C# background worker, winform. "Cross-thread operation not valid:"

Question

I'm trying to get background worker functioning in the most basic way with a windows form, for instance get a background process to change text in a label.. I got the basic background worker code here.. http://www.albahari.com/threading/part3.aspx Heres the code in my form.. Trying to make it so you press a button and then background worker thread is spawned which changes text in the label

    using System;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.Data;
    using System.Drawing;
    using System.Linq;
    using System.Text;
    using System.Windows.Forms;

       namespace WindowsFormsApplication4
    {
        public partial class Form1 : Form
        {
            public Form1()
            {
                InitializeComponent();
            }


            BackgroundWorker _bw = new BackgroundWorker();

             void backgroundio()
            {
                _bw.DoWork += bw_DoWork;
                _bw.RunWorkerAsync("Message to worker");

            }

             void bw_DoWork(object sender, DoWorkEventArgs e)
            {
                // This is called on the worker thread
                label1.Text = (string)(e.Argument);        // writes "Message to worker"
                // Perform time-consuming task...
            }

             void button1_Click(object sender, EventArgs e)
             {
                 backgroundio();
             }

        }
    }

For the label1.Text = (string)(e.Argument); I get this error.

Cross-thread operation not valid: Control 'label1' accessed from a thread other than the thread it was created on.

Thank for any help !! :)

Actually while I'm here can somebody explain this line?

 _bw.DoWork += bw_DoWork;

I dont get how += makes any sense in this context. how can you add those things?

Solution 1

Q1

Your bw_doWork method is a static. This means there is only one of those methods for all instances of your class. That method cannot access instance-specific properties or fields or methods. That explains the compiler error.

If you change that method to NOT be static it will allow you to reference label1 within it.

---

Q2.

The syntax you refer to is a shortcut to add an event handler to that event.

It just means "Add this handler to the list of handlers for the given event." The long-handed way of doing it is with AddEventHandler.

http://msdn.microsoft.com/en-us/library/system.reflection.eventinfo.addeventhandler.aspx

Q3

The cryptic message you get at runtime indicates that you cannot update the UI objects on the non-uI thread. (The bg worker implies a different thread.) The solution is to perform the update you want on the UI thread.

void bw_DoWork(object sender, DoWorkEventArgs e)
{
    // This is called on the worker thread
    UpdateLabel((string)e.Argument));
      ...more work here...
}

void UpdateLabel(string s)
{
    if (this.label1.InvokeRequired)
    {
        // It's on a different thread, so use Invoke.
        this.BeginInvoke (new MethodInvoker(() => UpdateLabel(s));
    }
    else
    {
        // It's on the same thread, no need for Invoke
        this.label1.Text = s;
    }
}

To learn more about it, http://msdn.microsoft.com/en-us/library/ms171728(v=vs.90).aspx

Solution 2

In the DoWork-event you should perform the work you want to be executed on a different thread, ie. "the background work that won't freeze your application".

Then you have a Progress-event (I think, something similar at least) - this is where you update your GUI, ie. change the text or your label. Don't perform any heavy work here, it's your main thread.

In your DoWork-event, ie. the background thread, you may report the progress to the main thread using a method on your BackgroundWorker object (I don't remember the name of the method, something like Report, Progress or something), the Progress-event will then be called on the main thread.

Comments

[Guye Incognito]

Thanks! To all: I edited my question after this, Cheeso isn't mad!!

[Cheeso]

ok, so you've accepted the answer, but you still have a question. I will edit my answer.

[Guye Incognito]

ah! the stuff here albahari.com/threading/part3.aspx is starting to make sense to me now!! :D