C#

class MainForm
{
    Thread LenghtyOperationThread;

    //Some Code
    
    private void buttonCreateReport_Click(object sender, EventArgs e)
    {
      LengthyOperationThread = new Thread(() => { CreateReport(); });
      RunLengthyThread();
      ShowReport();
    }

    //Other code
}

private void RunLengthyThread()
{
   WaitForm wf = new WaitForm();
   this.Enabled = false;
   wf.Show();
   LengthyOperationThread.Start();
   LengthyOperationThread.Join();
   wf.Close();
   this.Enabled = true;
}

private void RunLengthyThread()
{
   WaitForm wf = new WaitForm();
   Thread WaitFormThread = new Thread(() => {
                WaitForm wf = new WaitForm();
                wf.ShowDialog();
            });

   LengthyOperationThread.Start();
   LengthyOperationThread.Join();

   try
   {
     WaitFormThread.Abort();
   }
   catch{ }
}