Threading Basics

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.

Answer 2

You cannot use a method or property of a Form control from a different thread than the thread that created (called new) the control.

To do that just do:

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

    public void SetText(string text)
    {

        if (this.textBox1.InvokeRequired())
        {
            Action<string> auxDelegate = SetText;
            this.BeginInvoke(auxDelegate,text);
        }
        else
        {
            this.textBox1.Text = text;
        }
    }

What the method is doing with the beginInvoke is just calling again the SetText method from the thread that created the control.