C# winforms startup (Splash) form not hiding

Probably you just want to close the splash form, and not send it to back.

I run the splash form on a separate thread (this is class SplashForm):

class SplashForm
{
    //Delegate for cross thread call to close
    private delegate void CloseDelegate();

    //The type of form to be displayed as the splash screen.
    private static SplashForm splashForm;

    static public void ShowSplashScreen()
    {
        // Make sure it is only launched once.

        if (splashForm != null)
            return;
        Thread thread = new Thread(new ThreadStart(SplashForm.ShowForm));
        thread.IsBackground = true;
        thread.SetApartmentState(ApartmentState.STA);
        thread.Start();           
    }

    static private void ShowForm()
    {
        splashForm = new SplashForm();
        Application.Run(splashForm);
    }

    static public void CloseForm()
    {
        splashForm.Invoke(new CloseDelegate(SplashForm.CloseFormInternal));
    }

    static private void CloseFormInternal()
    {
        splashForm.Close();
        splashForm = null;
    }
...
}

and the main program function looks like this:

[STAThread]
static void Main(string[] args)
{
    SplashForm.ShowSplashScreen();
    MainForm mainForm = new MainForm(); //this takes ages
    SplashForm.CloseForm();
    Application.Run(mainForm);
}