在文件解锁并且可以读取和重命名之前，阻塞线程的最简单方法是什么？例如，.NET Framework中是否存在WaitOnFile（）？
我有一个服务，它使用FileSystemWatcher查找要传输到FTP站点的文件，但文件创建事件在另一个进程写完文件之前触发。
理想的解决方案会有一个超时期限，因此线程在放弃之前不会永久挂起。
编辑：在尝试了下面的一些解决方案后，我最终更改了系统，以便所有文件都写入Path.GetTempFileName()，然后执行File.Move()到最后地点。一旦FileSystemWatcher事件被触发，该文件就已经完成。
答案 0 :(得分：57)
从Eric的回答开始，我提出了一些改进，使代码更加紧凑和可重用。希望它有用。
FileStream WaitForFile (string fullPath, FileMode mode, FileAccess access, FileShare share)
{
for (int numTries = 0; numTries < 10; numTries++) {
FileStream fs = null;
try {
fs = new FileStream (fullPath, mode, access, share);
return fs;
}
catch (IOException) {
if (fs != null) {
fs.Dispose ();
}
Thread.Sleep (50);
}
}
return null;
}
答案 1 :(得分：37)
这是我在related question上给出的答案：
/// <summary>
/// Blocks until the file is not locked any more.
/// </summary>
/// <param name="fullPath"></param>
bool WaitForFile(string fullPath)
{
int numTries = 0;
while (true)
{
++numTries;
try
{
// Attempt to open the file exclusively.
using (FileStream fs = new FileStream(fullPath,
FileMode.Open, FileAccess.ReadWrite,
FileShare.None, 100))
{
fs.ReadByte();
// If we got this far the file is ready
break;
}
}
catch (Exception ex)
{
Log.LogWarning(
"WaitForFile {0} failed to get an exclusive lock: {1}",
fullPath, ex.ToString());
if (numTries > 10)
{
Log.LogWarning(
"WaitForFile {0} giving up after 10 tries",
fullPath);
return false;
}
// Wait for the lock to be released
System.Threading.Thread.Sleep(500);
}
}
Log.LogTrace("WaitForFile {0} returning true after {1} tries",
fullPath, numTries);
return true;
}
答案 2 :(得分：15)
这是执行此操作的通用代码，与文件操作本身无关。这是一个如何使用它的例子：
WrapSharingViolations(() => File.Delete(myFile));
或
WrapSharingViolations(() => File.Copy(mySourceFile, myDestFile));
您还可以定义重试次数和重试之间的等待时间。
注意：不幸的是，基础Win32错误（ERROR_SHARING_VIOLATION）没有暴露在.NET中，因此我根据反射机制添加了一个小的hack函数（IsSharingViolation）来检查它。
/// <summary>
/// Wraps sharing violations that could occur on a file IO operation.
/// </summary>
/// <param name="action">The action to execute. May not be null.</param>
public static void WrapSharingViolations(WrapSharingViolationsCallback action)
{
WrapSharingViolations(action, null, 10, 100);
}
/// <summary>
/// Wraps sharing violations that could occur on a file IO operation.
/// </summary>
/// <param name="action">The action to execute. May not be null.</param>
/// <param name="exceptionsCallback">The exceptions callback. May be null.</param>
/// <param name="retryCount">The retry count.</param>
/// <param name="waitTime">The wait time in milliseconds.</param>
public static void WrapSharingViolations(WrapSharingViolationsCallback action, WrapSharingViolationsExceptionsCallback exceptionsCallback, int retryCount, int waitTime)
{
if (action == null)
throw new ArgumentNullException("action");
for (int i = 0; i < retryCount; i++)
{
try
{
action();
return;
}
catch (IOException ioe)
{
if ((IsSharingViolation(ioe)) && (i < (retryCount - 1)))
{
bool wait = true;
if (exceptionsCallback != null)
{
wait = exceptionsCallback(ioe, i, retryCount, waitTime);
}
if (wait)
{
System.Threading.Thread.Sleep(waitTime);
}
}
else
{
throw;
}
}
}
}
/// <summary>
/// Defines a sharing violation wrapper delegate.
/// </summary>
public delegate void WrapSharingViolationsCallback();
/// <summary>
/// Defines a sharing violation wrapper delegate for handling exception.
/// </summary>
public delegate bool WrapSharingViolationsExceptionsCallback(IOException ioe, int retry, int retryCount, int waitTime);
/// <summary>
/// Determines whether the specified exception is a sharing violation exception.
/// </summary>
/// <param name="exception">The exception. May not be null.</param>
/// <returns>
/// <c>true</c> if the specified exception is a sharing violation exception; otherwise, <c>false</c>.
