Question

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;
using System.Threading;


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

        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        public void countToLots()
        {
            for (int i = 0; i < 10000000; i++)
            {
                textBox1.Text = "Counting to 10000000, value is " + i + Environment.NewLine;
            }
        }

        public void countToZero()
        {
            for (int i = 10000000; i > 0; i--)
            {
                textBox2.Text = "Counting to 0, value is " + i + Environment.NewLine;
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Thread countUp = new Thread(new ThreadStart(countToLots));
            Thread countDown = new Thread(new ThreadStart(countToZero));
            countUp.Start();
            countDown.Start();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            textBox3.Text = "Bobby bob bob " + Environment.NewLine;
        }
    }
}

I really need to try and get the hang of this – i just dont understand the theory behind why i get an error message. Could someone help me out please?

Cross-thread operation not valid:
Control ‘textBox1’ accessed from a
thread other than the thread it was
created on.

Answer 1

UI controls have “thread affinity”; they do not want to be touched by anything except the UI thread; that includes reading and writing properties. The assignment to .Text should be done from the UI thread, either by using Invoke, or BackgroundWorker.

For example:

public void countToLots()
{
    for (int i = 0; i < 10000000; i++)
    {
        // running on bg thread
        textBox1.Invoke((MethodInvoker) delegate {
            // running on UI thread
            textBox1.Text = "Counting to 10000000, value is "
                      + i + Environment.NewLine;
        });
        // running on bg thread again
    }
}

But note that this type of thread switching has overhead. You should not call back every iteration – you should (for example) update the UI every [n] iterations – in the above, every 10000 for example.