poke poke - 1 year ago 112
C# Question

InvalidOperationException when assigning text to a TextField

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()

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
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."
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)

Answer Source

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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