CODEWITHSUNDEEP
c#wpf
Henry Hunt
Henry Hunt

Reputation: 181

How to make thread safe calls from FileSystemWatcher

I am writing an application with two parts, one part downloads data and lists its sources in a file which is being monitored by the other part which, every 15 minutes when the data is downloaded therefore updating the file, it loads the file contents and removes the old data. I currently have this code:

   private void FileSystemWatcher_Changed(object sender, FileSystemEventArgs e)
   {
       try
       {
           fsw.EnableRaisingEvents = false;
           MessageBox.Show("File Changed: " + e.FullPath);
           _times.Clear();

           XmlDocument dataset = new XmlDocument();
           dataset.Load(@"C:\Users\Henry\AppData\Local\{9EC23EFD-F1A4-4f85-B9E9-729CDE4EF4C7}\cache\DATA_RAINOBS\dataset.xml");
           for (int x = 0; x < dataset.SelectNodes("//Times/Time").Count; x++)
           {
               _times.Add(
                   new Time()
                   {
                       Original = dataset.SelectNodes("//Times/Time/@original")[x].InnerText,
                       Display = dataset.SelectNodes("//Times/Time/@display")[x].InnerText,
                       Directory = dataset.SelectNodes("//Times/Time/@directory")[x].InnerText + "_LORES.png"
                   });
           }
           times.SelectedIndex = 0;
       }
       finally { fsw.EnableRaisingEvents = true; }
   }

But when I run it, I get a System.NotSupportedException and from further information I know that it is because I am trying to manipulate a list from a separate thread created by the FileSystemWatcher.

I have literally done hardly any programming using multiple threads so I have no idea what to do. It would be very helpful if someone could modify the code above so that it is thread safe and will work properly because then I will have something to learn from and it won't be wrong. How do I make this work?

Upvotes: 2

Views: 2076

Answers (2)

Rohit Vats
Rohit Vats

Reputation: 81313

Your _times collection is binded with GUI part so error must be at line

_times.Clear();

WPF has constraint that you cannot modify source collection which is binded to GUI from thread other than UI thread. Refer to my answer over here for details.

As stated above you can modify it from only UI thread so consider dispatching this stuff on UI dispatcher which will queue this on UI thread. Get UI dispatcher like this:

App.Current.Dispatcher.Invoke((Action)delegate
{
   _times.Clear();
});

Also make sure any subsequent calls related to UI is dispatched on UI thread. May be wrap it under one call only:

XmlDocument dataset = new XmlDocument();
dataset.Load(@"C:\Users\Henry\AppData\Local\{9EC23EFD-F1A4-4f85-B9E9-
               729CDE4EF4C7}\cache\DATA_RAINOBS\dataset.xml");
App.Current.Dispatcher.Invoke((Action)delegate
{
   _times.Clear();
   for (int x = 0; x < dataset.SelectNodes("//Times/Time").Count; x++)
   {
      _times.Add(
           new Time()
           {
               Original = dataset.SelectNodes("//Times/Time/@original")
                                [x].InnerText,
               Display = dataset.SelectNodes("//Times/Time/@display")
                                [x].InnerText,
               Directory = dataset.SelectNodes("//Times/Time/@directory")
                                [x].InnerText + "_LORES.png"
           });
   }
   _times.SelectedIndex = 0;
});

Upvotes: 0

Luaan
Luaan

Reputation: 63772

You have to use Invoke on the control (or its owner). The method will then be queued for processing on the control's thread, rather than the FSW's thread.

On WPF, this is handled by a dispatcher for this, eg. 

times.Dispatcher.Invoke(() => { yourCode; });

If you're expecting your code to take some time, you might consider only doing the invoke with a full list of items at the end, rather than invoking the whole operation.

Upvotes: 1

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