Question

.Net’s odd locking semantics are bugging me again.

I’m launching a thread, the child thread in turns starts a form. The parent thread should wait until the form is created.

My first attempt was to use a Monitor to watch the Form variable:

private void OpenForm()
{
    if (FormThread == null)
    {
        Monitor.Enter(Form);
        FormThread = new Thread(FormStub);
        FormThread.SetApartmentState(ApartmentState.STA);
        FormThread.Start();
        Monitor.Wait(Form);
        Monitor.Exit(Form);
    }
}

private void FormStub()
{
    Form = new ConnectorForm();
    Monitor.Enter(Form);
    Monitor.PulseAll(Form);
    Monitor.Exit(Form);
    Application.Run(Form);
}

… This throws an exception. Monitor.Enter() fails, since Form == null.

I could very easily create a dummy integer or something (I actually think I’ll canabalize the FormThread variable), but I was wondering if there was a more elegant solution.

Answer 1

Better synchronisation primitive for this case:

private ManualResetEvent mre = new ManualResetEvent(false);

private void OpenForm()
{
    if (FormThread == null)
    {
        FormThread = new Thread(FormStub);
        FormThread.SetApartmentState(ApartmentState.STA);
        FormThread.Start();
        mre.WaitOne();
    }
}

private void FormStub()
{
    Form = new ConnectorForm();
    mre.Set();
    Application.Run(Form);
}