/// </returns>
public static bool IsSharingViolation(IOException exception)
{
if (exception == null)
throw new ArgumentNullException("exception");
int hr = GetHResult(exception, 0);
return (hr == -2147024864); // 0x80070020 ERROR_SHARING_VIOLATION
}
/// <summary>
/// Gets the HRESULT of the specified exception.
/// </summary>
/// <param name="exception">The exception to test. May not be null.</param>
/// <param name="defaultValue">The default value in case of an error.</param>
/// <returns>The HRESULT value.</returns>
public static int GetHResult(IOException exception, int defaultValue)
{
if (exception == null)
throw new ArgumentNullException("exception");
try
{
const string name = "HResult";
PropertyInfo pi = exception.GetType().GetProperty(name, BindingFlags.NonPublic | BindingFlags.Instance); // CLR2
if (pi == null)
{
pi = exception.GetType().GetProperty(name, BindingFlags.Public | BindingFlags.Instance); // CLR4
}
if (pi != null)
return (int)pi.GetValue(exception, null);
}
catch
{
}
return defaultValue;
}
答案 3 :(得分：13)
我为这些事情汇总了一个助手类。如果您可以控制访问该文件的所有内容，它将起作用。如果你期待其他一些事情的争论，那么这是毫无价值的。
using System;
using System.IO;
using System.Threading;
/// <summary>
/// This is a wrapper aroung a FileStream. While it is not a Stream itself, it can be cast to
/// one (keep in mind that this might throw an exception).
/// </summary>
public class SafeFileStream: IDisposable
{
#region Private Members
private Mutex m_mutex;
private Stream m_stream;
private string m_path;
private FileMode m_fileMode;
private FileAccess m_fileAccess;
private FileShare m_fileShare;
#endregion//Private Members
#region Constructors
public SafeFileStream(string path, FileMode mode, FileAccess access, FileShare share)
{
m_mutex = new Mutex(false, String.Format("Global\\{0}", path.Replace('\\', '/')));
m_path = path;
m_fileMode = mode;
m_fileAccess = access;
m_fileShare = share;
}
#endregion//Constructors
#region Properties
public Stream UnderlyingStream
{
get
{
if (!IsOpen)
throw new InvalidOperationException("The underlying stream does not exist - try opening this stream.");
return m_stream;
}
}
public bool IsOpen
{
get { return m_stream != null; }
}
#endregion//Properties
#region Functions
/// <summary>
/// Opens the stream when it is not locked. If the file is locked, then
/// </summary>
public void Open()
{
if (m_stream != null)
throw new InvalidOperationException(SafeFileResources.FileOpenExceptionMessage);
m_mutex.WaitOne();
m_stream = File.Open(m_path, m_fileMode, m_fileAccess, m_fileShare);
}
public bool TryOpen(TimeSpan span)
{
if (m_stream != null)
throw new InvalidOperationException(SafeFileResources.FileOpenExceptionMessage);
if (m_mutex.WaitOne(span))
{
m_stream = File.Open(m_path, m_fileMode, m_fileAccess, m_fileShare);
return true;
}
else
return false;
}
public void Close()
{
if (m_stream != null)
{
m_stream.Close();
m_stream = null;
m_mutex.ReleaseMutex();
}
}
public void Dispose()
{
Close();
GC.SuppressFinalize(this);
}
public static explicit operator Stream(SafeFileStream sfs)
{
return sfs.UnderlyingStream;
}
#endregion//Functions
}
它使用命名的互斥锁。那些希望访问该文件的人试图获取对指定互斥锁的控制权，该互斥锁共享该文件的名称（将'\'变为'/'）。您可以使用Open（），它将停止直到可以访问互斥锁，或者您可以使用TryOpen（TimeSpan），它尝试获取给定持续时间的互斥锁，如果无法在时间跨度内获取，则返回false。这很可能在使用块中使用，以确保正确释放锁，并且在处理此对象时将正确处理流（如果打开）。
我做了一个快速测试，大约有20件事要对文件进行各种读/写操作，并且没有看到任何损坏。显然它不是很先进，但它应该适用于大多数简单的情况。
答案 4 :(得分：5)
对于这个特定的应用程序，直接观察文件将不可避免地导致难以追踪的bug，特别是当文件大小增加时。以下是两种不同的策略。
答案 5 :(得分：4)
我用了一段时间的技术之一是编写自己的函数。基本上捕获异常并使用计时器重试，您可以在指定的持续时间内触发。如果有更好的方法，请分享。
答案 6 :(得分：3)
来自MSDN：
OnCreated事件很快就会引发 在创建文件时。如果是文件 被复制或转移到 观看目录，OnCreated事件 将立即提出，然后 通过一个或多个OnChanged事件。
可以修改FileSystemWatcher，使其在“OnCreated”事件期间不执行读取/重命名，而是：
答案 7 :(得分：2)
在大多数情况下，像@harpo建议的简单方法会起作用。您可以使用此方法开发更复杂的代码：
答案 8 :(得分：2)
广告转移流程触发器文件SameNameASTrasferedFile.trg 在文件传输完成后创建。
然后设置FileSystemWatcher，它将仅在* .trg文件上触发事件。
答案 9 :(得分：1)
我不知道你用什么来确定文件的锁状态，但是这样的事情应该这样做。
while (true)
{
try {
stream = File.Open( fileName, fileMode );
break;
}
catch( FileIOException ) {
// check whether it's a lock problem
Thread.Sleep( 100 );
}
}
答案 10 :(得分：0)
一个可能的解决方案是，将filesystemwatcher与一些轮询结合起来，
为文件上的每个更改通知，并在收到通知时检查是否存在 按照当前接受的答案中的说明锁定：https://stackoverflow.com/a/50800/6754146 打开文件流的代码从答案中复制并稍加修改：
public static void CheckFileLock(string directory, string filename, Func<Task> callBack)
{
var watcher = new FileSystemWatcher(directory, filename);
FileSystemEventHandler check =
async (sender, eArgs) =>
{
string fullPath = Path.Combine(directory, filename);
try
{
// Attempt to open the file exclusively.
