This blog defines how to close a Window Form.
//closing button

private void close_btn_Click(object sender, EventArgs e)

{

DialogResult dialog = MessageBox.Show("do u want to close", "Exit", MessageBoxButtons.YesNo);

if (dialog == DialogResult.Yes)

{

Application.Exit();

}

else if (dialog == DialogResult.No)

{

// e.Cancel = true;

}

}