");
string output3 = Helper.ReadFileTextNoEncoding(@"c:\temp\test.txt");

其它参考3


这是一个被接受的答案的powershell版本。


function IsFileLocked($filename) {

    $result = $false

    $fileinfo = [System.IO.FileInfo] (gi $filename).fullname

    try {
        $stream = $fileInfo.Open([System.IO.FileMode]"Open",[System.IO.FileAccess]"ReadWrite",[System.IO.FileShare]"None")
        $stream.Dispose()
    } catch [System.IO.IOException] {
        $result = $true
    }

    $result
}

其它参考4


也许你可以使用FileSystemWatcher并观察Changed事件。[25]


我自己没有使用它,但它可能值得一试。如果filesystemwatcher在这种情况下变得有点沉重,我会选择try/catch/sleep循环。

其它参考5


我所知道的唯一方法是使用Win32独占锁API,它不是太快,但存在例子。


对于一个简单的解决方案,大多数人只需尝试/捕获/睡眠循环。

其它参考6


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");


希望这可以帮助!

其它参考7


只需按预期使用例外。接受文件正在使用中,然后重复尝试,直到您的操作完成。这也是最有效的,因为在行动之前你不会浪费任何检查状态的周期。


例如,使用下面的功能


TimeoutFileAction(() => { System.IO.File.etc...; return null; } );


可重复使用的方法,在2秒后超时


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);
}

其它参考8


您可以返回一个任务,该任务在可用时立即为您提供流。这是一个简化的解决方案,但它是一个很好的起点。它是线程安全的。


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();
    }
}


您可以像往常一样使用此流:


using (var stream = await FileStreamGetter.GetStreamAsync())
{
    Console.WriteLine(stream.Length);
}

其它参考9


上面接受的答案遇到一个问题,即如果文件已经打开以使用FileShare.Read模式进行写入,或者如果文件具有只读属性,则代码将不起作用。这个修改过的解决方案工作最可靠,需要牢记两件事(对于公认的解决方案也是如此):



  1. 对于使用写共享模式打开的文件不起作用

  2. 这不会考虑线程问题,因此您需要将其锁定或单独处理线程问题。



牢记这一点,这将检查文件是否锁定以进行写入或已锁定以防止读取:


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;
}

其它参考10


以下是一些代码,据我所知,它与接受的答案相同,但代码较少:


    public static bool IsFileLocked(string file)
    {
        try
        {
            using (var stream = File.OpenRead(file))
                return false;
        }
        catch (IOException)
        {
            return true;
        }        
    }


但是我认为以下列方式执行它会更加健壮:


    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);
    }

其它参考11


根据我的经验,您通常希望这样做,然后保护您的文件以做一些奇特的事情,然后使用受保护的文件。如果您只想使用这样一个文件,您可以使用Jeremy Thompson在答案中解释的技巧。但是,如果您尝试在许多文件上执行此操作(例如,当您重新执行此操作时)写一个安装程序),你会受到很大的伤害。


这可以解决的一个非常优雅的方法是使用这样一个事实:如果文件系统中的一个文件被使用,你的文件系统将不允许你更改文件夹名称。将文件夹保存在同一个文件系统中并且它ll像魅力一样工作。


请注意,您应该了解可以利用的明显方法。毕竟,文件不会被锁定。另外,请注意,还有其他原因导致您的Move操作失败。显然,正确的错误处理(MSDN)可以在这里提供帮助。


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);
}


对于个别文件,我坚持使用Jeremy Thompson发布的锁定建议。

其它参考12


您可以使用我的库访问多个应用程序中的文件。


您可以从nuget安装它:Install-Package Xabe.FileLock


如果您想了解更多相关信息,请查看
https://github.com/tomaszzmuda/Xabe.FileLock[26]


ILock fileLock = new FileLock(file);
if(fileLock.Acquire(TimeSpan.FromSeconds(15), true))
{
    using(fileLock)
    {
        // file operations here
    }
}


fileLock.Acquire方法仅在可以锁定此对象的独占文件时才返回true。
但是上传文件的app也必须在文件锁中做。
如果对象不可访问,则metod返回false。

其它参考13


我有兴趣看看是否会触发任何WTF反应。我有一个创建并随后从控制台应用程序启动PDF文档的过程。但是,我正在处理一个脆弱的地方,如果用户多次运行该过程,在没有先关闭先前生成的文件的情况下生成相同的文件,应用程序将抛出异​​常并死亡。这是一个相当频繁的情况,因为文件名基于销售报价数字。


我决定依靠自动递增的文件版本控制,而不是以这种不合理的方式失败:


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);
    }
}


可能会对catch块给予更多关注,以确保我捕获正确的IOException。我可能也会在启动时清除应用程序存储,因为这些文件无论如何都是临时的。


我意识到这超出了OP的简单检查文件是否正在使用的问题的范围,但这确实是我到达这里时想要解决的问题,所以它可能对其他人有用。

其它参考14


尝试将文件移动/复制到临时目录。如果可以,它没有锁定,您可以安全地在临时目录中工作而不会获得锁定。其他只是尝试在x秒内再次移动它。

其它参考15


我使用此解决方法,但我有一个时间跨度,当我检查文件锁定与IsFileLocked函数和我打开文件时。在这个时间跨度中,其他一些线程可以打开文件,因此我将获得IOException。


所以,我为此添加了额外的代码。在我的情况下,我想加载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!!!!!");
            }
        }


你怎么看?我可以换一些东西吗?也许我根本不必使用IsFileBeingUsed函数?


谢谢

其它参考16


我有一个类似的问题并做了一些似乎工作的东西,它使用异常处理虽然......


我把一个计数器继续尝试100次以阻止无限循环。


见下文...


    private void uploadFiles(string filename)
    {
        try
        {
            string fromFileAndPath = Properties.Settings.Default.Path + "\\" + filename;
            string toFileAndPath = Properties.Settings.Default.CopyLocation + "\\" + filename;
            if (!File.Exists(toFileAndPath))
            {
                FileInfo imgInfo = new FileInfo(fromFileAndPath);
                bool copied = false;
                int counter = 0;
                while (!copied && counter < 100) //While was added as I was getting "The process cannot access the file because it is being used by another process" errors.
                {
                    try
                    {
                        counter++;
                        imgInfo.CopyTo(toFileAndPath);
                        copied = true;
                    }
                    catch
                    {
                        //If it cannot copy catch
                    }
                }
                if (counter > 100)
                    throw new Exception("Unable to copy file!");
                Thread.Sleep(1);
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show("An error occurred: " + ex.Message, "Error!", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }
    }