C#

private static Form1 form = null;

 public Form1()
 {
     InitializeComponent();
     form = this;
 }

 private void Form1_Load(object sender, EventArgs e)
 {
     textBox2.Text = "Initialising.....";
     this.Show();
     Refresh();

form.UpDateText("Checking validity of " + FileName);

public void UpDateText(string ThisText)
 {
     using (StreamWriter logfile = new StreamWriter("D:\\TeklaZipping.log", true))
     {
         logfile.WriteLine(ThisText);
         logfile.Close();
     }
     Form1 frm1 = new Form1();
     frm1.textBox1.Text += ThisText + System.Environment.NewLine;
     frm1.Refresh();
 }

tps://www.codejourney.net/2019/02/csharp-8-default-interface-methods/

namespace ThisProg
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            form = this;
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            textBox2.Text = "Initialising.....";
            Refresh();
            button1.Enabled= false;
        }
        public static void FinishOff(int ErrorLevel)
        {
            button1.Enabled = true; // this line gives error CS0120 An object reference is required for the non-static field, method, or property 'Form1.button1'
        }
        private void button1_Click(object sender, EventArgs e)
        {
            System.Environment.Exit(1);
        }
    }
}