How can a new Form be run on a different thread in C#?

Question

I'm just trying to run a new thread each time a button click even occurs which should create a new form. I tried this in the button click event in the MainForm:

private void button1_Click(object sender, EventArgs e)
{
    worker1 = new Thread(new ThreadStart(thread1));
    worker2 = new Thread(new ThreadStart(thread2));

    worker1.Start();
    worker2.Start();
}

private void thread1()
{
    SubForm s = new SubForm();
    s.Show();
}

private void thread2()
{
    SubForm s = new SubForm();
    s.Show();
}

The code in the Subform button click event goes like this:

private void button1_Click(object sender, EventArgs e)
{
    int max;
    try
    {
        max = Convert.ToInt32(textBox1.Text);
    }
    catch
    {
        MessageBox.Show("Enter numbers", "ERROR");
        return;
    }

    progressBar1.Maximum = max;

    for ( long i = 0; i < max; i++)
    {
        progressBar1.Value = Convert.ToInt32(i);
    }          
}

Is this the right way? Because I'm trying to open two independent forms, operations in one thread should not affect the other thread.

Or is BackGroundworker the solution to implement this? If yes, can anyone please help me with that?

Answer 1

You do not need to run forms in separate threads. You can just call s.Show() on multiple forms normally. They will not block each other.

Of course, if you’re doing something else, like some sort of calculation or other task that takes a long while, then you should run that in a separate thread, but not the form.

Here is a bit of code that will let you create a progress bar that shows progress for a long process. Notice that every time to access the form from inside the thread, you have to use .Invoke(), which actually schedules that invocation to run on the GUI thread when it’s ready.

public void StartLongProcess()
{
    // Create and show the form with the progress bar
    var progressForm = new Subform();
    progressForm.Show();
    bool interrupt = false;

    // Run the calculation in a separate thread
    var thread = new Thread(() =>
    {
        // Do some calculation, presumably in some sort of loop...
        while ( ... )
        {
            // Every time you want to update the progress bar:
            progressForm.Invoke(new Action(
                () => { progressForm.ProgressBar.Value = ...; }));

            // If you’re ready to cancel the calculation:
            if (interrupt)
                break;
        }

        // The calculation is finished — close the progress form
        progressForm.Invoke(new Action(() => { progressForm.Close(); }));
    });
    thread.Start();

    // Allow the user to cancel the calculation with a Cancel button
    progressForm.CancelButton.Click += (s, e) => { interrupt = true; };
}

Answer 2

Although I'm not 100% aware of anything that says running completely seperate forms doing completely isolated operations in their own threads is dangerous in any way, running all UI operations on a single thread is generally regarded as good practice.

You can support this simply by having your Subform class use BackgroundWorker. When the form is shown, kick off the BackgroundWorker so that it processes whatever you need it to.

Then you can simply create new instances of your Subform on your GUI thread and show them. The form will show and start its operation on another thread.

This way the UI will be running on the GUI thread, but the operations the forms are running will be running on ThreadPool threads.

Update

Here's an example of what your background worker handlers might look like - note that (as usual) this is just off the top of my head, but I think you can get your head around the basic principles.

Add a BackgroundWorker to your form named worker. Hook it up to the following event handlers:

void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
    // Executed on GUI thread.
    if (e.Error != null)
    {
        // Background thread errored - report it in a messagebox.
        MessageBox.Show(e.Error.ToString());
        return;
    }

    // Worker succeeded.
}

void worker_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
    // Executed on GUI thread.
    progressBar1.Value = e.ProgressPercentage;
}

void worker_DoWork(object sender, DoWorkEventArgs e)
{
    // Executed on ThreadPool thread.
    int max = (int)e.Argument;

    for (long i = 0; i < max; i++)
    {
        worker.ReportProgress(Convert.ToInt32(i));
    }
}

Your click handler would look something like:

void button1_Click(object sender, EventArgs e)
{
    int max;

    try
    {
        // This is what you have in your click handler,
        // Int32.TryParse is a much better alternative.
        max = Convert.ToInt32(textBox1.Text);
    }
    catch
    {
        MessageBox.Show("Enter numbers", "ERROR");
        return;
    }

    progressBar1.Maximum = max;

    worker.RunWorkerAsync(max);
}

I hope that helps.

Answer 3

Try this. It runs the new Form on its own thread with its own message queues and what not. Run this code:

new Thread(new ThreadStart(delegate
{
   Application.Run(new Form());
})).Start();

Use Thread.CurrentThread.GetHashCode() to test that is runs on different thread.

Answer 4

It's possible to run different forms on different threads. There are two caveats I'm aware of:

1. Neither form may be an MDI client of the other. Attempting to make a form an MDI client of another when the forms have different threads will fail.
2. If an object will be sending events to multiple forms and all forms use the same thread, it's possible to synchronize the events to the main thread before raising it. Otherwise, the event must be raised asynchronously and each form must perform its own synchronization mechanism for incoming events.

Obviously it's desirable not to have any window's UI thread get blocked, but using separate threads for separate windows may be a nice alternative.