using (FileStream fs = new FileStream(fullPath,
FileMode.Open, FileAccess.ReadWrite,
FileShare.None, 100))
{
fs.ReadByte();
watcher.EnableRaisingEvents = false;
// If we got this far the file is ready
}
watcher.Dispose();
await callBack();
}
catch (IOException) { }
};
watcher.NotifyFilter = NotifyFilters.LastWrite;
watcher.IncludeSubdirectories = false;
watcher.EnableRaisingEvents = true;
//Attach the checking to the changed method,
//on every change it gets checked once
watcher.Changed += check;
//Initially do a check for the case it is already released
check(null, null);
}
通过这种方式，您可以检查文件是否已锁定，并在关闭指定的回调时收到通知，这样可以避免过度激进的轮询，只有在实际关闭时才能完成工作
答案 11 :(得分：0)
这里有一个与上面类似的答案，只是我添加了一个检查以查看文件是否存在。
bool WaitForFile(string fullPath)
{
int numTries = 0;
while (true)
{
//need to add this line to prevent infinite loop
if (!File.Exists(fullPath))
{
_logger.LogInformation("WaitForFile {0} returning true - file does not exist", fullPath);
break;
}
++numTries;
try
{
// Attempt to open the file exclusively.
using (FileStream fs = new FileStream(fullPath,
FileMode.Open, FileAccess.ReadWrite,
FileShare.None, 100))
{
fs.ReadByte();
// If we got this far the file is ready
break;
}
}
catch (Exception ex)
{
_logger.LogInformation(
"WaitForFile {0} failed to get an exclusive lock: {1}",
fullPath, ex.ToString());
if (numTries > 10)
{
_logger.LogInformation(
"WaitForFile {0} giving up after 10 tries",
fullPath);
return false;
}
// Wait for the lock to be released
System.Threading.Thread.Sleep(500);
}
}
_logger.LogInformation("WaitForFile {0} returning true after {1} tries",
fullPath, numTries);
return true;
}
答案 12 :(得分：-1)
我这样做与Gulzar一样，只是继续尝试循环。
实际上我甚至不打扰文件系统观察者。每分钟对网络驱动器轮询一次新文件很便宜。
答案 13 :(得分：-1)
只需将已更改事件与NotifyFilter NotifyFilters.LastWrite 一起使用：
var watcher = new FileSystemWatcher {
Path = @"c:\temp\test",
Filter = "*.xml",
NotifyFilter = NotifyFilters.LastWrite
};
watcher.Changed += watcher_Changed;
watcher.EnableRaisingEvents = true;
答案 14 :(得分：-1)
添加Outlook附件时遇到了类似的问题。 “使用”节省了一天。
string fileName = MessagingBLL.BuildPropertyAttachmentFileName(currProp);
//create a temporary file to send as the attachment
string pathString = Path.Combine(Path.GetTempPath(), fileName);
//dirty trick to make sure locks are released on the file.
using (System.IO.File.Create(pathString)) { }
mailItem.Subject = MessagingBLL.PropertyAttachmentSubject;
mailItem.Attachments.Add(pathString, Outlook.OlAttachmentType.olByValue, Type.Missing, Type.Missing);
答案 15 :(得分：-3)
作为一个选项如何：
private void WaitOnFile(string fileName)
{
FileInfo fileInfo = new FileInfo(fileName);
for (long size = -1; size != fileInfo.Length; fileInfo.Refresh())
{
size = fileInfo.Length;
System.Threading.Thread.Sleep(1000);
}
}
当然，如果在创建时预先分配了文件大小，那么你会得到误报。