private Control findTextBox(string name)
        {
            foreach (Control ctl in this.Controls)
            {
                if (ctl.GetType() == typeof(TextBox) && ctl.Name.Equals(name))
                {
                    return ctl;
                }
            }
            return null;
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            Control ctl = null;
            string name = null;
            for (int i = 1; i <= 9; i++){
                name = "t" + i.ToString();
                ctl = this.findTextBox(name);
                if (ctl != null){
                    ctl.Text = "irgendein Text..";
                }
            }
        }