FileSystemWatcher触发多次Change事件的解决办法

迈不过友情╰ 2022-04-03 06:58 350阅读 0赞

最近要用到FileSystemWatcher来监控某个目录中的[文件][Link 1]是否发生改变，如果改变就[执行][Link 2]相应的操作。但在[开发][Link 3][过程][Link 4]中，发现FileSystemWatcher在文件创建或[修改][Link 5]后，会[触发][Link 6]多个Created或Changed事件，具体原因就是处理文件的过程中执行了多次[文件系统][Link 7]操作，触发了多次事件。具体[可以][Link 8]参看[微软][Link 9]的关于FileSystemWatcher这[方面][Link 10]的[解释][Link 11]：[Troubleshooting FileSystemWatcher Components][][ ][Troubleshooting FileSystemWatcher Components]，[另外][Link 12]我在网上发现 [Consolidate Multiple FileSystemWatcher Events][] 关于这方面的[解决][Link 13]办法，比较[实用][Link 14]，方便快速引入到[项目][Link 15]中。

# 来自MSDN的问题说明 #

## Troubleshooting FileSystemWatcher Components ##

**Visual Studio .NET 2003**

其他版本

![ImageSprite.png][]

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You may encounter the following situations while working with the **FileSystemWatcher** component:

### UNC Path Names Not Accepted on Windows NT 4.0 Computers ###

If you are working with a **FileSystemWatcher** component on a Windows NT version 4.0 computer and trying to set its path to monitor file system activity on a different Windows NT version 4.0 computer, you will not be able to specify a UNC-based path value in the [Path][] property to point to the computer in question. You can only set UNC-based values when working on Windows 2000 computers.

### Cannot Watch Windows 95 or Windows 98 Directories ###

If you set your **FileSystemWatcher **component to reference a directory on a Windows 95 or Windows 98 computer, you will receive an error about an invalid directory path when the project runs. When using **FileSystemWatcher**, you cannot watch directories on computers running Windows 95 or Windows 98.

### Multiple Created Events Generated for a Single Action ###

You may notice in certain situations that a single creation event generates multiple [Created][] events that are handled by your component. For example, if you use a**FileSystemWatcher** component to monitor the creation of new files in a directory, and then test it by using Notepad to create a file, you may see two **Created** events generated even though only a single file was created. This is because Notepad performs multiple file system actions during the writing process. Notepad writes to the disk in batches that create the content of the file and then the file attributes. Other applications may perform in the same manner. Because **FileSystemWatcher** monitors the operating system activities, all events that these applications fire will be picked up.

> **Note   **Notepad may also cause other interesting event generations. For example, if you use the  **ChangeEventFilter** to specify that you want to watch only for attribute changes, and then you write to a file in the directory you are watching using Notepad, you will raise an event . This is because Notepad updates the **Archived** attribute for the file during this operation.

### Unexpected Events Generated on Directories ###

Changing a file within a directory you are monitoring with a **FileSystemWatcher** component generates not only a [Changed][] event on the file but also a similar event for the directory itself. This is because the directory maintains several types of information for each file it contains — the names and sizes of files, their modification dates, attributes, and so on. Whenever one of these attributes changes, a change is associated with the directory as well.

# 解决方案 #

The .NET framework provides a `FileSystemWatcher` class that can be used to monitor the file system for changes. My requirements were to monitor a directory for new files or changes to existing files. When a change occurs, the application needs to read the file and immediately perform some operation based on the contents of the file.

While doing some manual testing of my initial implementation it was very obvious that the`FileSystemWatcher` was firing multiple events whenever I made a change to a file or copied a file into the directory being monitored. I came across the following in the MSDNdocumentation’s [Troubleshooting FileSystemWatcher Components][]

> ### Multiple Created Events Generated for a Single Action ###
> 
> You may notice in certain situations that a single creation event generates multiple Created events that are handled by your component. For example, if you use a FileSystemWatcher component to monitor the creation of new files in a directory, and then test it by using Notepad to create a file, you may see two Created events generated even though only a single file was created. This is because Notepad performs multiple file system actions during the writing process. Notepad writes to the disk in batches that create the content of the file and then the file attributes. Other applications may perform in the same manner. Because FileSystemWatcher monitors the operating system activities, all events that these applications fire will be picked up.
> 
> Note: Notepad may also cause other interesting event generations. For example, if you use the ChangeEventFilter to specify that you want to watch only for attribute changes, and then you write to a file in the directory you are watching using Notepad, you will raise an event. This is because Notepad updates the Archived attribute for the file during this operation.

I did some searching and was surprised that .NET did not provide any kind of wrapper around the FileSystemWatcher to make it a bit more user friendly. I ended up writing my own wrapper that would monitor a directory and only throw one event when a new file was created, or an existing file was changed.

