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{
  // im Form-Konstruktor
  VisibleChanged += Form_VisibleChanged; //  bzw. direkt im Designer
  Visible = true
}

void Form_VisibleChanged(object sender, EventArgs e)
{
  VisibleChanged -= Form_VisibleChanged;

  splashScreen.Dispose(); // oder Hide
}                    

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    public partial class Hauptform : Form
    {
        Splash spScreen;

        public Hauptform()
        {
            InitializeComponent();

            bwSplash.WorkerReportsProgress = true;
            bwSplash.WorkerSupportsCancellation = true;

            spScreen = new Splash();
        }
        
        private void Hauptform_Load(object sender, EventArgs e)
        {
            if (bwSplash.IsBusy != true)
            {
                spScreen.Show();
                
                this.WindowState = FormWindowState.Minimized;
                this.ShowInTaskbar = false;

                bwSplash.RunWorkerAsync();
            }
        }

        private void bwSplash_DoWork(object sender, DoWorkEventArgs e)
        {
            BackgroundWorker worker = sender as BackgroundWorker;

            for (int i = 0; i <= 100; i++)
            {
                if (worker.CancellationPending == true)
                {
                    e.Cancel = true;
                    break;
                }
                else
                {
                    System.Threading.Thread.Sleep(500);
                }
            }
        }

        private void bwSplash_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            spScreen.Close();

            this.WindowState = FormWindowState.Normal;
            this.ShowInTaskbar = true;
        }
    }