C# Windows Form created by EventHandler disappears immediately

Question

I don't know why this is happening, but when I create a new form inside an EventHandler, it disappears as soon as the method is finished.

Here's my code. I've edited it for clarity, but logically, it is exactly the same.

static void Main()
{
    myEventHandler = new EventHandler(launchForm);
    // Code that creates a thread which calls
    // someThreadedFunction() when finished.
}

private void someThreadedFunction()
{
    //Do stuff

    //Launch eventhandler
    EventHandler handler = myEventHandler;
    if (handler != null)
    {
        handler(null, null);
        myEventHandler = null;
    }
}

private void launchForm(object sender, EventArgs e)
{
    mf = new myForm();
    mf.Show();
    MessageBox.Show("Do you see the form?");
}

private myForm mf;
private EventHandler myEventHandler;

The new form displays as long as the MessageBox "Do you see the form?" is there. As soon as I click OK on it, the form disappears.

What am I missing? I thought that by assigning the new form to a class variable, it would stay alive after the method finished. Apparently, this is not the case.

Answer 1

Is

dbf.Show();

a typo? Is it supposed to be this instead?

mf.Show();

Is it possible that there is another form that you are showing other than the one you intend to show?

Answer 2

I think if you create a form on a thread, the form is owned by that thread. When creating any UI elements, it should always be done on the main (UI) thread.

Answer 3

I believe the problem is that you are executing the code within the handler from your custom thread, and not the UI thread, which is required because it operates the Windows message pump. You want to use the Invoke method here to insure that the form gets and shown on the UI thread.

private void launchForm(object sender, EventArgs e)
{
    formThatAlreadyExists.Invoke(new MethodInvoker(() =>
    {
        mf = new myForm();
        mf.Show();
        MessageBox.Show("Do you see the form?");
    }));
}

Note that this assumes you already have a WinForms object (called formThatAlreadyExists) that you have run using Application.Run. Also, there may be a better place to put the Invoke call in your code, but this is at least an example of it can be used.

Answer 4

this looks as if you are not on the form sta thread so once you show the form it is gone and the thread finishes it's job it kills it self since there is nothing referenceing the thread. Its not the best solution out there for this but you ca use a showdialog() rather than a show to accomplish it keeping state if you need a code example i use this exact same process for a "loading...." form

public class Loading
{
    public delegate void EmptyDelegate();
    private frmLoadingForm _frmLoadingForm;
    private readonly Thread _newthread;
    public Loading()
    {
        Console.WriteLine("enteredFrmLoading on thread: " + Thread.CurrentThread.ManagedThreadId);
        _newthread = new Thread(new ThreadStart(Load));
        _newthread.SetApartmentState(ApartmentState.STA);
        _newthread.Start();
    }

    public void Load()
    {
        Console.WriteLine("enteredFrmLoading.Load on thread: " + Thread.CurrentThread.ManagedThreadId);
        _frmLoadingForm = new frmLoadingForm();
        if(_frmLoadingForm.ShowDialog()==DialogResult.OK)
        {

        }
    }


    /// <summary>
    /// Closes this instance.
    /// </summary>
    public void Close()
    {
        Console.WriteLine("enteredFrmLoading.Close on thread: " + Thread.CurrentThread.ManagedThreadId);
        if (_frmLoadingForm != null)
        {
            if (_frmLoadingForm.InvokeRequired)
            {
                _frmLoadingForm.Invoke(new EmptyDelegate(_frmLoadingForm.Close));
            }
            else
            {
                _frmLoadingForm.Close();
            }
        }
        _newthread.Abort();
    }
}
public partial class frmLoadingForm : Form
{

    public frmLoadingForm()
    {
        InitializeComponent();
    }
}

Answer 5

You created a window on a non UI thread. When the thread aborts it will take your window along with it. End of story. Perform invoke on the main form passing a delegate which will execute the method that creates the messagebox on the UI thread. Since the MessageBox is a modal window, if dont want the launchForm method to block the background thread, create a custom form with the required UI and call show() on it, not ShowDialog().