Question

i copied the following example Microsoft Thread Example

Which gives the code below
but i get an error on the line “this.progressBar1.Value = newval;” stating that “Cross-thread operation not valid: Control ‘progressBar1’ accessed from a thread other than the thread it was created on.”

what could be the issue?
thanks
damo

C# Code

public partial class Form1 : Form
{         
    public Form1()
    {
        InitializeComponent();
    }
    private void Form1_Load(object sender, EventArgs e)
    {
        Thread trd = new Thread(new ThreadStart(this.ThreadTask));
        trd.IsBackground = true;
        trd.Start();
    }
    private void ThreadTask()
    {
        int stp;
        int newval;
        Random rnd = new Random();
        while (true)
        {
            stp = this.progressBar1.Step * rnd.Next(-1, 2);
            newval = this.progressBar1.Value + stp;
            if (newval > this.progressBar1.Maximum)
                newval = this.progressBar1.Maximum;
            else if (newval < this.progressBar1.Minimum)
                newval = this.progressBar1.Minimum;
            this.progressBar1.Value = newval;
            Thread.Sleep(100);
        }
    }
    private void button1_Click(object sender, EventArgs e)
    {
        MessageBox.Show("This is the main thread");
    }
}

Answer 1

You have to invoke new delegate:

    delegate void ThreadTaskDelegate();
    private void ThreadTask()
    {
        if (this.InvokeRequired)
        {
            ThreadTaskDelegate del = new ThreadTaskDelegate(ThreadTask);
            this.Invoke(del, null);
        }
        else
        {
            int stp;
            int newval;
            Random rnd = new Random();
            while (true)
            {
                stp = this.progressBar1.Step * rnd.Next(-1, 2);
                newval = this.progressBar1.Value + stp;

                if (newval > this.progressBar1.Maximum)
                    newval = this.progressBar1.Maximum;
                else if (newval < this.progressBar1.Minimum)
                    newval = this.progressBar1.Minimum;

                this.progressBar1.Value = newval;

                Thread.Sleep(100);
            }
        }


    }

Happy coding! 🙂