Question

I am in the process of writing an application that communicates with several devices through GPIB commands, running a test on some equipment. I’ve set up a class, TestProcedure, which will start a new thread, and run the testing. Throughout testing, I’ve set up several custom events to send information back to the GUI. Here is an example of a simple event:

    public event InformationEventHandler<string> TestInfoEvent;
    /// <summary>
    /// Event raised when information should be passed back to the main testing form.
    /// </summary>
    /// <param name="s">Information to send back to form.</param>
    private void OnInfo(string s)
    {
        if (TestInfoEvent != null)
            TestInfoEvent(this, s);
    }

Which would be handled through the GUI, updating a text box like this:

TheTestProcedure.TestInfoEvent += new TestProcedure.InformationEventHandler<string> 
                                 (InfoOccurred);
....
private void InfoOccurred(Object sender, string s)
{
    this.textBox1.Text = s + Environment.NewLine + this.textBox1.Text;
    if (this.textBox1.Text.Length > 10000)
        this.textBox1.Text = this.textBox1.Text.Remove(1000);
}

This event handling seems to be working fine. I haven’t received any cross threading issues, and overall it’s been working as expected. However, on another form I just added a similar event handler, which throws a cross-thread exception. The event fires, sending a simple class with a bit of information that I display in an InputTextBox (A custom ComponentOne control). The particular control does not have a .Invoke method, so I’m looking for alternative solutions to access it asynchronously.

So my question is, are event handlers safe to access controls on a form? If not, how do event handlers fire, and could somebody help educate me, or provide some linked information, as to how an event handler communicates with form controls? Do I need to lock the event?

Answer 1

Controls on the UI thread may only be accessed from the UI thread – any access from other threads is bound to cause issues. You need to use InvokeRequired and BeginInvoke() to marshal an event to the right thread if it’s not already there.

Example