C# threading issue

Question

To play a bit with threading, delegates and backgroundworkers, I'm putting together a few small applications, I'm having a bit of trouble with one of them. I've a Windows form, with a textbox, a button and a richttext. When I press the button, the text in the textbox is used as a paramter to instantiate a class, like this:

public partial class Form1 : Form
{
    private BackgroundWorker backgroundWorker;

    public Form1()
    {
        InitializeComponent();            
    }

    private void button1_Click(object sender, EventArgs e)
    {   
        backgroundWorker = new BackgroundWorker();
        backgroundWorker.DoWork += new DoWorkEventHandler(worker_DoWork);
        backgroundWorker.RunWorkerAsync();
    }

    void worker_DoWork(object sender, DoWorkEventArgs e)
    {
        new Thread((ThreadStart)delegate()
        {
            this.BeginInvoke((ThreadStart)delegate()
            {
                foreach (string line in textBox1.Lines)
                {  
                    Dig digger = new Dig(line, textBox1.Text);
                    digger.DomainChecked += new Dig.DomainCheckedHandler(OnUpdateTicker);

                    string response = digger.GetAllInfo();

                    richTextBox1.AppendText(response);
                    Application.DoEvents();
                }
            });
        }).Start();
    }

    void OnUpdateTicker(string msg)
    {
        new Thread((ThreadStart)delegate()
        {
            this.BeginInvoke((ThreadStart)delegate()
            {
                label4.Text = msg;
                Application.DoEvents();
            });
        }).Start();            
    }
}

When debugging I run into a 'textBox1.Lines' threw an exception of type 'Microsoft.VisualStudio.Debugger.Runtime.CrossThreadMessagingException' Any tips on how to solve this problem?

Answer 1

First, there is no need to create new threads inside DoWork; the whole idea with the BackgroundWorker is that DoWork is executed on a separate thread. Second, since DoWork is executed on a separate thread and UI controls can be modified only on the UI thread, you need to invoke those updates correctly. So, a rewritten version of worker_DoWork could look like this:

void worker_DoWork(object sender, DoWorkEventArgs e)
{
    foreach (string line in textBox1.Lines)
    {  
        Dig digger = new Dig(line, textBox1.Text);
        digger.DomainChecked += new Dig.DomainCheckedHandler(OnUpdateTicker);
        string response = digger.GetAllInfo();
        richTextBox1.Invoke((Action) delegate { richTextBox1.AppendText(response); });
    }
}

Note how the code does not explicitly spawn any new threads, and also how the AppendText method call is done through a Control.Invoke call, forcing it to execute on the UI thread.

Answer 2

The main reason is that the textbox is not owned by the background thread.

Your UI thread owns all the UI objects, and you're spinning up a background thread when a button is pressed. That background thread should not have access to any UI objects.

If you want the value of the textbox to be used, you'll need to pass it to your background thread another way.

Have a look here for an explanation (and solution).

Answer 3

You can only update controls on the main thread from the main thread itself, unless you explicitly tell your program that it's ok to do, by using the .Invoke method of the control.

From: http://www.albahari.com/threading/part3.aspx

Control.Invoke

In a multi-threaded Windows Forms application, it's illegal to call a method or property on a control from any thread other than the one that created it. All cross-thread calls must be explicitly marshalled to the thread that created the control (usually the main thread), using the Control.Invoke or Control.BeginInvoke method. One cannot rely on automatic marshalling because it takes place too late – only when execution gets well into unmanaged code, by which time plenty of internal .NET code may already have run on the "wrong" thread – code which is not thread-safe.