In order to consolidate the multiple FileSystemWatcher events down to a single event, I save the timestamp when each event is received, and I check back every so often (using a Timer) to find paths that have not caused additional events in a while. When one of these paths is ready, a single `Changed` event is fired. An additional benefit of this technique is that the event from the FileSystemWatcher is handled very quickly, which could help prevent its internal buffer from filling up.

Here is the code for a `DirectoryMonitor` class that consolidates multiple Win32 events into a single `Change` event for each change:

## 解决方案代码 ##

using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.IO;
    using System.Threading;
    
    namespace ShareReadFile
    {
        public delegate void FileSystemEvent(String path);
    
        public interface IDirectoryMonitor
        {
            event FileSystemEvent Change;
            void Start();
        }
    
        public class DirectoryMonitor : IDirectoryMonitor
        {
            private readonly FileSystemWatcher m_fileSystemWatcher = new FileSystemWatcher();
            private readonly Dictionary<string, DateTime> m_pendingEvents = new Dictionary<string, DateTime>();
            private readonly Timer m_timer;
            private bool m_timerStarted = false;
    
            public DirectoryMonitor(string dirPath)
            {
                m_fileSystemWatcher.Path = dirPath;
                m_fileSystemWatcher.IncludeSubdirectories = false;
                m_fileSystemWatcher.Created += new FileSystemEventHandler(OnChange);
                m_fileSystemWatcher.Changed += new FileSystemEventHandler(OnChange);
    
                m_timer = new Timer(OnTimeout, null, Timeout.Infinite, Timeout.Infinite);
            }
    
            public event FileSystemEvent Change;
    
            public void Start()
            {
                m_fileSystemWatcher.EnableRaisingEvents = true;
            }
    
            private void OnChange(object sender, FileSystemEventArgs e)
            {
                // Don't want other threads messing with the pending events right now
                lock (m_pendingEvents)
                {
                    // Save a timestamp for the most recent event for this path
                    m_pendingEvents[e.FullPath] = DateTime.Now;
    
                    // Start a timer if not already started
                    if (!m_timerStarted)
                    {
                        m_timer.Change(100, 100);
                        m_timerStarted = true;
                    }
                }
            }
    
            private void OnTimeout(object state)
            {
                List<string> paths;
    
                // Don't want other threads messing with the pending events right now
                lock (m_pendingEvents)
                {
                    // Get a list of all paths that should have events thrown
                    paths = FindReadyPaths(m_pendingEvents);
    
                    // Remove paths that are going to be used now
                    paths.ForEach(delegate(string path)
                    {
                        m_pendingEvents.Remove(path);
                    });
    
                    // Stop the timer if there are no more events pending
                    if (m_pendingEvents.Count == 0)
                    {
                        m_timer.Change(Timeout.Infinite, Timeout.Infinite);
                        m_timerStarted = false;
                    }
                }
    
                // Fire an event for each path that has changed
                paths.ForEach(delegate(string path)
                {
                    FireEvent(path);
                });
            }
    
            private List<string> FindReadyPaths(Dictionary<string, DateTime> events)
            {
                List<string> results = new List<string>();
                DateTime now = DateTime.Now;
    
                foreach (KeyValuePair<string, DateTime> entry in events)
                {
                    // If the path has not received a new event in the last 75ms
                    // an event for the path should be fired
                    double diff = now.Subtract(entry.Value).TotalMilliseconds;
                    if (diff >= 75)
                    {
                        results.Add(entry.Key);
                    }
                }
    
                return results;
            }
    
            private void FireEvent(string path)
            {
                FileSystemEvent evt = Change;
                if (evt != null)
                {
                    evt(path);
                }
            }
        }
    }

# [FileSystemWatcher用法详解][FileSystemWatcher]    #

[http://blog.csdn.net/hwt0101/article/details/8469285][FileSystemWatcher]

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[Troubleshooting FileSystemWatcher Components]: http://msdn.microsoft.com/en-us/library/xcc1t119%28VS.71%29.aspx
[Link 12]: http://www.haogongju.net/tag/%E5%8F%A6%E5%A4%96
[Consolidate Multiple FileSystemWatcher Events]: http://spin.atomicobject.com/2010/07/08/consolidate-multiple-filesystemwatcher-events
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[Link 16]: http://msdn.microsoft.com/zh-cn/library/xcc1t119%28v=vs.71%29.aspx#feedback
[Path]: http://msdn.microsoft.com/zh-cn/library/system.io.filesystemwatcher.path%28v=vs.71%29.aspx
[Created]: http://msdn.microsoft.com/zh-cn/library/system.io.filesystemwatcher.created%28v=vs.71%29.aspx
[Changed]: http://msdn.microsoft.com/zh-cn/library/system.io.filesystemwatcher.changed%28v=vs.71%29.aspx
[FileSystemWatcher]: http://blog.csdn.net/hwt0101/article/details/8469285