this clode is closing all forms code example

Example 1: open a new form with a button and close the last one

this.Hide();
    var form2 = new Form2();
    form2.Closed += (s, args) => this.Close(); 
    form2.Show();

Example 2: c# open a new form and close current

{
    this.Hide();
    Form1 f1 = new Form1();
    f1.ShowDialog();
    this.Close();
}