Form is not available and doesn't update while a loop is in progress

Question

I have a method that has a for loop in it. In the for loop I want to update some label's text on the mainform, but the changes are only done after the loop ends.

I tried to do it on another thread like this:

Thread firstthread = new Thread(new ThreadStart(myMethod));
firstthread.Start();

When I did that I got an InvalidOperationException because of trying to access controls on another thread or something like that.

How should I update the labels(or other controls) on the mainform from a loop while the loop is in progress?

This is an olsd favourite, here is one reference... http://osherove.com/blog/2006/3/1/the-3-ways-to-create-a-thread-safe-gui-with-net-20-with-one.html — Justin Harvey, Oct 08 '12 at 13:41

score 4 · Accepted Answer · answered Oct 08 '12 at 13:47

4

You should use a BackgroundWorker. Place your long running loop inside of the DoWork event handler; it will run in a background thread and not block the UI thread. You can set ReportProgress to true and then attach an event handler to that to allow you to update a label (or whatever else) periodically. The ProgressReported event runs in the UI thread. You can also add a handler to the Completed event which runs in the UI thread as well.

You can look at the MSDN page for BackgroundWorker for details and code samples.

answered Oct 08 '12 at 13:47

Servy

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Soon I'm going to deal with that solution, but it's not that simple. – davidgereb Oct 08 '12 at 19:42
Well, usually it really is that simple. If it's not, then you'll need to provide more details about what complicates your particular case. – Servy Oct 08 '12 at 19:49

Marco Mp · Answer 2 · 2012-10-08T13:52:00.937

You should check the Invoke and BeginInvoke methods on the Form (for Windows.Forms) or on the Dispatcher object of the window (for WPF).

For example:

this.BeginInvoke(new Action(() => this.Text = "ciao"));

changes the title bar of the form.

BeginInvoke is asynchronous - it doesn't wait for the change to happen - while Invoke is synchronous and blocks until the change is done. Unless you have specifically that need, I would suggest using BeginInvoke which reduces the chances of an accidental deadlock.

This will allow you to update UI from a concurrent thread - and works whatever threading mechanism you are using (TPL tasks, plain Thread, etc.).

score 1 · Answer 3 · answered Oct 08 '12 at 14:19

As Servy said, you can use something like in this simple example:

public partial class Form1 : Form
{
    BackgroundWorker bgw;
    public Form1()
    {
        InitializeComponent();
        bgw = new BackgroundWorker();
        bgw.DoWork += new DoWorkEventHandler(bgw_DoWork);
        bgw.ProgressChanged += new ProgressChangedEventHandler(bgw_ProgressChanged);
        bgw.WorkerReportsProgress = true;
    }

    void bgw_ProgressChanged(object sender, ProgressChangedEventArgs e)
    {
        string text = (string)e.UserState;
        SetValue(text);//or do whatever you want with the received data
    }
    void SetValue(string text)
    {
        this.label1.Text = text;
    }

    void bgw_DoWork(object sender, DoWorkEventArgs e)
    {
        for (int i = 0; i < 10000; i++)
        {
            string text = "Value is " + i.ToString();
            bgw.ReportProgress(1, text);
            Thread.Sleep(1000);
        }
    }

    private void button1_Click(object sender, EventArgs e)
    {
        bgw.RunWorkerAsync();
    }

}

I'm calling ReportProgress method many times in a for loop but it just do it once — davidgereb, Oct 08 '12 at 19:41
I didn't tried your code, just something like that I found [link](http://www.dotnetperls.com/backgroundworker) before you answered and now it works but I think the problem is the way I want to update, but that's tomorrow problem, I think I'll sleep on it. — davidgereb, Oct 08 '12 at 20:09

Form is not available and doesn't update while a loop is in progress

3 Answers3