I am having a weird problem with C#, I can't yet explain. I actually planned to create a simple file viewer that automatically updates the displayed text as soon as the file is changed, but it turns out more complicated than I thought.
I have the following basic code:
public partial class Main : Window
{
private FileSystemWatcher watcher;
private String filePath = "C:\\";
private String fileName = "example.txt";
public Main()
{
InitializeComponent();
this.txtFile.Text = File.ReadAllText(filePath + fileName);
this.watcher = new FileSystemWatcher(filePath);
this.watcher.Changed += new FileSystemEventHandler(watcher_Changed);
this.watcher.NotifyFilter = NotifyFilters.LastAccess | NotifyFilters.LastWrite;
this.watcher.Filter = fileName;
this.watcher.EnableRaisingEvents = true;
}
void watcher_Changed(object sender, FileSystemEventArgs e)
{
String text = File.ReadAllText(e.FullPath);
this.txtFile.Text = text; // <-- here comes the exception (line 42)
}
}
Now the assignment to the (WPF) TextField in the changed handler raises a System.InvalidOperationException
exception, telling me that the object is already used in a different thread. So why do I get that exception and more importantly, what do I have to do, to make this working?
By the way, I get the exception regardless of the string I'm assigning.
Full text of the exception, but in german:
System.InvalidOperationException wurde nicht behandelt. Message="Der aufrufende Thread kann nicht auf dieses Objekt zugreifen, da sich das Objekt im Besitz eines anderen Threads befindet." Source="WindowsBase" StackTrace: bei System.Windows.Threading.Dispatcher.VerifyAccess() bei System.Windows.DependencyObject.SetValue(DependencyProperty dp, Object value) bei System.Windows.Controls.TextBox.set_Text(String value) bei LiveTextViewer.Main.watcher_Changed(Object sender, FileSystemEventArgs e) in ...\Main.xaml.cs:Zeile 42. bei System.IO.FileSystemWatcher.CompletionStatusChanged(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* overlappedPointer) bei System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* pOVERLAP) InnerException:
FileSystemWatcher is raising this event on a different thread. You can't access user interface elements from a different thread than the one on which it was created.
You'll need to use the Dispatcher to push the call that sets the text back onto the UI thread:
void watcher_Changed(object sender, FileSystemEventArgs e)
{
String text = File.ReadAllText(e.FullPath);
this.Dispatcher.Invoke(DispatcherPriority.Normal, new Action(() =>
{
this.txtFile.Text = text;
// Do all UI related work here...
}));
}
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