Help with Cross-thread operation not valid

Posted on 2009-04-10
Last Modified: 2012-05-06
I have a simple application that is monitoring a folder in VB.NET.
When a file is dropped into the folder I want the buttons on the form to become enabled. When I try to access the buttons on the form to enable them I receive the following error message.

Cross-thread operation not valid: Control 'btnExtractXML' accessed from a thread other than the thread it was created on.

Sub onChanged(ByVal from As Object, ByVal e As FileSystemEventArgs) Handles watcher.Created
       btnExtractXML.Enabled = False
       txtPickFile.Enabled = True
       btnPickFile.Enabled = True
       btnRun.Enabled = True
    End Sub

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Question by:ross13
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LVL 53

Accepted Solution

Dhaest earned 500 total points
ID: 24114827
How to solve "Cross thread operation not valid"

How can I update my user interface from a thread that did not create it?

Updating the UI from a Secondary Thread
LVL 53

Expert Comment

ID: 24114830
The BackgroundWorker class makes this kind of thing very simple. It does all the marshalling to the UI thread for you.

To use winforms control from separate thread you should call Control.Invoke method. In your case this will be TextBox.Invoke
The code may look like this:
delegate void OutputUpdateDelegate(string data);
public void UpdateTextBox(string data)
if ( txtOutput.InvokeRequired )
  txtOutput.Invoke(new OutputUpdateDelegate(OutputUpdateCallback),
  new object[] { data });
  OutputUpdateCallback(data); //call directly
private void OutputUpdateCallback(string data)
 txtOutput.Text += data;
UpdateTextBox method can be called from other threads 

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LVL 53

Expert Comment

ID: 24114832
You can't access the form's controls through other threads than the one who created it. You can fix it by calling invoke.

Example of using an invoke and delegate can be found here:
How can I update my user interface from a thread that did not create it?

Update UI the easy way using anonymous delegates!F4A38E96E598161E!652.entry
LVL 53

Expert Comment

ID: 24114834
Perhaps you should take a look at this article: Updating the UI from a Secondary Thread
LVL 86

Expert Comment

by:Mike Tomlinson
ID: 24116267
It looks like your FileSystemWatcher is called "watcher":

    Sub onChanged(ByVal from As Object, ByVal e As FileSystemEventArgs) Handles watcher.Created

How and where from are you creating it?  What is the entry point into your application?  Can you show us some code?


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