Show modal dialog (top most) in new thread - multithreading

Question

A modal dialog with top most property set to true, doesn't appear as top most when shown within a new thread. Example code:
Thread thread = new Thread(KickOffForm);
thread.SetApartmentState(ApartmentState.STA);
thread.Start();
private void KickOffForm(object state)
{
Thread.Sleep(5000); // Mimics logic that takes place before form is shown
var form = new Form2();
form.ShowDialog();
}
The modal dialog appears as top most if the form is instantiated at the beginning of the thread. Example code:
Thread thread = new Thread(KickOffForm);
thread.SetApartmentState(ApartmentState.STA);
thread.Start();
private void KickOffForm(object state)
{
var form = new Form2();
Thread.Sleep(5000); // Mimics logic that takes place before form is shown
form.ShowDialog();
}
The above code is executed within a class that is instantiated when exe starts.
Why would the form appear as top most when instantiated at the beginning of the thread and not if instantiated later on?

Answer 1

Forms can only be modal to the thread they are created and owned by.
If you want to display a modal dialog that stops interaction with your main form, you must create the dialog on the main UI thread.
This must be so, because each thread runs it's own message loop. One thread knows nothing about any message loop in another thread.

Answer 2

Maybe you could invoke the dialog window in a correct thread:
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void button1_Click( object sender, EventArgs e )
{
Thread thread = new Thread( KickOffForm );
thread.SetApartmentState( ApartmentState.STA );
thread.Start();
}
private void KickOffForm( object state )
{
var form = new Form2();
Thread.Sleep( 5000 ); // Mimics logic that takes place before form is shown
this.Invoke( (Action)(() => { form.ShowDialog(); }) );
}
}